OpenEXR chunks

After the headers, an OpenEXR file holds one offset table per part and then the chunks of pixels. All numbers are little-endian; the sizes of the basic types are on the attribute types page.

Offset tables

Each entry is an unsigned long: the offset of a chunk from the start of the file.

In a single-part file with no chunkCount, the number of entries comes from dataWindow, tiles and the compression.
In a multi-part file, the number of entries is chunkCount.
Scan line chunks are listed by increasing y, tiles by increasing y order.

Part number

In a multi-part file (version bit 12 set), every chunk starts with an unsigned long part number. Part 0 is the first header’s part. Other files have no part number.

Scan line chunk

Field	Type	Description
y	int	The top scan line of the block. The first block matches the data window’s minimum y.
Pixel data size	int	Bytes of pixel data.
Pixel data	bytes	The pixels, compressed if that made them smaller.

Uncompressed pixels are stored top to bottom. Within each scan line the channels are in alphabetical order, each channel’s values together, left to right. The last block can hold fewer lines if the data window height is not a whole number of blocks.

Compression	Scan lines per block
`NO_COMPRESSION`	1
`RLE_COMPRESSION`	1
`ZIPS_COMPRESSION`	1
`ZIP_COMPRESSION`	16
`PIZ_COMPRESSION`	32
`PXR24_COMPRESSION`	16
`B44_COMPRESSION`	32
`B44A_COMPRESSION`	32
`DWAA_COMPRESSION`	32
`DWAB_COMPRESSION`	256
`HTJ2K256_COMPRESSION`	256
`HTJ2K32_COMPRESSION`	32

Tile chunk

Field	Type	Description
Tile coordinates	4 × int	tileX, tileY, levelX, levelY.
Pixel data size	int	Bytes of pixel data.
Pixel data	bytes	As for a scan line, using the tile’s width and height.

Tiles at the right or bottom edge of a level can be narrower or shorter than the others.

Deep scan line chunk

Deep images store a variable number of samples per pixel. A deep scan line chunk is:

Field	Type	Description
Part number	unsigned long	Only in multi-part files.
y	int	The scan line.
Packed size of the pixel offset table	not stated in the layout document	Bytes of compressed offset table.
Packed size of the sample data	not stated in the layout document	Bytes of compressed sample data.
Unpacked size of the sample data	unsigned long	Bytes once expanded.
Pixel offset table	compressed	The sample counts.
Sample data	compressed	The samples.

A deep tile chunk has the same fields, with tile coordinates in place of y. The pixel offset table has one int per column in the data window: entry n is the total sample count of column n and all columns to its left.

Unpacked sample data stores all samples of the first channel, then all of the second, and so on. With ZIP_COMPRESSION, up to 16 scan lines are stored together, with the channels grouped within each scan line.

Deep parts may use only NO_COMPRESSION, RLE_COMPRESSION, ZIPS_COMPRESSION or ZIP_COMPRESSION.

A complete file

The OpenEXR project’s sample is a 415-byte, uncompressed, single-part scan line file, 4 by 3 pixels, with channels G (half) and Z (float). Its offset table holds 319, 351 and 383, one per scan line, and each block has 24 bytes of pixel data: 4 × 2 bytes of G and 4 × 4 bytes of Z.