Resource Interchange File Format - Compatibility Issues - INFO Chunk Placement Problems

INFO Chunk Placement Problems

For cataloguing purposes, the optimal position for the INFO chunk is near the beginning of the file. However, since the INFO chunk is optional, it is often omitted from the detailed specifications of individual file formats, leading to some confusion over the correct position for this chunk within a file.

When dealing with large media files, the expansion or contraction of the INFO chunk during tag-editing can result in the following "data" section of the file having to be read and rewritten back to disk to accommodate the new header size. Since media files can be gigabytes in size, this is a potentially disk-intensive process. One workaround is to "pad out" the leading INFO chunk using dummy data (using a "dummy chunk" or "pad chunk") when the file is created. Later editing can then expand or contract the "dummy" field to keep the total size of the file header constant: an intelligently written piece of software can then overwrite just the file header when tagging data is changed, without modifying or moving the main body of the file.

Some programs have tried to address the problem by placing the INFO chunk at the end of a media file, after the main body of the file. This has resulted in two different conventions for chunk placement, with the attendant risk that some combinations of software can cause a file's INFO data to be ignored or permanently overwritten during editing. More sophisticated programs will take into account the possibility of "unexpected" chunk placement in files and respond accordingly. For instance, when the audio-editing program Audacity encounters a .WAV file with end-placed INFO data, it will correctly identify and read the data, but on saving, will relocate the INFO chunk back to the file header.

Although CorelDRAW 10 nominally uses a RIFF file structure, the program's initial release placed the INFO chunk at the end, so that any embedded preview bitmap would not be displayed under Windows' file manager by default. A "patch" utility supplied with the program fixes this problem.

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