Tribes: Ascend/FileSpecs/TextureCache

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As Tribes: Ascend uses the Unreal 3 engine, it makes use of the content cooking functionality[1] that it provides. This compresses the images and stores them all within one file. A second file is used as an index into the main cache file to extract the images. Ascend uses a slight variation of Unreal's cooking methods. In addition to applying standard image compression techniques, they use zlib compression on it before storing it.

The file specifications for the two texture cache files are as follows:

Contents

texture.cache.hdr.rtc

This file contains information about all of the images in the main cache file. Below is the file's structure. The first four bytes state how many files are in the image cache. For each of those images, there is an image data structure which contains the image dimensions, compression information, and the number of mipmaps. For each mipmap, there is a mipmap chunk structure that gives the offsets into the main texture cache file. The first chunk is the main image and has the dimensions found in the image structure fields. The remaining chunk dimensions are found by continually dividing the original dimensions by two.

File Structure

Name Size Description
File Count 4 bytes The number of images in the texture cache
Image data structure
Texture Name Pascal String (4 byte length field) The texture file's name
Flags 2 bytes Compression flags
Width 2 bytes The width of the image
Height 2 bytes The height of the image
Mipmap Count 4 bytes The number of mipmap chunks (including the image itself)
Mipmap chunk structure
Unknown 1 byte Always set to "1"
Offset 4 bytes The byte offset into the data file
Length 4 bytes The length of the data to read from the data file
Decompressed Size 4 bytes The actual size of the image, uncompressed

Flags

Hex Code Description
0x205 DXT1 compression
0x207 DXT5 compression

texture.cache.data.rtc

This file contains all of the textures used in the game, and their mipmaps. Each block of data in this file is compressed using zlib compression[2].

Name Size Description
Zlib header 2 bytes 0x9C78, indicating that the block is under zlib compression
Compressed data Variable The length of this data comes from the "Length" field of the Mipmap chunk structure above

After decompressing[3] the entire block (header included) using the "Length" and "Decompressed Size" fields the image data can be found. The images themselves are compressed using DXT compression. Using the "Flags" field from the Mipmap chunk structure, the appropriate DXT decompression routines should be applied.

Tools

An image extraction tool can be found here: http://j-software.dk/tribestex.zip [4] It works pretty well, but has minor issues with the alpha layers.

Repacking

It is theoretically possible to insert new image data back into the texture cache files. One simply modifies the image, makes smaller copies for the mipmaps, zlib compress each one, append it to the main data file, and update the cache header with the new offsets. No tools exist that currently do this. This is partly because the updater will erase all of the changes when a new patch is released. One person was attempting to use the Unreal Development Kit and have it re-execute the cooking routines on the new images, but they received a Cease and Desist letter from HiRez Studios.

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