Another ImageMagick Use Case: Process Oscilloscope Screenshots

28-Aug-2026 (last update 29-Aug-2026)

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.

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.

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.

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.

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.