Patent Description:
Some background information can be found in <CIT> which relates to a printer calibration method in which a printer prints a first target on a page. The page is rotated <NUM> degrees and reloaded into the printer. The printer prints a second target onto the same side of the page. The two targets produce information on image alignment to the page. Some further background information can be found in <CIT> and <CIT>.

Features of the present disclosure are illustrated by way of example and are not limited in the following figure(s), in which like numerals indicate like elements, in which:.

In imaging devices, such as inkjet printers, laser printers, photocopiers, facsimile devices, or the like a print medium may be fed from a source and positioned relative to the print engine for printing. The print medium may include various types of materials such as opaque and transparent medium including paper sheets, plastic sheets such as transparencies, vellum sheets, envelopes, cardstock, or the like. In order to ensure proper alignment of the image on the print medium, the printer may include mechanisms for mechanical correction of image placement errors including, for example, lateral placement errors as well as skew. However, the mechanisms for mechanical correction may be costly and may introduce complexity to the printer. In particular, the components for mechanical correction of skew may be complex. As such, for low cost systems, mechanical correction of placement errors may be cost prohibitive.

In addition to mechanical correction, placement errors may be corrected using electronic image compensation methods. For example, once an amount of placement error is known, a value corresponding to the amount of placement error may be entered into the printer such that placement of the image may be digitally corrected during image processing. However, while this method does not require the mechanical components to mechanically correct placement errors, additional components, such as optical sensors or the like, may be used to detect the position of the print medium in order to calculate the amount of compensation required. These additional components may introduce cost and complexity to the printer, and may be cost prohibitive in low cost printers.

Disclosed herein are apparatuses, methods, and computer readable media for determining skew correction values. As discussed herein, the skew correction values may be obtained using a skew correction vernier printed on a print medium, which may reduce or eliminate costly mechanisms for mechanical correction of image placement errors or optical sensors for image detection.

A vernier (also referred to herein as a vernier scale) may include two scales printed on the print medium, each having a different fixed spacing. The scales may be printed on the same side of the print medium and may be positioned relative to a desired edge of the print medium for skew detection (e.g., the leading edge and/or side edge). The skew correction value may be obtained by folding the print medium so that the two scales overlap and a measurement may be visually taken from the vernier scale. For example, when the print medium is folded such that respective corners (or the leading edge or side edge) are aligned and the two scales overlap with each other, a skew correction value for the leading edge may be obtained by identifying markings on two scales that most closely overlap with each other. One of the scales may include shift values that may range between a certain negative value and a certain positive value, in which a negative value may correspond to a particular negative shift value, a positive value may correspond to a particular positive shift value, and a zero value may correspond to an image that is not skewed. In other words, a zero value may correspond to the print medium, and thus the feed mechanisms, being aligned correctly with the imaging mechanisms that printed the two scales on the print medium. A print engine may use the identified skew correction value to correct for a skew in the print medium during printing onto the print medium.

By using a vernier to obtain skew correction values as disclosed herein, imaging devices may be fabricated without complex components (e.g., a mechanism for mechanical alignment of print media and/or optical sensors) for automatically detecting and correcting for skew may not be needed. As a result, the imaging devices may be fabricated with fewer mechanical components, which may enable the imaging devices to operate in a relatively efficient and simple manner. This may also result in the imaging devices consuming a lower amount of energy while still enabling for skew correction during printing operations.

Reference is first made to <FIG> and <FIG>. <FIG> depicts a block diagram of an example apparatus <NUM> that may print indicia on a print medium, in which the indicia may indicate a skew correction value that may be used to identify a level of skew in the print medium. <FIG> depicts an example print medium <NUM> on which the example apparatus <NUM> depicted in <FIG> may print indicia that may correlate to a skew correction value for a first edge of the print medium. It should be understood that the example apparatus <NUM> depicted in <FIG> and the example print medium <NUM> depicted in <FIG> may include additional features and that some of the features described herein may be removed and/or modified without departing from the scopes of the apparatus <NUM> or print medium <NUM>.

