Pixel-measure screenshots to diagnose layout differences, don't eyeball
Helping a user dial in the visual look of an app/reader to match a target screenshot when feels-different complaints arise
Visual inspection of screenshots produces wrong-direction recommendations. I made three eyeballing mistakes in a row comparing two reader screenshots: suggested shrinking line length (the target actually had narrower margins, not wider); suggested swapping to Inter (target was Proxima Nova-ish; Inter would have moved farther from it); identified column gap as the only difference when word spacing also diverged. Switching to pixel-level measurement via a ~30-line PIL+numpy script — load → grayscale → threshold to ink/no-ink → column-wise sum to find text-vs-gap column runs → run-length encode → classify gaps by width (outer margin, inter-column gutter, inter-word, inter-letter) → row-wise sum for baselines/line-height — produced concrete numbers that drove the correct fixes (Apple Books had 2.4x wider column gap and 45% wider word spacing). Letter widths and word/letter gap ratio fall out for free. The whole script fits in one Bash heredoc.
When the user says 'this feels different' comparing two screenshots, do not recommend changes from visual inspection — write a ~30-line numpy/PIL projection script that measures margins, gaps, letter widths, and line spacing, then let the actual pixel numbers drive the recommendation.