2.6.1.2. V4L2_PIX_FMT_PISP_COMP1_RGGB (‘PC1R’), V4L2_PIX_FMT_PISP_COMP1_GRBG (‘PC1G’), V4L2_PIX_FMT_PISP_COMP1_GBRG (‘PC1g’), V4L2_PIX_FMT_PISP_COMP1_BGGR (‘PC1B), V4L2_PIX_FMT_PISP_COMP1_MONO (‘PC1M’), V4L2_PIX_FMT_PISP_COMP2_RGGB (‘PC2R’), V4L2_PIX_FMT_PISP_COMP2_GRBG (‘PC2G’), V4L2_PIX_FMT_PISP_COMP2_GBRG (‘PC2g’), V4L2_PIX_FMT_PISP_COMP2_BGGR (‘PC2B), V4L2_PIX_FMT_PISP_COMP2_MONO (‘PC2M’)

2.6.1.2.1. Raspberry Pi PiSP compressed 8-bit Bayer formats

2.6.1.2.1.1. Description

The Raspberry Pi ISP (PiSP) uses a family of three fixed-rate compressed Bayer formats. A black-level offset may be subtracted to improve compression efficiency; the nominal black level and amount of offset must be signalled out of band. Each scanline is padded to a multiple of 8 pixels wide, and each block of 8 horizontally-contiguous pixels is coded using 8 bytes.

Mode 1 uses a quantization and delta-based coding scheme which preserves up to 12 significant bits. Mode 2 is a simple sqrt-like companding scheme with 6 PWL chords, preserving up to 12 significant bits. Mode 3 combines both companding (with 4 chords) and the delta scheme, preserving up to 14 significant bits.

The remainder of this description applies to Modes 1 and 3.

Each block of 8 pixels is separated into even and odd phases of 4 pixels, coded independently by 32-bit words at successive locations in memory. The two LS bits of each 32-bit word give its “quantization mode”.

In quantization mode 0, the lowest 321 quantization levels are multiples of FSD/4096 and the remaining levels are successive multiples of FSD/2048. Quantization modes 1 and 2 use linear quantization with step sizes of FSD/1024 and FSD/512 respectively. Each of the four pixels is quantized independently, with rounding to the nearest level. In quantization mode 2 where the middle two samples have quantized values (q1,q2) both in the range [384..511], they are coded using 9 bits for q1 followed by 7 bits for (q2 & 127). Otherwise, for quantization modes 0, 1 and 2: a 9-bit field encodes MIN(q1,q2) which must be in the range [0..511] and a 7-bit field encodes (q2-q1+64) which must be in [0..127].

Each of the outer samples (q0,q3) is encoded using a 7-bit field based on its inner neighbour q1 or q2. In quantization mode 2 where the inner sample has a quantized value in the range [448..511], the field value is (q0-384). Otherwise for quantization modes 0, 1 and 2: The outer sample is encoded as (q0-MAX(0,q1-64)). q3 is likewise coded based on q2. Each of these values must be in the range [0..127]. All these fields of 2, 9, 7, 7, 7 bits respectively are packed in little-endian order to give a 32-bit word with LE byte order.

Quantization mode 3 has a “7.5-bit” escape, used when none of the above encodings will fit. Each pixel value is quantized to the nearest of 176 levels, where the lowest 95 levels are multiples of FSD/256 and the remaining levels are multiples of FSD/128 (level 175 represents values very close to FSD and may require saturating arithmetic to decode).

Each pair of quantized pixels (q0,q1) or (q2,q3) is jointly coded by a 15-bit field: 2816*(q0>>4) + 16*q1 + (q0&15). Three fields of 2, 15, 15 bits are packed in LE order {15,15,2}.

An implementation of a software decoder of compressed formats is available in Raspberry Pi camera applications code base.