The apparatus <NUM> may be a computing device, a server computer, a laptop computer, or the like. In other examples, the apparatus <NUM> may be part of an imaging device, such as a printer, a multifunction machine, or the like. In any of these examples, the apparatus <NUM> may control operations of the imaging device to, for instance, print indicia on a print medium for skew correction. As shown in <FIG>, the apparatus <NUM> may include a processor <NUM> that may control operations of the apparatus <NUM>. The processor <NUM> may be a semiconductor-based microprocessor, a central processing unit (CPU), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), and/or other suitable hardware device.

The apparatus <NUM> may also include a non-transitory computer readable medium <NUM> that may have stored thereon machine readable instructions <NUM>-<NUM> (which may also be termed computer readable instructions) that the processor <NUM> may execute. The non-transitory computer readable medium <NUM> may be an electronic, magnetic, optical, or other physical storage device that includes or stores executable instructions, where the term "non-transitory" does not encompass transitory propagating signals. The non-transitory computer readable medium <NUM> may be, for example, Random Access memory (RAM), an Electrically Erasable Programmable Read-Only Memory (EEPROM), a storage device, an optical disc, and the like. The non-transitory computer readable medium <NUM> may also be referred to as a memory.

In some examples, instead of the non-transitory computer readable medium <NUM>, the apparatus <NUM> may include hardware logic blocks that may perform functions similar to the instructions <NUM>-<NUM>. In yet other examples, the apparatus <NUM> may include a combination of instructions and hardware logic blocks to implement or execute functions corresponding to the instructions <NUM>-<NUM>. In any of these examples, the processor <NUM> may implement the hardware logic blocks and/or execute the instructions <NUM>-<NUM>. As discussed herein, the apparatus <NUM> may also include additional instructions and/or hardware logic blocks such that the processor <NUM> may execute operations in addition to or in place of those discussed above with respect to <FIG> and <FIG>.

As shown in <FIG>, the processor <NUM> may fetch, decode, and execute the instructions <NUM> to print a first indicia <NUM> at a first region on a first side of a print medium <NUM>. In addition, the processor <NUM> may fetch, decode, and execute the instructions <NUM> to print a second indicia <NUM> at a second region on the first side of the print medium <NUM>. The second region may be spaced a prescribed distance from the first region along a first edge <NUM> of the print medium <NUM>. In addition, a relative position of the first indicia <NUM> to the second indicia <NUM> may correlate to a skew correction value for the first edge <NUM> of the print medium <NUM> as discussed herein.

According to examples, the print medium <NUM> may be fed through an imaging device in a direction indicated by the arrow <NUM>. In addition, as the print medium <NUM> is fed through the imaging device, the processor <NUM> may control an imaging component, such as an inkjet printhead, a toner application mechanism, or the like, to print the first indicia <NUM> and the second indicia <NUM>. As depicted in <FIG>, and by way of particular example, the first indicia <NUM> may be printed at an upper left region of the print medium <NUM> near a first corner <NUM> of the print medium <NUM> and the second indicia <NUM> may be printed at an upper right region of the print medium <NUM> near a second corner <NUM> of the print medium <NUM>.

The first indicia <NUM> and the second indicia <NUM> may form a pair of indicia that together enable measurement of a skew correction value for a particular edge of the print medium <NUM>. For example, an edge of the print medium <NUM> in the feed direction (e.g., <NUM>) may be referred to as a leading edge <NUM>. The first indicia <NUM> may be printed at a distance d1 from the leading edge <NUM> and the second indicia <NUM> may be printed at a distance d2 from the leading edge <NUM>, as illustrated in <FIG>. When the print medium <NUM> is properly aligned to the imaging components, e.g., fed parallel to the feed direction <NUM>, the distance d1 and the distance d2 may be equal to each other. However, when the print medium <NUM> is not properly aligned to the imaging components, e.g., fed at an angle with respect to the feed direction <NUM>, the distance d1 may differ from the distance d2. As a result, a difference between d1 and d2 may indicate a placement error of the print medium <NUM>. In addition, the difference between the distances d1 and d2 may correlate to an amount of skew or image twist that may result in the misaligned print medium <NUM>.

