%%
 title: "Another ImageMagick Use Case: Process Oscilloscope Screenshots"
 date: "28-Aug-2026"
 %%
 
 # Another ImageMagick Use Case: Process Oscilloscope Screenshots
 
 It's been a while since the last post (time flies, right?), a lot has happened
 but recently I bought a portable oscilloscope, the OWON HDS25S (2CH, 25 MHz, 250
 MSa/s, 8 kSa memory depth), maybe I will talk about it in a future post, but in
 this one I want to talk about another use case I found for ImageMagick: process
 oscilloscope screenshots to change the dark color scheme to a lighter one,
 making it suitable for reports and if desired, for printing.
 
 <!--
 <div class="centered-img">
     <img style="width: 38%" src="hds25s.png" alt="OWON HDS25s">
 </div>
 -->
 
 <div class="side-by-side">
     <img style="max-width: 33%"  src="hds25s.webp" alt="Owon HDS25S front">
     <img style="max-width: 55%" src="hds25s_back_trim.webp" alt="Owon HDS25S back">
 </div>
 
 Of course, ImageMagick has an infinite number of uses: converting images from
 one format to another, compressing images, applying filters, etc. But with these
 posts, I focus on those uses that are more complex or specific to a particular
 use case.
 
 So, even though I recently bought the oscilloscope, I've already used it
 extensively for assignments and lab reports. In particular, I found the
 oscilloscope's screenshot feature very useful for reports. I also like that two
 screenshots fit nicely side by side on a PDF page because of the screen's 4:3
 aspect ratio.
 
 <div class="side-by-side">
   <img src="sine_250.png" alt="Sine with frecuency 250 Hz and dark scheme">
   <img src="sine_637.png" alt="Sine with frequency 637 Hz and dark scheme">
 </div>
 
 As seen in the pictures above, the screenshots are clear and look nice, but when
 using them in reports, for my personal taste, they look better with a lighter
 color scheme. This also makes sense if the report needs to be printed, as it
 uses much less ink since there is no need to print the background.
 
 To convert the screenshots from a dark to a light color scheme, I created a
 shell script that applies a series of ImageMagick filters to the input file.
 Part of the resulting shell script can be seen below.
 
 ```shell
 WIDTH=$(magick "$INPUT" -format '%w' info:)
 HEIGHT=$(magick "$INPUT" -format '%h' info:)
 TRIG_X=$((WIDTH * 975 / 1000))
 
 magick "$INPUT" -negate -colorspace Gray \
     +sigmoidal-contrast 8x65%            \
     -level 20%,90% gray.png
 
 magick "$INPUT" -alpha off -fuzz 5%     \
     -fill transparent +opaque '#F81C00' \
     -fill '#ED0000' -opaque '#F81C00'   \
     red_mask.png
 
 magick "$INPUT" -alpha off -fuzz 5%     \
     -fill transparent +opaque '#00FC00' \
     -fill '#007A3D' -opaque '#00FC00'   \
     green_mask.png
 
 magick "$INPUT" -alpha off -fuzz 5%                   \
     -fill transparent +opaque '#68C4F0'               \
     -fill '#FC0000' -opaque '#68C4F0'                 \
     -region "$((WIDTH-TRIG_X))x${HEIGHT}+${TRIG_X}+0" \
     -fill "#FF6900" -opaque '#FC0000'                 \
     +region blue_mask.png
 
 magick "$INPUT" -alpha off -fuzz 5%                   \
     -fill transparent +opaque '#F8E400'               \
     -fill '#005FFF' -opaque '#F8E400'                 \
     -region "$((WIDTH-TRIG_X))x${HEIGHT}+${TRIG_X}+0" \
     -fill "#FF6900" -opaque '#005FFF'                 \
     +region yellow_mask.png
 
 magick "$INPUT" -alpha off -fuzz 5%     \
     -fill transparent +opaque '#F87C20' \
     -fill "#FF6900" -opaque '#F87C20'   \
     orange_mask.png
 
 magick "$INPUT" -alpha off -fuzz 5%     \
     -fill transparent +opaque '#10F8F0' \
     -fill '#FF00FF' -opaque '#10F8F0'   \
     cyan_mask.png
 
 magick gray.png     -colorspace sRGB         \
     red_mask.png    -compose Over -composite \
     blue_mask.png   -compose Over -composite \
     yellow_mask.png -compose Over -composite \
     green_mask.png  -compose Over -composite \
     orange_mask.png -compose Over -composite \
     cyan_mask.png   -compose Over -composite \
     "$OUTPUT"
 ```
 
 The script works, by creating a color-inverted greyscale version of the input
 image and saving it as `gray.png`. Then, for the main colors used by the
 interface: red, blue, green, orange, cyan and yellow, it creates a mask in which
 only that particular color appears, while the rest of the colors are made
 transparent. It then replaces that color with the desired new color. For
 example, for channel one's yellow `#F8E400`, `yellow_mask.png` is created with
 the yellow color replaced with blue `#005FFF`.
 
 At the end, all the masks are combined on top of the previously created inverted
 greyscale image, resulting in the final image shown below.
 
 <div class="side-by-side">
   <img src="sine_250_light.png" alt="Sine with frecuency 250 Hz and light scheme">
   <img src="sine_637_light.png" alt="Sine with frequency 637 Hz and light scheme">
 </div>
 
 It is true that simply inverting the image would have been enough, but I think
 it was worth the effort to write the script, given that the colors can be
 specified arbitrarily and the final result is better.