Fraps

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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.

Header

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

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:

Y plane:

 y0 y1  y4 y5
 y2 y3  y6 y7

Cb plane:

 cb0    cb1

Cr plane:

 cr0    cr1

Version 1

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

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.

Frame header

 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.

Plane format

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

Version 3 Fraps data, according to investigations, appears to be identical to v5 and can be decoded by the same algorithm.

Version 4

Version 4 Fraps data, according to investigations, appears to be identical to v2 and can be decoded by the same algorithm.

Version 5

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.