In some examples, the print medium <NUM> may include a third indicia <NUM> and a fourth indicia <NUM> printed relative to a side edge <NUM> for measuring skew correction values relative to the side edge <NUM>. The third indicia <NUM> may be printed in a region adjacent to the first corner <NUM> at a distance d3 from the side edge <NUM>, and the second indicia <NUM> may be printed in a region adjacent to a third corner <NUM> at a distance d4 from the side edge <NUM>. When the print medium <NUM> is properly aligned to the imaging components, e.g., fed parallel to the feed direction <NUM>, the distance d3 and the distance d4 may be equal to each other. However, when the print medium <NUM> is not properly aligned to the imaging components, e.g., fed at an angle with respect to the feed direction <NUM>, the distance d3 may differ from the distance d4. As a result, a difference between d3 and d4 may indicate a placement error of print medium <NUM> and may correlate to an amount of skew or image twist that may result in the misaligned print medium <NUM>.

The first indicia <NUM> and the second indicia <NUM> may be part of a vernier (also referred to herein as a vernier scale). In other words, the first indicia <NUM> and the second indicia <NUM> may be printed to have respective scales <NUM>, <NUM> as shown in <FIG>, in which the first scale <NUM> of the first indicia <NUM> differs from the second scale <NUM> of the second indicia <NUM>. As shown in <FIG>, the first and second scales <NUM>, <NUM> may include respective markings, which may be lines or another appropriate type of indication, and may be disposed to be substantially parallel to the leading edge <NUM> and arranged in a direction substantially perpendicular to the leading edge <NUM>. The markings of the first and second scales <NUM>, <NUM> may be graduated at different spacings with respect to each other to form a vernier scale.

Generally speaking, a vernier (vernier scale) may provide visual indications that may enable more accurate measurement readings than may be possible by human estimation. The vernier scale may include a main scale and a subsidiary scale that is positioned relative to the main scale. The relative positions and spacings of the subsidiary scale to the main scale may enable increased resolution of measurement readings using mechanical interpolation. For example, in <FIG>, the first scale <NUM> may be a main scale having markings that have a first fixed spacing and the second scale <NUM> may be a subsidiary scale (vernier) having markings that have a second fixed spacing that is different from the first fixed spacing. In some examples, the second indicia <NUM> (e.g., subsidiary scale <NUM> or vernier) may have <NUM> divisions equal in distance to <NUM> divisions on the first indicia <NUM> (e.g., main scale <NUM>). In other examples, the second indicia <NUM> and the first indicia <NUM> may have other divisions.

As shown in <FIG>, the first and second scales <NUM> and <NUM> may be printed on a common side of the print medium <NUM> with respect to each other and may be positioned relative to a desired edge, e.g., the leading edge <NUM>, of the print medium <NUM> for skew detection. The first scale <NUM> may also be printed with skew correction values <NUM> that may be used to determine the level of skew occurring in print medium <NUM> during an imaging operation on the print medium <NUM>. As shown, the skew correction values <NUM> may range between a certain negative value (-<NUM>) and a certain positive value (<NUM>), in which a negative value may correspond to a particular negative shift value, a positive value may correspond to a particular positive shift value, and a zero value may correspond to an image that is not skewed. It should be noted that the values shown in <FIG> for the skew correction values <NUM> are for purposes of illustration and should thus not be construed as limiting the present disclosure to those values.

