GIMP: Cross-Hatch for E-Book Illustrations
by TheOuterLinux (https://theouterlinux.gitlab.io)
Last updated: 2025/05/27

GIMP 2.10
G'MIC-Qt 3.5.5 for GIMP 2.10

Use the following method to get a cross-hatch effect for an image or
photo. Then, if you would like, learn how to convert the result to SVG 
using Inkscape for adding to an E-book, such as ePub.

GIMP

    1. Open photo or image

G'MIC

    2. Weighted RGB To Grayscale filter

           Green Weight(%): 33.33
           Blue Weight(%):  33.33

           Min-Max Value
               Min Value Weight(%): 0
               Max Value Weight(%): 0
            
           Post Process Adjustment
               Value Shift:   0
               Midgray Shift: 0

       CLICK OK

GIMP

    2. Colors --> Curves...
           - Adjust curve to lower contrast but not too much as you will
             still need dark areas for the G'MIC Threshold filter to
             look okay. The photo or image should look like an old B&W
             photo that has faded over time with little contrast.

G'MIC

    3. Threshold Sketch filter

           Input Image Settings
               Threshold Low:    124
               Threshold Medium: 172
               Threshold High:   134
               Threshold Max:    225
               Blur Amount:      0

           Output Etch Settings

               Horizontal Amount:  100
               Cross-Hatch Amount: 51
               Vertical 1 Amount:  0
               Vertical 2 Amount:  10

               Horizontal Length: 0
               Vertical 1 Length: 0
               Vertical 2 Length: 0
               Flip Cross-Hatch:  False

               Curve Amount: 2
               Gamma:        0.3981
               Fast Resize:  True
               Color Image:  False

       CLICK OK
       
GIMP

    4. Export as PNG

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BONUS: Bitmap to SVG

You can also use the resulting image to convert it to a pure black and
white SVG using Inkscape, the point being useful for easy scaling when
adding to E-books, such as ePub.

Inkscape

    1. Open exported PNG image that was edited using the above.
    
    2. File --> Document Properties --> Resize to content
    
    3. With the image selected, Path --> Trace Bitmap...
    
    4. Use Single Scan's "Brightness cutoff" Detection mode for pure 
       black and white.
       
       Suggestion...
       
           Threshold:           0.886
           Invert Image:        False
           Speckles:            False; 0
           Smooth corners:      True; 1.00
           Optimize:            True; 0.200
           User-assisted trace: False
           Live updates:        True
       
    5. After playing with the adjustments, click Apply and then 
       immediately CTRL+x (cut) the result. This will leave you with
       the original image. Select and delete that image. Paste (CTRL+v)
       the SVG back; no need to move it in place because of next step.
       
    6. File --> Document Properties --> Resize to content
    
    7. You may want to add a white box behind the entire cross-hatch.
       HOWEVER, make this box transparent via 50% alpha. Some ePubs
       do not show this white box if using an "invert colors" mode,
       typically to get white text on a black background. BUT, this is
       not always the case and therefore having the background rectangle
       set to 50% alpha will make it easy on the eyes while still able
       to show the image. If the reader is using white text on a white
       or any other color background, the 50% alpha should still look
       fine or not be noticed at all.
       
    8. Select all (CTRL+a) and then group everything together (CTRL+g).
    
    9. Save as "Plain SVG"; "Inkscape SVG" may still work, this is just
       to be sure.

Sigil

    Sigil is a free and open-source WYSIWYG (What You See Is What You 
    Get) ePub editor. You can add SVGs just the same as you can add PNGs
    to the Images section in the side-panel. ePubs are essentially HTML
    files packed inside of a ZIP with the ePub extension. Therefore,
    though it may be slightly more taxing on the device you are reading
    the book on, SVGs are scalable and therefore you would not need to
    worry about supplying high definition images for future proofing as
    the SVG will always be scalable to match to current "standard"
    definitions of screens.