BACKGROUND [0001] Some print apparatuses perform printing operations whereby print agent, or printing fluid, is delivered onto a printable substrate, such as a sheet of paper or cardboard, to form a printed image. Once printed, the printed sheet may be transported to a stacking region where sheets of printable substrate that have been printed can be stacked in a pile before post-printing processing (e.g. cutting) is performed. [0002] To achieve effective post-processing of the stacked sheets, it may be intended that the sheets are stacked uniformly, such that an edge (e.g, a leading edge) of the sheets are aligned with one another in the stack.

BRIEF DESCRIPTION OF DRAWINGS

[0003] Examples will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0004] Figure 1 is a schematic illustration of an example of stack analysis apparatus and a stack of substrate sheets;

[0005] Figure 2 is a flowchart of an example of a substrate stack assessment method; [0006] Figure 3 is an example of a graph showing pixel luminance for pixels in an image of a stack of substrate sheets;

[0007] Figure 4 is a further example of a graph showing pixel luminance for pixels in an image of a stack of substrate sheets;

[0008] Figure 5 is a further example of a graph showing pixel luminance for pixels in an image of a stack of substrate sheets;

[0009] Figure 6 is a further example of a graph showing pixel luminance for pixels in an image of a stack of substrate sheets;

[0010] Figure 7 is an example of three graphs showing the deviation of an edge of each substrate sheet from a reference position; [0011] Figure 8 is a flowchart of a further example of a substrate stack assessment method; [0012] Figure 9 is a schematic illustration of an example of stack analysis apparatus;

[0013] Figure 10 is a schematic illustration of a further example of a stack analysis apparatus; and [0014] Figure 11 is a schematic illustration of a machine-readable medium in communication with a processor.

DETAILED DESCRIPTION

[0015] When creating a stack of sheets of substrate or media, such as paper, cardboard or the like, it may be intended that the sheets are stacked neatly with at least one edge of each sheet aligned and flush against the edge of other sheets in the stack. This may be the case, for example, when sheets of printable substrate are to be stacked after they have been printed as part of a printing operation, in other examples, sheets of unprinted substrate may be stacked, for example if sheets of printable substrate are prepared prior to a printing operation. As used herein, the terms “printable substrate” and “printable media” are used interchangeably, and are intended to refer to items that can be printed on using a print apparatus.

[0016] Following a printing operation, it may be intended that sheets of printable substrate that have been printed (e.g. substrate sheets on which print agent has been deposited) are to be processed further, for example by having edges trimmed or by being cut in some way. To achieve a consistent cut through ail of the intended sheets in the stack, it may be intended that the edges of the sheets that are to be cut are aligned with one another, in a print apparatus, sheets of substrate may proceed from a substrate feeder, through the print apparatus, into a stacking region. During the passage through the print apparatus, the speed at which the substrate sheets move varies, and the sheets are exposed to various mechanical forces. Furthermore, the condition of substrate sheets provided into the feeder of the print apparatus may vary. For example, environmental conditions (e.g. humidity) or mechanical forces may damage or change the properties of a substrate sheet before it passes through the print apparatus. Substrate sheets having different physical properties may not behave as intended when they reach the stacking region and, as a result, substrate sheets may not be stacked uniformly with aligned edges.

[0017] Examples disclosed herein provide a mechanism by which a stack of sheets of substrate can be monitored using an optical assessment system, or stack assessment system, A camera of the system captures an image of an edge of a stack of substrate sheets and, following various processing operations, it may be determined if any sheets within the stack of sheets are misaligned or, more specifically, if an edge of any sheets in the stack deviates from a reference position. In some examples, the extent to which a sheet deviates from a reference position may also be determined. If it is determined that any sheets deviate from the reference position by more than a defined threshold amount, then appropriate action may be taken, such as pausing the printing operation, and/or making an appropriate adjustment to the print apparatus, or a component thereof, to improve the alignment of sheets stacked in the stacking region, [0018] Referring now to the drawings, Figure 1 is a schematic illustration of an example apparatus 100, such as a substrate stack analysis apparatus, and a stack 102 of sheets of substrate to be analyzed. In this example, the substrate sheets may comprise sheets of printable substrate, such as paper, cardboard, plastics, ceramics, fabric, glass and the like. In this example, the sheets in the stack 102 each have a first edge and a second edge, such that the first edges of the sheets accumulate at a first end 102a of the stack 102, and the second edges of the sheets accumulate at a second end 102b of the stack. The apparatus 100 comprises at least one image capture device, or image capture module 104, such as a camera and/or a CCD, to capture an Image of at least one end of the stack 102 of sheets. In some examples, the apparatus 100 may include a single camera 104 to capture an image of both ends 102a and 102b of the stack 102, in the example shown, however, the apparatus 100 includes a first camera 104 to capture an image of the first end 102a of the stack 102, and a second camera 106 to capture an image of the second end 102b of the stack.