The level of skew may be obtained by folding the print medium <NUM> such that the two scales <NUM>, <NUM> overlap each other. A measurement from the vernier scale (skew correction values <NUM>) may then be determined. For example, when the print medium <NUM> is folded such that respective corners are aligned and the first and second scales <NUM>, <NUM> overlap each other, a level of skew for the leading edge <NUM> may be obtained by identifying the markings on the first and second scales <NUM>, <NUM> that most closely overlap with each other. In addition, the skew correction value <NUM> corresponding to the markings of the first and second scales <NUM>, <NUM> that most closely overlap with each other may be read and may be identified as denoting the level of skew for the leading edge <NUM>. Similarly, the value <NUM> corresponding to the markings in the scales <NUM>, <NUM> that most closely overlap with each other may identify a level of shift that is to be applied to a print medium to compensate for the identified skew. As noted herein, when the value <NUM> is "<NUM>", a shift may not be needed as the scales <NUM>, <NUM> may be considered as being aligned with each other.

As shown in <FIG>, the third indicia <NUM> and the fourth indicia <NUM> may be part of a vernier and may thus be printed to have a third scale <NUM> and a fourth scale <NUM>, respectively. Similarly to the first and second scales <NUM>, <NUM>, the third and fourth scales <NUM>, <NUM> of the third indicia <NUM> and the fourth indicia <NUM> may differ from each other. In addition, the third and fourth scales <NUM>, <NUM> may include respective markings, which may be lines or another appropriate type of indication, and may be disposed to be substantially parallel to the side edge <NUM> and arranged in a direction substantially perpendicular to the side edge <NUM>. The markings of the third and fourth scales <NUM>, <NUM> may be graduated at different spacings with respect to each other to form a vernier scale. In addition, third scale <NUM> may be a main scale and the fourth scale <NUM> may be a subsidiary scale of the vernier scale. The third scale <NUM> may have markings that have a first fixed spacing and the fourth scale <NUM> may be a subsidiary scale (vernier) having markings that have a second fixed spacing that is different from the first fixed spacing. In some examples, the fourth indicia <NUM> (e.g., subsidiary scale <NUM> or vernier) may have <NUM> divisions equal in distance to <NUM> divisions on the third indicia <NUM> (e.g., main scale <NUM>). In other examples, the fourth indicia <NUM> and the third indicia <NUM> may have other divisions.

As shown in <FIG>, the third scale <NUM> and the fourth scale <NUM> may be printed on a common side of the print medium <NUM> with respect to each other and may be positioned relative to a desired edge, e.g., the side edge <NUM>, of the print medium <NUM> for skew detection. The third scale <NUM> may also be printed with skew correction values <NUM> that may be used to determine the level of skew occurring in print medium <NUM> during an imaging operation on the print medium <NUM>. It should be noted that the values shown in <FIG> for the skew correction values <NUM> are for purposes of illustration and should thus not be construed as limiting the present disclosure to those values. The level of skew may be obtained by folding the print medium <NUM> such that the third and fourth scales <NUM>, <NUM> overlap each other. A measurement from the vernier scale may then be determined. For example, when the print medium <NUM> is folded so that respective corners are aligned and the third and fourth scales <NUM>, <NUM> overlap each other, a level of skew for the side edge <NUM> may be obtained by identifying the markings on the third and fourth scales <NUM>, <NUM> that most closely overlap with each other. In addition, the skew correction value <NUM> corresponding to the markings of the third and fourth scales <NUM>, <NUM> that most closely overlap with each other may be read and may be identified as denoting the level of skew for the side edge <NUM>. Similarly, the value <NUM> corresponding to the markings in the scales <NUM>, <NUM> that most closely overlap with each other may identify a level of shift that is to be applied to a print medium to compensate for the identified skew. As noted herein, when the value <NUM> is "<NUM>", a shift may not be needed as the scales <NUM>, <NUM> may be considered as being aligned with each other.

Various examples of different levels of overlap between the pair of first and second scales <NUM>, <NUM> (or equivalently, the third and fourth scales <NUM>, <NUM>) are depicted in <FIG>. As shown in <FIG>, a vernier scale <NUM> may be formed when the print medium <NUM> is folded such that the markings of the second scale <NUM> overlap the markings of the first scale <NUM>. The vernier scale <NUM> may be visible through the folded print medium <NUM>. In some examples, the folded print medium <NUM> may be held up to a light source (not shown) to improve visibility of the vernier scale <NUM>. It should be understood that the description of the vernier scale <NUM> pertaining to the first and second scales <NUM>, <NUM> to determine the level of skew of a leading edge <NUM> may equally pertain to the third and fourth scales <NUM>, <NUM> to determine the level of skew of a side edge <NUM> of the print medium <NUM>.

