parallax/jsPDF · error · Error

Invalid graphics extension block.

Error message

Invalid graphics extension block.

What it means

Thrown by GifReader while parsing a Graphics Control Extension (0xF9) sub-block when the block size byte is not 4 or the terminator byte at p+4 is not 0. Per spec a GCE is exactly 4 bytes followed by a 0x00 block terminator; any deviation indicates a malformed or truncated GIF.

Source

Thrown at src/libs/omggif.js:510

              loop_count = buf[p++] | (buf[p++] << 8);
              p++; // Skip terminator.
            } else {
              // We don't know what it is, just try to get past it.
              p += 12;
              while (true) {
                // Seek through subblocks.
                var block_size = buf[p++];
                // Bad block size (ex: undefined from an out of bounds read).
                if (!(block_size >= 0)) throw Error("Invalid block size");
                if (block_size === 0) break; // 0 size is terminator
                p += block_size;
              }
            }
            break;

          case 0xf9: // Graphics Control Extension
            if (buf[p++] !== 0x4 || buf[p + 4] !== 0)
              throw new Error("Invalid graphics extension block.");
            var pf1 = buf[p++];
            delay = buf[p++] | (buf[p++] << 8);
            transparent_index = buf[p++];
            if ((pf1 & 1) === 0) transparent_index = null;
            disposal = (pf1 >> 2) & 0x7;
            p++; // Skip terminator.
            break;

          case 0xfe: // Comment Extension.
            while (true) {
              // Seek through subblocks.
              var block_size = buf[p++];
              // Bad block size (ex: undefined from an out of bounds read).
              if (!(block_size >= 0)) throw Error("Invalid block size");
              if (block_size === 0) break; // 0 size is terminator
              // console.log(buf.slice(p, p+block_size).toString('ascii'));
              p += block_size;
            }

View on GitHub (pinned to a3930ce03a)

Solutions

1. Re-encode the GIF with a conformant encoder (ImageMagick, gifsicle, ffmpeg).
2. Validate file integrity / re-download if truncation is suspected.
3. Wrap GifReader construction in try-catch and fall back to a re-encoded copy.

Example fix

// before
var reader = new GifReader(buffer);
var frame0 = reader.frameInfo(0);
// after
try {
  var reader = new GifReader(buffer);
} catch (e) {
  // re-encode via a known-good tool then retry
  reader = new GifReader(reencodedBuffer);
}

function looksIntactGif(buf){ return buf && buf.length >= 6 && buf[0]===0x47 && buf[1]===0x49 && buf[2]===0x46; }

try { var reader = new GifReader(buffer); } catch (e) { if (/Invalid graphics extension block/.test(e.message)) { reader = new GifReader(reencodedBuffer); } else throw e; }

When it happens

Trigger: Decoding a GIF whose GCE has the wrong block size (corruption); a truncated stream where bytes read as undefined; bit-flip damage in the extension header.

Common situations: Re-compressed or re-saved GIFs by a buggy encoder; files truncated during download; hand-crafted/edited GIFs with incorrect extension framing.

Related errors

• Unknown graphic control label: 0x" + buf[p - 1].toString(16)
• Invalid GIF 87a/89a header.
• Width/Height invalid.
• Invalid code/color length, must be power of 2 and 2 .. 256.
• Background index out of range.

AI-assisted analysis of parallax/jsPDF@a3930ce03a (2026-08-13). Data as JSON: /api/errors/14355823e2c5e705. Report an issue: GitHub.