[0019] The apparatus 100 may comprise a light source 108 to illuminate the stack 102. In some examples, the apparatus 100 may comprise multiple sources 108, such as a first light source to illuminate the first end 102a of the stack 102 and a second light source to illuminate the second end 102b of the stack. In the example shown in Figure 1 , a single light source 108 is provided to illuminate both ends 102a and 102b of the stack 102,

[0020] The apparatus 100 may further comprise a processor 110 which may be in communication with the first camera 104, the second camera 106 and/or the light source(s) 108. The processor 110 may control components with which it is in communication, and/or may receive data from components such as the cameras. Various components of the apparatus 100, including the processor 110, may also be in communication with other components within and/or external to the apparatus 100, In some examples, the processor 110 may be located remotely from the apparatus 100 and may, for example, comprise processing circuitry accessible via a cloud-computing environment. Among other functions, the processor 110 may be used to process data, such as image data, acquired using the camera or cameras 104, 106. For example, the processor 110 may perform blocks of the methods disclosed below, in order to assess the stack 102. [0021] Figure 2 is a flowchart of an example of a method 200. The method 200 may, for example, comprise a stack assessment method, in some examples, the method 200 may be considered to be a computer-implemented method, performed, for example, using processing circuitry or a processor, such as the processor 110. The method 200 comprises, at block 202, capturing an image of a stack 102 of sheets of printable substrate. In some examples, block 202 may involve capturing multiple images (e.g. video) of the stack 102. For example, video may be captured at 120 frames per second (fps) or any other appropriate frame rate. In examples where multiple images are captured, processing functions described below may be performed in respect of each captured image, or in respect of a subset of the captured images, such as every 10 images. Any appropriate image resolution may be used for capturing images, such as 1024 x 768 pixels.

[0022] The captured image may show parts of the stack 102 in addition to the end(s) of the stack. For example, other parts of the stacking region, or components of a print apparatus may be visible in the image in addition to the stack of sheets. Thus, in order to assess whether or not the sheets are stacked adequately or, more specifically, whether or not the edges of the sheets are aligned with one another, the method 200 may involve identifying the relevant part of the image to be assessed. This may be referred to as determining a region of interest, whereby the region of interest includes the end of the stack 102, and may include part or all of the stack, but may exclude some other parts of the captured image that are not part of the stack, such as the background of the image. The captured image or images each comprise a plurality of rows of pixels and a plurality of columns of pixels. Each pixel may have an associated luminance relating to the amount of light emitted from that pixel. According to examples disclosed herein, the relevant parts of the image (i.e. the region of the interest) may be determined by analyzing the luminance of pixels in the Image, and determining which rows and columns of pixels in the image correspond to the stack 102 and which do not.

[0023] Thus, at block 204, the method 200 comprises calculating pixel luminance values for pixels in each of a plurality of rows of pixels and pixel luminance values for pixels in each of a plurality of columns of pixels in the captured image, in other words, at block 204, the luminance values of pixels in each row and in each column in the Image are calculated or determined. Various metrics may be used as a measure of the luminance in each row and column. For example, the luminance of all pixels in a row or column may be summed to obtain a total luminance for each row and for each column. In other examples, the luminance of pixels in each row and each column may be averaged. Thus, in such examples, calculating the pixel luminance values may comprise calculating an average pixel luminance for each of the plurality of rows of pixels and an average pixel luminance for each of the plurality of columns of pixels in the captured image.