When folded, each of the markings on the first scale <NUM> may form a pair with a marking on the second scale <NUM>. A pair of markings that most closely overlaps each other may correspond to a skew correction value for the print medium <NUM>. In the examples shown in <FIG>, in the leftmost vernier scale <NUM>, the bottommost markings of the first scale <NUM> and the second scale <NUM> are shown as most closely overlapping each other. As a result, an amount of skew of the leading edge <NUM> may be associated with a skew correction value of -<NUM> as indicated by the label <NUM>. In the vernier scale <NUM>, the markings corresponding to the skew correction value of -<NUM> are depicted as most closely overlapping each other. As a result, an amount of skew of the leading edge <NUM> may be associated with a skew correction value of -<NUM> as indicated by the label <NUM>. In the vernier scale <NUM>, the markings corresponding to the skew correction value of <NUM> are depicted as most closely overlapping each other. As a result, an amount of skew of the leading edge <NUM> may be associated with a skew correction value of <NUM> as indicated by the label <NUM>, which may mean that the leading edge <NUM> is not skewed or is slightly skewed with respect to the imaging components. The remaining verniers in <FIG> show different levels of skew.

To determine the level of skew along the leading edge <NUM>, the print medium <NUM> may be folded vertically inward such that the second corner <NUM> overlaps the first corner <NUM> and the leading edge <NUM> is aligned, e.g., collinear. In some examples, the first indicia <NUM> and/or the second indicia <NUM> may be printed in regions near opposite edges of the print medium <NUM> (e.g., near corners <NUM>, <NUM>) or away from the first and/or second corners <NUM>, <NUM>. For example, the first indicia <NUM> may be positioned near the first corner <NUM> while the second indicia <NUM> may be positioned near a center of the print medium <NUM>, while being at a distance d2 from the leading edge <NUM>. In this case, the print medium <NUM> may be folded such that the first indicia <NUM> is overlapped with the second indicia <NUM> while ensuring that overlapped portions of the leading edge <NUM> are collinear. Accuracy of the measurement from the vernier scale may depend on the distance between the pair of indicia <NUM>, <NUM> as well as how accurately the overlapped portions of the leading edge <NUM> are aligned.

Similarly, the print medium <NUM> may be folded horizontally inward so that the third corner <NUM> overlaps the first corner <NUM> and the side edge <NUM> is aligned, e.g., collinear. In some examples, the third indicia <NUM> and/or the fourth indicia <NUM> may be printed in regions near opposite edges of the print medium <NUM> (e.g., near corners <NUM>, <NUM>) or away from the first and/or third corners <NUM>, <NUM>. For example, the third indicia <NUM> may be positioned near the first corner <NUM> while the fourth indicia <NUM> may be positioned near a center of the print medium <NUM>, while being at a distance d4 from the side edge <NUM>. In this case, the print medium <NUM> may be folded such that the third indicia <NUM> is overlapped with the fourth indicia <NUM> while ensuring that overlapped portions of the side edge <NUM> are collinear. Accuracy of the measurement from the vernier scale may depend on the distance between the pair of indicia <NUM>, <NUM> as well as how accurately the overlapped portions of the side edge <NUM> are aligned.

In some examples, the vernier for the leading edge <NUM> (e.g., indicia <NUM>, <NUM>) may be printed together with the vernier for the side edge <NUM> (e.g., indicia <NUM>, <NUM>). In these examples, the third indicia <NUM> may be positioned adjacent to the first indicia <NUM> as illustrated in <FIG>, or positioned at other locations along the side edge <NUM>. In addition, verniers for the leading edge <NUM> and the side edge <NUM> may be printed together to, for instance, determine skew correction values for both the leading edge <NUM> and the side edge <NUM> concurrently.

