Apple ProRes

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ProRes Introduction

Apple ProRes is a family of proprietary video codecs used for storing and editing high definition video data in Apple's Final Cut Pro. Apple's official whitepaper lists the codec's key features as being:

  • intra-only codecs
  • 4:2:2 / 4:4:4:4 source material
  • 10-bit (12-bit for ProRes 4444) sample depth
  • variable bitrate

ProRes 422 Standard Definition / High Quality codec

ProRes 422 SD/HQ is the same codec operating on two different bitrates (flavours). Two different FOURCCs are used in order to indicate each flavour:

Flavour name FOURCC Bitrate
Standard Definition (SD) 'apcn' 145 Mbps
High Quality (HQ) 'apch' 220 Mbps

ProRes algorithm is based on the Discrete cosine transform (further DCT) and utilizes the following compression techniques:

The bitstream of the ProRes 422 has been designed to provide the following additional features:

  • frame-level multi-threaded encoding/decoding depending on available CPU cores
  • spatial scalability providing the possibility to decode a video at different partial resolutions (1/2, 1/4, 1/8 of the full size and so on). ProRes is capable of saving CPU cycles while decoding at smaller resolutions due to a special bitstream layout enabling partial bitstream access and parsing.

Frame layout

A typical ProRes 422 frame has the following layout:

       Frame container atom
           Frame header
            Picture 1
 Picture 2 (interlaced frames only)

Frame container atom

At the beginning of each frame the frame container atom is located. It has the classical QuickTime atom structure with the ID set to the undocumented ProRes frame type ID:

Field size Field name Description
4 bytes size frame size in bytes
4 bytes type 'icpf' ("image codec prores frame"?)

All data is stored in the big-endian format. The value of the field "size" must match frame size from the movie container.

Frame header

A frame header stores description information, such as frame dimension, frame structure (progressive/interlaced), color information and the like. All data is stored in the big-endian format.

Field size Field name Value Description
2 bytes hdrSize size of this header in bytes. Must be at least 28 bytes long.
2 bytes padding1 0 reserved and set to "0".
4 bytes vendorID? 'apl0' Ignored in all known decoders.
2 bytes frameWidth Width of encoded frame.
2 bytes frameHeight Height of encoded frame.
1 byte frameFlags

layout: AAxxBBxx where

  • bits AA = sample depth?
  • bits BB = frame type:
    • "0" - progressive
    • "1" - interlaced (top-field first)
    • "2" - interlaced (bottom-field first)
Frame structure flags.
3 bytes padding2 0 Ignored.
1 byte colorMatrix
  • "1" = ITU-R BT.709-2 / SMPTE 274M-1995 / SMPTE 296M-1997
  • "6" = ITU-R BT.601-4 / SMPTE 170M-1994 / SMPTE 293M-1996
Color matrix ID for color conversion between YUV and RGB (see below).
2 bytes padding3 0 Ignored.