feat: implement indexed PNG (color type 3) decoding support by Copilot · Pull Request #1 · nnmrts/image

Copilot · 2026-03-10T01:08:48Z

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Co-authored-by: nnmrts <20396367+nnmrts@users.noreply.github.com>

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Pull request overview

Adds decode support for PNG/APNG indexed color (color type 3) by parsing palette chunks and expanding packed palette indices into RGBA output, including sub-byte bit depths and per-entry transparency.

Changes:

Extend unfilterAndConvert to handle indexed scanlines (1/2/4/8-bit) and apply palette lookup into RGBA.
Parse PLTE/tRNS in PNG/APNG decoders and thread a combined RGBA palette into the scanline conversion step.
Add new unit tests covering indexed PNG variants and document the feature in the changelog.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
`src/formats/png_base.ts`	Adds indexed-palette conversion logic and adjusts scanline sizing/filter bpp for sub-byte indexed data.
`src/formats/png.ts`	Parses `PLTE`/`tRNS`, builds RGBA palette, passes it into unfilter/convert for PNG decode.
`src/formats/apng.ts`	Mirrors palette parsing/building and threads palette into APNG frame decode.
`test/formats/png.test.ts`	Adds fixture builder + tests for indexed PNG (8-bit, tRNS, 4-bit, 1-bit).
`CHANGELOG.md`	Notes indexed PNG decoding support additions.

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Copilot · 2026-03-10T01:34:01Z

src/formats/png.ts

+    if (colorType === 3 && plte) {
+      const numColors = plte.length / 3;
+      palette = new Uint8Array(numColors * 4);
+      for (let i = 0; i < numColors; i++) {
+        palette[i * 4] = plte[i * 3];


PLTE/tRNS handling needs basic validation for correctness and robustness: (1) for color type 3, PLTE is mandatory—if missing, decoding should fail with a clear error; (2) PLTE length must be a multiple of 3 and no more than 256 entries (768 bytes); and (3) the indexed bitDepth should be restricted to 1/2/4/8. Without these checks, malformed inputs can produce silent corruption or large/invalid palette allocations.

Copilot · 2026-03-10T01:34:01Z

src/formats/apng.ts

+    if (colorType === 3 && plte) {
+      const numColors = plte.length / 3;
+      palette = new Uint8Array(numColors * 4);
+      for (let i = 0; i < numColors; i++) {
+        palette[i * 4] = plte[i * 3];


Same as PNG: when building the palette, validate PLTE (multiple of 3, <= 256 entries) and enforce that indexed images (color type 3) must have a PLTE chunk and a valid indexed bitDepth (1/2/4/8). This avoids silent corruption and potential memory issues from oversized/malformed palettes.

Copilot · 2026-03-10T01:34:01Z

test/formats/png.test.ts

+test("PNG: indexed color (type 3) - 8-bit depth, no transparency", async () => {
+  const format = new PNGFormat();
+
+  // 2x2 image with 4 distinct palette colors
+  const palette: [number, number, number][] = [
+    [255, 0, 0], // index 0: red
+    [0, 255, 0], // index 1: green
+    [0, 0, 255], // index 2: blue
+    [255, 255, 0], // index 3: yellow
+  ];
+  const indices = [0, 1, 2, 3]; // row-major: [red, green, blue, yellow]
+
+  const pngData = await buildIndexedPNG(2, 2, 8, palette, null, indices);
+  const decoded = await format.decode(pngData);


The new indexed-PNG tests cover positive cases, but there are no negative tests for malformed indexed images (e.g., color type 3 with missing PLTE, PLTE length not divisible by 3, or pixel indices that exceed the palette size). Adding a couple of assertRejects cases would help lock in spec-compliant error handling for the new code paths.

Copilot · 2026-03-10T01:34:02Z

src/formats/png_base.ts

+    // For indexed color with sub-byte bit depths, scanline bytes != width * bytesPerPixel
+    const scanlineLength = colorType === 3 && bitDepth < 8
+      ? Math.ceil(width * bitDepth / 8)
+      : width * bytesPerPixel;
+


scanlineLength is only corrected for indexed (color type 3) sub-byte images. Valid grayscale PNGs (color type 0) can also have bit depths 1/2/4, but with the current calculation (width * bytesPerPixel) the decoder will read too many bytes per row and misalign subsequent scanlines. Consider computing scanline bytes generically as ceil(width * bitsPerPixel / 8) for any packed format (at least color types 0 and 3), or explicitly rejecting unsupported bit-depth/color-type combinations before unfiltering.