[0024] The method 200 comprises, at block 206, determining, based on the calculated values, a region of interest within the image that shows a plurality of stacked sheets whose alignment is to be assessed. An example of how the region of interest may be determined is discussed below with reference to Figures 3 to 5.

[0025] Once the region of interest has been determined, the method 200 comprises, at block 208, determining, based on an assessment of each row of pixels within the region of interest of the image, a deviation, from a reference position, of an edge of each sheet of printable substrate in the plurality of stacked sheets. Examples of how the determination of block 208 may be performed are discussed below.

[0026] Figure 3 shows an example of an image 302 of a part of a stack 102 of substrate sheets. The image 302 also shows other parts of a print apparatus in which the stack 102 is located. The values along the bottom of the image 302 represent the columns of pixels in the image, and the values on the side of the image represent the rows of pixels in the image. Values are provided similarly in the images of Figures 4 to 7. Figure 3 also includes a graph 304 showing a relative luminance associated with each row of pixels in the image 302. While, in examples discussed herein, a relative luminance is measured, in other examples, a different measure of luminance (e,g. absolute luminance) may be used. In this example, the graph 304 shows an average relative luminance (i.e. a grey level or grey value ranging from 0 to 255) for each row in the image. The pixel row numbers along the axis of the graph 304 correspond to the pixel rows shown in the image 302. In the image 302, it can be seen that the pixels in rows 0 to around 250 are relatively dark and, therefore, are shown in the graph 304 to have a relatively low average relative luminance ranging from around 10 to 30. From around pixel row 250 onwards, the measured or calculated average relative luminance is relatively high, at around 80 to 120. It may be determined, therefore, that the significant change in relative luminance at around pixel row 250 may correspond to a row of pixels in the image where a top sheet of the stack 102 appears, because the pixels in that row of the image represent something (e.g. a substrate sheet) other than the background that is shown in the previous row (e.g. the row above in this example). The change in relative luminance may be referred to as a discontinuity. In some examples, the presence of the top sheet in the stack 102 may be determined when the change in the average relative pixel luminance between two adjacent rows, or between two rows within a defined number of rows (e.g. between two rows that are within 5 pixels of one another) in the image meets or exceeds a defined threshold amount. Thus, in some examples, determining (block 206) the region of interest may comprise determining a position of a top sheet of the plurality of stacked sheets. The position of the top sheet may be determined by determining that the relative pixel luminance values for pixels in a first row of pixels differ from the relative pixel luminance values for pixels in a second row of pixels by more than a threshold amount. The first row pixels may appear within a defined distance or number of pixels of the second row of pixels. In some examples, the first of pixels may be adjacent to the second pixels. In some examples, an edge detection operator, such as a Canny edge detector may be used in identifying the upper edge of the stack 102 in the image 302, which corresponds to the top sheet in the stack. [0027] in order to check that the change in relative luminance between the rows is due to the appearance in the image 302 of the edge of the stack 102, and not merely noise (e.g. high-luminance pixels from the background of the image), the relative luminance (e.g. the average relative luminance) of rows near to the discontinuity may be considered. For example, the average relative luminance of a number of rows of pixels (e.g. 10 pixels) towards the stack direction (i.e. in this example in the direction of increasing pixel number) is analyzed, if the average relative iuminance of each pixei row differs from (e.g. is above) the average relative iuminance of the row before the discontinuity by more than a defined threshold, then the discontinuity is considered to be “valid”. In other words, it is determined that the discontinuity represents a row corresponding to the edge (e.g. the top sheet) of the stack 102. Otherwise, the discontinuity is disregarded and the next discontinuity appearing in the graph 304 is considered.

