INPUT COMPRESSION TEST · OCTOBER 12, 2026

Coloring Page Maker: Can JPEG Compression Add Extra Lines?

JPEG compression can change the edges a coloring page maker traces. Start with the original file when you have it. A PNG copy prevents another lossy save, but converting an already compressed JPEG to PNG cannot restore missing detail. Our controlled test below shows changed marks; it does not establish that every JPEG makes a worse coloring page.

Try your original photo →

What we changed, and what stayed the same

On October 12, 2026, we created a 600 × 400-pixel RGB fixture with a blue rectangle, a yellow circle and a black bar on white. We saved the original as PNG and made two JPEG copies with Pillow's quality parameter set to 90 and 10, both with subsampling = 2. Those values are encoder settings, not percentages of image quality.

After decoding the three files to RGBA, we ran the unchanged blurGray and makeLineArt functions with the Detailed preset. The public app.js matched our local code byte for byte. We did not upload these fixtures through the interface, resize them, or export printable pages. This isolates decoding/compression and tracing, rather than a complete user workflow.

All counts cover the same 600 × 400 = 240,000 output pixels. The last column counts positions where JPEG-trace gray is below 250 and the matched PNG-trace gray is exactly 255. It includes faint marks and displaced contours. It is not a count of proven compression defects, and the lower dark-pixel count is not a quality score.

In an initial 10 × 230-pixel strip beside the rectangle, all three outputs had zero marks below 250. That check did not show extra marks there. We retained the zero result, then compared the whole matched canvas to describe the differences without pretending that the first region supported a positive finding.

The low-quality copy changes the trace visibly around the colored shapes. The high-quality copy also differs at some positions. Since a line may shift or become lighter, adding and removing traced pixels can happen together. These files support “compression changes this trace,” rather than “JPEG always adds more lines.”

Inspect the exact files

Inputs: original PNG, JPEG quality 90, JPEG quality 10. Outputs: PNG trace, quality-90 trace, quality-10 trace. Zoom near the circle and rectangle edges; do not use the resized article preview for pixel counting.

What to do with a photo you already have

1. Keep the untouched original, and compare it using the same crop and line style.
2. If a shared or downloaded copy has blocky edges, request the original when possible. Our synthetic test does not diagnose your particular file.
3. Use PNG for subsequent lossless saves when practical. This avoids a new JPEG encoding step; it does not undo an earlier one.
4. Judge the actual printable outline. Choose clear, useful contours, rather than the file with the most dark pixels.

This single fixture is not a test of every camera, JPEG encoder, line style, or physical printer. It does not measure children's preferences or Google rankings. For missing detail from a different cause, read the blurry-input experiment. For export format choices, read PNG versus PDF downloads.

Watch the test

Watch the narrated compression experiment with English captions and a downloadable MP4.

Sources and scope

Reproduction settings, original fixture, actual output PNGs and public code provide the experiment's evidence. Pillow documents its JPEG quality and subsampling parameters. Benchmark pages reviewed in the required order: Mimi Panda, iColoring, Canva. Our selected contribution is a reproducible input-compression comparison for this site's algorithm; it is not a test of their private converters.