The skew correction value <NUM> and/or <NUM> corresponding to the most closely overlapping markings may be identified as discussed herein with respect to <FIG>. According to examples, the identified skew correction value <NUM> or <NUM> may be entered into an imaging device for the imaging device to apply the identified skew correction value <NUM> or <NUM> during imaging on a subsequent print medium. For instance, the imaging device may digitally compensate for the skew according to the identified skew correction value <NUM>, <NUM> by, for instance, modifying the placement of printing fluid, toner, or the like, onto the print medium in a future job to correct for the skew in the print medium. In some examples, a user may enter the identified skew correction value <NUM>, <NUM> into the imaging device via, for instance, a control panel of the imaging device, a computing device external to the imaging device, or the like.

Various manners in which the processor <NUM> may operate are discussed in greater detail with respect to the method <NUM> depicted in <FIG>. Particularly, <FIG> depicts a flow diagram of an example method <NUM> for printing a vernier scale to obtain a skew correction value of a printing medium <NUM>. It should be understood that the method <NUM> depicted in <FIG> may include additional operations and that some of the operations described therein may be removed and/or modified without departing from the scope of the method <NUM>. The description of the method <NUM> is made with reference to the features depicted in <FIG> for purposes of illustration.

At block <NUM>, the processor <NUM> may print a first scale <NUM> on a first side of a print medium <NUM>. At block <NUM>, the processor <NUM> may print a second scale <NUM> on the first side of the print medium <NUM>. The processor <NUM> may print the second scale <NUM> to be aligned with the first scale <NUM> along a common axis. As discussed herein, the second scale <NUM> may have a different spacing than the first scale <NUM> to form a vernier scale such that the first scale <NUM> may overlap the second scale <NUM> when the print medium <NUM> is folded along a leading edge <NUM> of the print medium <NUM> to a position in which a first portion of the leading edge <NUM> is collinear with a second portion of the leading edge <NUM>. In some examples, multiple degrees of overlap between the first scale <NUM> and the second scale <NUM> may correlate to respective skew correction values <NUM> for the leading edge <NUM> of the print medium <NUM>. The level of skew of the leading edge <NUM> of the print medium <NUM> may be determined as discussed above with respect to <FIG>.

In some examples, the processor <NUM> may additionally or alternatively print a third scale <NUM> and a fourth scale <NUM> on the print medium <NUM>. The fourth scale <NUM> may be aligned with the third scale <NUM> along another common axis. As discussed herein, the fourth scale <NUM> may have a different spacing than the third scale <NUM> to form another vernier scale. In some examples, the processor <NUM> may print the third scale <NUM> to overlap the fourth scale <NUM> when the print medium <NUM> is folded along a second edge <NUM> of the print medium <NUM> to a position in which a first portion of the second edge <NUM> is collinear with a second portion of the second edge <NUM>. As discussed herein, multiple degrees of overlap between the third scale <NUM> and the fourth scale <NUM> may correlate to respective skew correction values <NUM> for the second edge <NUM> of the print medium <NUM>. The level of skew of the second edge <NUM> of the print medium <NUM> may be determined as discussed above with respect to <FIG>.

Some or all of the operations set forth in the method <NUM> may be included as utilities, programs, or subprograms, in any desired computer accessible medium. In addition, the method <NUM> may be embodied by computer programs, which may exist in a variety of forms. For example, the method <NUM> may exist as machine readable instructions, including source code, object code, executable code or other formats. Any of the above may be embodied on a non-transitory computer readable storage medium.

Examples of non-transitory computer readable storage media include computer system RAM, ROM, EPROM, EEPROM, and magnetic or optical disks or tapes. It is therefore to be understood that any electronic device capable of executing the above-described functions may perform those functions enumerated above.