[0028] in some examples, a bottom sheet of the region of interest may be determined in a similar way, namely by identifying a significant (e.g. more than a threshold amount) change in the relative Iuminance values (e.g. a discontinuity) between two rows of pixels. In other examples, including the example shown in Figure 3, the bottom sheet of a stack 102 whose alignment is to be assessed may be determined from an indication that the bottom/first sheet has been fed into the print apparatus from a feeder, an indication that the bottom/first sheet has been printed, and/or an indication that the bottom/first sheet has arrived in the stacking region. In some examples, the print apparatus, or a component (e.g. a processor) thereof, may send a trigger signal when a new printing operation has begun, so that it can be determined that the next sheet to arrive in the stacking region is the bottom/first sheet of the stack 102 to be assessed, in the example of Figure 3, sheets from three printing operations are shown stacked in the image 302. Sheets of a first printing operation 306 are at the bottom of the stack 102, sheets of a second printing operation 308 are next, and sheets of a third printing operation 310 (whose alignment is to be assessed in this example) are on top. It is noted that the sheets of the second printing operation 308 are of a different size to the sheets of the first and third printing operations 306, 310. From a signal indicating the start of the third printing operation 310 (e.g. a trigger signal provided by the printing apparatus), it can be determined where the bottom/first sheet of that printing operation appears in the image. In some examples, other information, such as information describing properties (e.g. thickness, size, and the like) of the sheets may be provided, so that the position of the bottom/first sheet in the image can be more accurately determined. For example, from an indication of the thickness of each sheet in pixels, and an indication of the number of sheets that have been printed and/or stacked in the stacking region during the printing operation, it is possible to determine where in the image the bottom sheet of the printing operation is located. This is likely to be the first sheet that was printed in the printing operation and, therefore, the first sheet to be stacked in the stack 102 to be analyzed, and will form the bottom of the region of interest. [0029] The vertical boundaries of the region of interest may also be determined in a similar way. Figure 4 shows an example of an image 402, which is a cropped portion of the image 302. Figure 4 also includes a graph 404 showing a relative luminance associated with each column of pixels in the image 402. in this example, the graph 404 shows an average relative luminance for each column in the image. The pixel column numbers along the axis of the graph 404 correspond to the pixel columns shown in the image 402. in the image 402, it can be seen that the pixels in columns 0 to around 450 are relatively dark and, therefore, are shown in the graph 404 to have a relatively low average relative luminance ranging from around 25 to 75. From around pixel column 450 onwards, the measured or calculated average relative luminance is relatively high. It may be determined, therefore, that the significant change in reiative luminance, or discontinuity, at around pixel column 450 may correspond to a column of pixels in the image where an edge of the stack 102 appears. A technique similar to that described above may be used to confirm that the discontinuity corresponds to the edge of the stack 102 rather than as a resuit of noise. For example, the reiative iuminance (e.g. the average relative luminance) of a number of columns of pixels (e.g. 10 pixels) towards the stack direction (i.e. in this example in the direction of increasing pixel number) may be analyzed and, if the average relative luminance of each pixel column differs from (e.g. is above) the average relative luminance of the row before the discontinuity by more than a defined threshold, then the discontinuity is considered to be valid. Thus, the identified column may be confirmed as the edge of the stack 102.

[0030] Using the above technique, a first pixel column of the region of interest may be determined. Namely, the first column of the region of interest may be considered to be the column corresponding to the start of the discontinuity (at around 430 pixels in the graph 404). in the example shown in Figure 4, the last pixel column of the region of interest may comprise the last column in the image (i.e. the pixel column at around 1010 pixels), if the camera used to capture the image captures just one end of the stack 102 rather than both ends of the stack.

[0031] in order to identify small deviations in the alignment of sheets in the stack 102, the region of interest may be extended or expanded beyond the end of the stack in a direction away from the stack. For example, the region of interest may be extended by around 20 pixels from the first column that correspond to the discontinuity shown in Figure 4. Figure 5 shows an example of an image 502, which is a cropped a portion of the image 402. Figure 5 also includes a graph 504 showing a relative luminance associated with each column of pixels in the image 502. More specifically, the graph 504 shows a maximum relative luminance of each column of pixels in the image 502.

