Using ImageMagick to Save the Character Screen for Diablo II

Sometimes I speedrun Diablo II and want to stream my runs and show my audience my current stats, such as my level and resistances. I could just run around all the time with my character screen open, but then I would be stuck with a half a screen all the time. If only there was a tool that would extract this information from memory and show it on stream. Oh wait — there is — and it’s called DiabloInterface and it’s awesome!

However, I use Linux and that kind of memory peeking doesn’t seem to be working in wine. Understandably, it’s a issue marked as wontfix. In a perfect world I would modify that code to work with /proc/pid/mem — but this world ain’t perfect and I’ve got Diablos to slay.

So, we need a simple solution. ImageMagick to the rescue! It provides the import command for grabbing screenshots, convert to crop out a portion of the image and compare command to see if the character screen is visible.

First take a screenshot with:

import -window 'Diablo II' /tmp/d2screen.bmp

A screenshot of Diablo II with the character screen open

The screenshot is saved as a BMP file to avoid wasting time compressing or decompressing to PNG.

Now let’s extract a portion of the image we will use to check if the character screen is open:

convert /tmp/d2screen.bmp -crop 67x20+89+143 /tmp/is_char_screen.bmp

An extracted portion of the Diablo II screenshot when the character screen is open

Now we can compare the two images and see if they are the same:

compare -metric RMSE strength_template.bmp /tmp/is_char_screen.bmp /dev/null 2> /dev/null

The compare command will use the exit status of 0 when they are similar images. We can use this to save the screenshot to a known location which OBS will automatically reload and it can be displayed on stream

An example of the character screen being used in a streaming overlay while not open

The end result is this bash script:

#!/usr/bin/env bash
while true; do
  # Grab the current Diablo II screen
  import -window 'Diablo II' /tmp/d2screen.bmp
# Extract the area where "Strength" is shown
  convert /tmp/d2screen.bmp -crop 67x20+89+143 /tmp/is_char_screen.bmp
# Check if it matches a template image
  compare -metric RMSE strength_template.bmp /tmp/is_char_screen.bmp /dev/null 2> /dev/null
if [ $? -eq 0 ]; then
    # Character screen is showing so copy the screenshot
    cp /tmp/d2screen.bmp charscreen.bmp
  fi
sleep 0.3s
done

Hopefully this helps someone else in their hacking endeavors!