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[WIP] Support for encodings using floating-point values #197

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[WIP] Support for encodings using floating-point values #197

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22 changes: 14 additions & 8 deletions ffv1.md
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Expand Up @@ -35,7 +35,7 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "S

`MSB`: Most Significant Bit, the bit that can cause the largest change in magnitude of the symbol.

`RCT`: Reversible Color Transform, a near linear, exactly reversible integer transform that converts between RGB and YCbCr representations of a `Pixel`.
`RCT`: Reversible Color Transform, a near linear, exactly reversible transform that converts between RGB and YCbCr representations of a `Pixel`.

`VLC`: Variable Length Code, a code that maps source symbols to a variable number of bits.

Expand Down Expand Up @@ -910,17 +910,23 @@ If state_transition_delta is not present in the FFV1 bitstream, all Range coder

### colorspace_type

`colorspace_type` specifies the color space encoded, the pixel transformation used by the encoder, the extra plane content, as well as interleave method.
`colorspace_type` specifies the color space encoded, the pixel transformation used by the encoder, the extra plane content, interleave method, as well as the format of each `Pixel`.

|value | color space encoded | pixel transformation | extra plane content | interleave method |
|-------|:------------------------|:------------------------|:------------------------|:------------------------|
| 0 | YCbCr | None | Transparency | `Plane` then `Line` |
| 1 | RGB | JPEG2000-RCT | Transparency | `Line` then `Plane` |
| Other | reserved for future use | reserved for future use | reserved for future use | reserved for future use |
|value | color space encoded | pixel transformation | extra plane content | interleave method | Format
|-------|:------------------------|:------------------------|:------------------------|:------------------------|--------
| 0 | YCbCr | None | Transparency | `Plane` then `Line` | Integer
| 1 | RGB | JPEG2000-RCT | Transparency | `Line` then `Plane` | Integer
| 2 | YCbCr | None | Transparency | `Plane` then `Line` | IEEE 754
| 3 | RGB | JPEG2000-RCT | Transparency | `Line` then `Plane` | IEEE 754
| Other | reserved for future use | reserved for future use | reserved for future use | reserved for future use | reserved for future use

Restrictions:

If `colorspace_type` is 1, then `chroma_planes` MUST be 1, `log2_h_chroma_subsample` MUST be 0, and `log2_v_chroma_subsample` MUST be 0.
If `colorspace_type` is 1 or 3, then `chroma_planes` MUST be 1, `log2_h_chroma_subsample` MUST be 0, and `log2_v_chroma_subsample` MUST be 0.

If `colorspace_type` is 2 or 3, then `bits_per_raw_sample` MUST be one of the values supported by IEEE 754 (at the moment of writing: 16, 32, 64, 80, 128, 256; in practice only values 16 and 32 are usable).

Note: IEEE 754 `Pixels` are handled exactly the same way as integer `Pixels` during the decoding process, except that the resulting values are flagged as floating-point numbers rather than integers.

### chroma_planes

Expand Down