[0032] in order to determine the first column of pixels containing a sheet of the stack 102, the maximum relative luminance of each pixel column in the image 502 is analyzed, as shown in the graph 504. In this way, the analysis is sensitive to any minor deviations. As can be seen in the graph 504, a peak or spike (i.e. a discontinuity) appears at around pixel column 10 of the image 502. This may be considered to be a “fine-tuning” process. In order to check that the identified discontinuity is “valid” rather than merely noise, techniques such as those discussed above may be implemented.

[0033] Once the region of interest has been established, each row of the region of interest may be analyzed. Figure 6 shows an example of an image 602, which is a cropped portion of the image 502, and which represents the region of interest to be analyzed. Figure 6 also includes a graph 604 representative of the number of pixels by which each sheet is displaced in the image 602. For example, a row in the image 602 corresponding to a sheet that is displaced from the edge of the region of interest (i.e. separated from pixel column 0) has a relatively higher value on the graph 604 than a row corresponding to a sheet that is nearer the edge of the region of interest (i.e. near pixel column 0). Thus, it can be seen that the misaligned sheets in the stack at around pixel rows 70 to 100 correspond to a peak (e.g. a discontinuity) in the graph 604 relative to the sheets appearing in rows either side of pixel rows 70 to 100. Similarly, a misalignment of sheets appearing in the image at around pixel rows 200 to 260 correspond to a relatively high value in the graph 604. Specifically, the sheet at approximately pixel row 100 appears to be displaced from the edge of the region of interest by around 12 pixels.

[0034] The data shown in the graph 604 may be used to determine whether or not any sheets in the stack 102 are displaced from other stack and also which particular sheets are displaced. If ail of the sheets were perfectly aligned with one another, the graph would include a straight line of constant displacement over the whole range of pixel rows.

[0035] Thus, as noted previously, block 208 of the method 200 uses an assessment of each row of pixels within the region of interest of the image to determine a deviation, from a reference position, of an edge of each sheet of printable substrate in the plurality of stacked sheets. Different reference positions may be used depending on the metric to be considered. Figure 7 shows three graphs showing the deviation of an edge of each sheet according to different metrics, in some examples, determining (block 208) deviation may comprise determining the deviation of the edge of each sheet relative to an average deviation of sheets in the plurality of stacked sheets. In this example, a deviation of each sheet in the stack from the edge of the region of interest (i.e. pixel column 0) may be calculated, and an average deviation over all of the sheets may be determined. Graph 702 in Figure 7 shows an example of the deviation of each row relative to an average deviation. In some examples, determining (block 208) deviation may comprise determining the deviation of the edge of each sheet relative to the position of the edge of the sheet with the lowest deviation from the reference position in the plurality of stacked sheets. In other words, the deviation of each sheet may be determined relative to pixel column 0. Graph 704 in Figure 7 shows an example of the deviation of each row relative to pixel column 0 (sometimes referred to as the “plumb”), in some examples, determining (at block 208) deviation may comprise determining the deviation of the edge of each sheet relative to the position of a bottom sheet in the plurality of stacked sheets. As noted above, the bottom sheet in the plurality of stacked sheets may be considered to be the first sheet of the printing operation. Graph 706 in Figure 7 shows an example of the deviation of each row relative to the bottom sheet of the stack 102. in other examples, other metrics may be used. [0036] Figure 8 is a flowchart of a further example of a substrate stack assessment method 800. The method 800, which may comprise a computer implemented method, may comprise a block or blocks of the method 200 discussed above. The method 800 may, in some examples, further comprise, at block 802, pre-processing the captured image prior to calculating the pixel luminance values (e.g. relative luminance values). Various pre-processing techniques may be used in order to modify the captured image improve the subsequent analysis. In some examples, the camera 104, 106 used to capture the image may comprise or be used in conjunction with a fisheye lens with may cause the captured image to be distorted, for example with curved edges. Thus, the pre-processing may, in some examples, comprise reducing a degree of distortion of the image resulting from the nature of an image capture device used to capture the image. For example, the pre-processing may comprise reducing a degree of fisheye distortion of the image.