Copilot · 2026-03-10T01:34:02Z

src/formats/png_base.ts

+          } else {
+            // No palette: treat index as grayscale value
+            rgba[outIdx] = index;
+            rgba[outIdx + 1] = index;
+            rgba[outIdx + 2] = index;
+            rgba[outIdx + 3] = 255;
+          }


For indexed PNGs (color type 3), the palette is required by the PNG spec. The current behavior silently falls back to treating the index as a grayscale value when palette is missing, which will produce incorrect output for otherwise-valid inputs and can hide malformed files. It would be better to throw a clear error when colorType === 3 and palette is undefined (ideally earlier during chunk parsing).

Copilot · 2026-03-10T01:34:02Z

src/formats/png_base.ts

+          if (palette) {
+            rgba[outIdx] = palette[index * 4];
+            rgba[outIdx + 1] = palette[index * 4 + 1];
+            rgba[outIdx + 2] = palette[index * 4 + 2];
+            rgba[outIdx + 3] = palette[index * 4 + 3];


Palette lookup does not validate that the decoded index is within the palette range. If the image references an out-of-range index, reads like palette[index * 4] will yield undefined and end up as 0, producing incorrect pixels instead of reporting a malformed PNG. Add a bounds check (e.g., index < palette.length / 4) and throw a descriptive error when violated.

nnmrts · 2026-03-10T01:34:37Z

@copilot apply changes based on the comments in this thread

…dback Co-authored-by: nnmrts <20396367+nnmrts@users.noreply.github.com>

Copilot · 2026-03-10T01:38:18Z

@copilot apply changes based on the comments in this thread

Applied all review changes in commit 9ab72f3:

png_base.ts: Fixed scanline length calculation to also handle grayscale (color type 0) sub-byte bit depths using Math.ceil(width * bitDepth / 8). Replaced the silent grayscale fallback for missing palette with a clear error. Added palette index bounds check that throws a descriptive error on out-of-range access.
png.ts / apng.ts: Added PLTE validation for color type 3: throws if PLTE is missing, if its length isn't a multiple of 3, if it exceeds 256 entries, or if the bit depth isn't 1/2/4/8.
test/formats/png.test.ts: Added 3 assertRejects negative tests covering: missing PLTE chunk, PLTE length not divisible by 3, and out-of-range palette index.

Initial plan

7b6d122

Copilot AI assigned Copilot and nnmrts Mar 10, 2026

pull-request-size bot added the size/XS label Mar 10, 2026

Copilot started work on behalf of nnmrts March 10, 2026 01:08 View session

feat: implement indexed PNG (color type 3) decoding support

f587084

Co-authored-by: nnmrts <20396367+nnmrts@users.noreply.github.com>

pull-request-size bot added size/L and removed size/XS labels Mar 10, 2026

Copilot AI changed the title ~~[WIP] Add support for indexed PNG color type 3~~ feat: implement indexed PNG (color type 3) decoding support Mar 10, 2026

Copilot finished work on behalf of nnmrts March 10, 2026 01:19

nnmrts requested a review from Copilot March 10, 2026 01:29

Copilot started reviewing on behalf of nnmrts March 10, 2026 01:29 View session

nnmrts marked this pull request as ready for review March 10, 2026 01:31

Copilot AI reviewed Mar 10, 2026

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Copilot started work on behalf of nnmrts March 10, 2026 01:34 View session

fix: add validation and error handling for indexed PNG per review fee…

9ab72f3

…dback Co-authored-by: nnmrts <20396367+nnmrts@users.noreply.github.com>

Copilot finished work on behalf of nnmrts March 10, 2026 01:38

nnmrts changed the base branch from main to feat-indexed-png March 10, 2026 01:59

nnmrts merged commit 071bcc6 into feat-indexed-png Mar 10, 2026

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feat: implement indexed PNG (color type 3) decoding support#1

feat: implement indexed PNG (color type 3) decoding support#1
nnmrts merged 3 commits intofeat-indexed-pngfrom
copilot/add-indexed-png-support

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Copilot AI commented Mar 10, 2026 •

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