Turning now to <FIG>, there is shown a block diagram of an example computer readable medium <NUM> that may have stored thereon machine readable instructions that when executed by a processor, may cause a print engine to print a vernier on a print medium <NUM> for determining a skew correction value for the print medium <NUM>. It should be understood that the computer readable medium <NUM> depicted in <FIG> may include additional instructions and that some of the instructions described herein may be removed and/or modified without departing from the scope of the computer readable medium <NUM> disclosed herein. The computer readable medium <NUM> may be a non-transitory computer readable medium. The term "non-transitory" does not encompass transitory propagating signals.

The computer readable medium <NUM> may have stored thereon machine readable instructions <NUM> that a processor, such as the processor <NUM>, depicted in <FIG>, may execute. The computer readable medium <NUM> may be an electronic, magnetic, optical, or other physical storage device that includes or stores executable instructions. The computer readable medium <NUM> may be, for example, Random Access memory (RAM), an Electrically Erasable Programmable Read-Only Memory (EEPROM), a storage device, an optical disc, and the like.

The processor may fetch, decode, and execute the instructions <NUM> to cause a print engine (e.g., of an imaging device) to print a first scale <NUM> of a vernier on a first side of a print medium <NUM>. The first scale <NUM> may be positioned near a side edge <NUM> of the print medium <NUM> and may have a plurality of first markings. The processor may fetch, decode, and execute the instructions <NUM> to cause the print engine to print a second scale <NUM> of the vernier on the first side of the print medium <NUM> positioned near a second edge <NUM> of the print medium <NUM>. The second scale <NUM> may be aligned with the first scale <NUM> along a common axis and may have a plurality of second markings. The second scale <NUM> may have a different spacing than the first scale <NUM>.

In some examples, the first scale <NUM> may overlap the second scale <NUM> when the print medium <NUM> is folded to a position along a leading edge <NUM> of the print medium <NUM> in which a first portion of the leading edge <NUM> is collinear with a second portion of the leading edge <NUM>. A skew correction value <NUM> of the leading edge <NUM> of the print medium <NUM> may correlate to a pair of a first marking and a second marking that most closely overlaps each other while the print medium <NUM> is folded along the leading edge <NUM> as discussed herein.

The processor may fetch, decode, and execute the instructions <NUM> to cause the print engine to print a third scale <NUM> of a second vernier on the first side of the print medium <NUM>. The third scale <NUM> may be positioned near a leading edge <NUM> of the print medium <NUM> and may have a plurality of third markings. The processor may fetch, decode, and execute the instructions <NUM> to cause the print engine to print a fourth scale <NUM> of the second vernier on the first side of the print medium <NUM> positioned near a trailing edge <NUM> of the print medium <NUM>. The fourth scale <NUM> may be aligned with the third scale <NUM> along a common axis and may have a plurality of fourth markings. The fourth scale <NUM> may have a different spacing than the third scale <NUM>.

In some examples, the third scale <NUM> may overlap the fourth scale <NUM> when the print medium <NUM> is folded to a position along a side edge <NUM> of the print medium <NUM> in which a first portion of the side edge <NUM> is collinear with a second portion of the side edge <NUM>. A skew correction value <NUM> of the side edge <NUM> of the print medium <NUM> may correlate to a pair of a third marking and a fourth marking that most closely overlaps each other while the print medium <NUM> is folded along the side edge <NUM> as discussed herein.

Claim 1:
A method comprising:
printing (<NUM>, <NUM>), by a print engine a first scale (<NUM>) on a first side of a print medium (<NUM>); and
printing (<NUM>, <NUM>), by the print engine a second scale (<NUM>) on the first side of the print medium, the second scale being aligned with the first scale along a common axis, characterised in that the second scale has a different spacing than the first scale; and the first scale is to overlap the second scale when the print medium is folded along a first edge (<NUM>) of the print medium to a position in which a first portion of the first edge is collinear with a second portion of the first edge, and wherein multiple degrees of overlap between the first scale and the second scale correlate to respective skew correction values for the first edge of the print medium.