[0037] in some examples, the image captured using the camera or cameras 104, 106 may comprise an image in the RGB color space. In such examples, the pre- processing may comprise converting the captured image to a grayscale image. For example, each pixel in the image may be converted into a greyscale (luminance or relative luminance) level ranging from 0 to 255. The red (R), green (G) and blue (B) in each pixel may be converted into luminance (corresponding to greyscale level) according to the following weights: [0038] Luminance, L = 0.30*R + 0.59*G + 0.11* B

[0039] in other examples, other weights may be used.

[0040] Referring again to Figure 8, the method 800 may comprise, at block 804, receiving an indication of the position of a bottom sheet of the plurality of stacked sheets. As noted previously, a component of a print apparatus may send a signal when the first sheet of a particular printing operation is sent to the stacking region and, based on the signal, the position of the first/bottom sheet may be determined. Determining (206) the region of interest may, in some examples, be based on the indication of the position of the first sheet, as described above. For example, the bottom sheet of the stack 102 may comprise the bottom of the region of interest. [0041] At block 806, the method 800 may comprise, responsive to determining that the deviation of an edge of a sheet of printable substrate exceeds a threshold distance from the reference position, performing an alert action. For example, if it is determined that the edge of a sheet is displaced from the reference position (e.g. from the average deviation, from pixel column 0, or from the edge of the bottom sheet) by more than a threshold amount (e.g. a defined number of pixels in the image) then action may be taken to notify an operator. Thus, in some examples, the alert action may comprise generating an alert signal for delivery to a user. For example, a processor may deliver an alert signal, a notification signal or message or display for presentation on a device, such as the print apparatus or a display screen.

[0042] Examples of the present disclosure also provide an apparatus, such as a stack analysis apparatus. Figure 9 is a simplified illustration of such a stack analysis apparatus 900. The stack analysis apparatus 900 comprises an image capture module 902 processing apparatus 904 in communication with the image capture module. The image capture module 902 is to capture an image of at least an edge end of a stack of items of printable media. The items are printable medium may, for example, comprise the sheets of printable substrate discussed above. The processing apparatus 904 may perform, or control other components to perform, functions described in the blocks of the methods 200, 800 discussed above, in some examples, the processing apparatus 904 is to receive, from the image capture module 902, an image of at least an end of the stack of items of printable media. In some examples, the processing apparatus 904 may receive an image of both ends of the stack of items. The processing apparatus 904 is to measure a luminance of pixels in each row of pixels and a luminance of pixels in each column of pixels in the captured image. In some examples, the processing apparatus 904 may measure an average luminance in each row of pixels and an average luminance in each column of pixels in the captured image. Based on the measured luminance values, the processing apparatus 904 is to identify a region within the image which shows a portion of the stack of items whose alignment is to be analyzed. The identified region may comprise the region of interest discussed above, in some examples, the processing apparatus 904 may identify, or define, the region by finding adjacent rows/columns (or nearly adjacent rows/columns - e.g. within 2 or 3 pixels) where the luminance value (e.g. the average luminance) differs by at least a defined threshold amount. This may be considered to be a discontinuity, as discussed above.

[0043] The processing apparatus 904 is to analyze each row of pixels in the identified region to establish the extent to which the edge each item of printable media shown in the region deviates from a reference position. If it is determined that an item of printable media deviates from the reference position by at least a defined threshold distance (e.g. number of pixels in the image), then appropriate action may be taken by the processing apparatus 904. For example, the processing apparatus 904 may generate an alert signal for delivery to an operator. Thus, if it is determined that the stack of items is not aligned to an appropriate specification, then appropriate action may be taken, such as pausing a printing operation to identify a cause of the misalignment.

