imageconfig

Image dimensions

Decode(io.ReaderAt) reads width, height and format without changing the source position. Callers upload the original bytes; they do not decode, resize or re-encode pixels. Config intentionally contains no color model. A successful result establishes dimensions, not the validity of all pixel data.

PNG, JPEG and GIF use the registered standard-library configuration readers. TIFF, BMP and WebP use independently implemented metadata readers with no external image codec dependency:

• TIFF: classic little/big-endian TIFF, scalar SHORT/LONG width and height in the first IFD. Other tag payloads and subsequent IFDs are never followed. BigTIFF is unsupported. The 16-bit entry count bounds traversal to 65535 entries; buffers do not grow with file offsets or tag payload sizes.
• BMP: Windows 40-, 108- and 124-byte DIB headers, including top-down images, supported palette depths, and the existing default bitfield masks. Palette reads are bounded to 1024 bytes. Pixel data is not read.
• WebP: VP8, VP8L and VP8X dimension headers, with RIFF bounds and chunk padding included in offset calculations. Unknown chunks are skipped using random access. Traversal stops after 4096 chunks; supporting more requires an explicit resource-limit review and a real compatibility case. VP8X dimensions describe the canvas, including for animated images.

These readers validate the metadata needed for dimensions. They deliberately do not parse TIFF compression/color metadata, WebP frame bodies or embedded EXIF/ICC profiles. Do not use them as complete-file validators. If pixel processing is introduced, select and review a separate codec at that boundary instead of extending these metadata readers into decoders.

Format references:

• https://www.adobe.io/content/dam/udp/en/open/standards/tiff/TIFF6.pdf
• https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-bitmapinfoheader
• https://developers.google.com/speed/webp/docs/riff_container