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The Fraps video codec is used to store in real-time data that is captured from high frame-rate computer games played on Windows PCs. There are 6 known versions of the codec: 0, 1, 2, 3, 4, and 5. All versions occur with the same FourCC.
A Fraps video frame begins with a header that is either 4 or 8 bytes in length:
byte 0 version bytes 1-2 unknown byte 3 flags
If bit 6 of byte 3 is set (byte & 0x40) then there are an additional 4 header bytes before the video data. The meaning of these 4 bytes is unclear.
Bit 7 of the flags byte indicates when set that the current frame is unchanged from the previous frame.
Version 0 Fraps data is planar YUV 4:2:0 video data stored in an interleaved manner. The payload bytestream is formatted at:
y0 y1 y2 y3 cb0 cr0 y4 y5 y6 y7 cb1 cr1
Which will map to:
y0 y1 y4 y5 y2 y3 y6 y7
Version 1 Fraps data is 24-bit BGR data stored in an upside-down order, similar to the way that Microsoft BMP files store their data.
Version 2 Fraps data stores YUV data with a Huffman scheme.
All data is stored in 32-bit little-endian words. If the total frame size is 8 bytes long then repeat the previous frame. Otherwise, proceed to frame decoding.
bytes 0- 7 should be ignored bytes 8-11 'FPSx' bytes 12-15 offset to the Y plane (minus 8), should always be 16 bytes 16-19 offset to the U plane (minus 8) bytes 20-23 offset to the V plane (minus 8)
Huffman tree construction
The Huffman tree is built by the canonical algorithm:
- sort symbols by their counts in ascending order
- create node by joining two first nodes (or leaves)
- place the node just before the node with a count greater than this new node.
WARNING If symbols have a zero count they should also be included in the tree. The Huffman tree for this codec always has 256 leaves.
First 256 words (4 byte each, 1 kB total) of plane data are symbol frequencies used to construct the Huffman tree. The rest of the data represents the bitstream packed into 32-bit words, MSB first.
Lines are coded as the difference from the previous lines. For Y plane the line with index - 1 should contain all zeroes; for chroma planes - all 128.
Version 3 Fraps data, according to investigations, appears to be identical to v5 and can be decoded by the same algorithm.
Version 4 Fraps data, according to investigations, appears to be identical to v2 and can be decoded by the same algorithm.
Version 5 Fraps data is the same as version 4, but encodes planar RGB24 upside-down. For better compression ratio red and blue components are stored as a differences to the green component.