[0044] In some examples, the image capture module 902 may comprise a fisheye lens. For example, a fisheye lens may comprise one of a plurality of optical elements used in conjunction with the image capture module 902. In such examples, the processing apparatus 904 may perform an image processing function on the captured image, so as to reduce a degree of distortion of the captured image resulting from the fisheye lens. For example, the image processing function may reduce the amount of curvature at the edges of the captured image. [0045] Figure 10 is a schematic illustration of a further example of a stack analysis apparatus 1000. The stack analysis apparatus 1000 may comprise components of the apparatus 900 discussed above. The stack analysis apparatus 1000 further comprises a printable media detection module 1002 and a print operation control module 1004, which may be operatively coupled to the processing apparatus 904. The printable media detection module 1002 is to generate, responsive to determining that an item of printable media has been received in a stacking area, a media detection signal to be delivered to the processing apparatus 904. For example, after an item of printable media has progressed from the media feed mechanism and through the print apparatus, the media detection signal may be generated when the printable media arrives in the stacking area, thereby serving as an indication of the precise time that the item of printable media joins the stack. The print operation control module 1004 is to generate, responsive to determining that a print operation has commenced, an operation commencement signal to be delivered to the processing apparatus. In this way, the processing apparatus 904 is notified when a new printing operation begins, and this information, together with the media detection signal indicating that an item of printable medium has been received in the stacking area, can be used to determine the position in the stack of the item of printable media that forms the bottom item of the stack of items whose alignment is to be assessed. Thus, the processing apparatus 904 may identify the region within the image further based on the media detection signal and the operation commencement signal, [0046] in some examples, the apparatus 1000 may comprise multiple image capture modules (e.g. cameras), for example to enable separate images to be captured of each end of the stack. In some examples, the image capture module 902 may comprise a first image capture module to capture an image of a first end of the stack of items of printable medium. The stack analysis apparatus 1000 may further comprise a second image capture module 1006 to capture an image of a second end of the stack of items of printable media. The stack analysis apparatus 1000 may further comprise a light source 1008 to illuminate the first end and the second end of the stack. The light source 1008 may, in some examples, comprise an LED, or a plurality of LEDs.

[0047] Examples of the present disclosure also provide a machine-readable medium. Figure 11 is a schematic illustration of an example of a machine-readable medium 1104 in communication with a processor 1102. The machine-readable medium 1104 comprises instructions which, when executed by the processor 1102, cause the processor to perform various functions, such as the functions described in the blocks of the methods 200, 800 discussed above. The machine-readable medium 1104 may comprise instructions (e.g. image acquisition instructions 1106) which, when executed, cause the processor 1102 to acquire an image of a pile of printable substrates. The machine-readable medium 1104 may comprise instructions (e.g. luminance calculation instructions 1108) which, when executed, cause the processor 1102 to calculate a pixel luminance for pixels in each row of pixels and a pixel luminance for pixels in each column of pixels In the captured image, in some examples, Instructions may cause the processor to calculate an average pixel luminance for each row of pixels and an average pixel luminance for each column of pixels in the captured image. The machine-readable medium 1104 may comprise instructions (e.g. region identification instructions 1110) which, when executed, cause the processor 1102 to identify, based on the calculated pixel luminance values, a region of interest within the image that shows an edge of at least a portion of the pile of plurality of printable substrates whose alignment is to be assessed. The machine-readable medium 1104 may comprise instructions (e.g. pixel analysis instructions 1112) which, when executed, cause the processor 1102 to analyze pixels in the region of interest to determine a position, relative to a reference position, of an edge of each printable substrate shown in the region. Based on the determined position of the edge of each printable substrate, it may be determined whether or not any of the substrates are significantly misaligned (e.g. displaced from the reference position by more than a defined amount) and, if such a determination is made, then appropriate action may be taken. [0048] Examples disclosed herein provide a mechanism by which misalignment of sheets of printable media in a stack (e.g. displacement of sheets from a reference position) may be identified using a camera, so that any appropriate action may be taken quickly (e.g. correcting any fault that is identified as causing the misalignment), so that stacking of the printable media may continue in an aligned manner. [0049] Examples in the present disclosure can be provided as methods, systems or machine readable instructions, such as any combination of software, hardware, firmware or the like. Such machine readable instructions may be included on a computer readable storage medium (including but is not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0050] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0051] The machine readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine readable instructions. Thus functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors. [0052] Such machine readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0053] Such machine readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams. [0054] Further, the teachings herein may be implemented In the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure. [0055] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.

[0056] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[0057] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.