QuickTime 4 Reference

Atom Types in QuickTime Image Files

There are two defined atom types: 'idsc' , which contains an image description, and 'idat' , which contains the image data. This is illustrated in Figure 16-1 . For a JPEG image, the image description atom contains a QuickTime image description describing the JPEG image's size, resolution, depth, and so on, and the image data atom contains the actual JPEG compressed data, as shown in Table 16-1 .

In QuickTime 4, there is a new optional atom type: 'iicc' , which can store a ColorSync profile. For more information about storing ColorSync profiles, refer to the section "Getting ColorSyncProfiles" .

Figure 1 An `idsc' atom followed by an `idat' atom

Table 1 A QuickTime Image file containing JPEG compressed data

`0000005E`	Atom size, 94 bytes
`69647363`	Atom type, `'idsc'`
`00000056`	Image description size, 86 bytes
`6A706567`	Compressor identifier, `'jpeg'`
`00000000`	Reserved, set to zero
`0000`	Reserved, set to zero
`0000`	Reserved, set to zero
`00000000`	Major and minor version of this data, zero if not applicable
`6170706C`	Vendor who compressed this data,`'appl'`
`00000000`	Temporal quality, zero (no temporal compression)
`00000200`	Spatial quality, codecNormalQuality
`0140`	Image width, 320
`00F0`	Image height, 240
`00480000`	Horizontal resolution, 72 dpi
`00480000`	Vertical resolution, 72 dpi
`00003C57`	Data size, 15447 bytes (use zero if unknown)
`0001`	Frame count, 1
`0C 50 68 6F 74 6F 20 2D 20 4A 50 45 47 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00`	Compressor name, "Photo - JPEG" (32 byte Pascal string)
`0018`	Image bit depth, 24
`FFFF`	Color look-up table ID, -1 (none)
`00003C5F`	Atom size, 15455 bytes
`69646174`	Atom type, `'idat'`
`FF D8 FF E0 00 10 4A 46 49 46 00 01 01 01 00 48 ...`	JPEG compressed data

A QuickTime Image file can also contain other atoms. For example, it can contain single-fork preview atoms.

The exact order and size of atoms is not guaranteed to match the example in Table 16-1 . Applications reading QuickTime image files should always use the atom size to traverse the file and ignore atoms of unrecognized types.

Like QuickTime movie files, QuickTime Image files are big-endian. However, image data is stored in the same byte order as usually specified by the particular compression format.