YUVColorTypes

This package is an add-on to ColorTypes, and supports YUV-like colors.

Unfortunately, the environment surrounding YUV-like colors has historically been confusing. In the first place, there is no consensus on even calling a color with one luminance component and two chroma components a YUV. Therefore, this package aims to handle well-defined and well-formed YUV-like colors in a "reasonable" manner.

This package does not provide a public API for the specific storage format of YUV colors. This is because YUV-like images are often used with chroma subsampling (e.g., YUV 4:2:0) and planar formats with channel-separated images.

Also, this package provides limited support for conversion between RGBs and YUVs, and does not provide general color management functionality.

Type design

Background

The ColorTypes.jl schemes for handling YUV-like colors include the following problems:

1. There are various profiles.
2. ColorTypes.jl does not provide abstract types for specific color models, except for AbstractRGB.
3. YUV-like components are often normalized in the range [0, 256) instead of [0, 1] or [-0.5, 0.5).
   1. YUV-like components are often handled as integer types, especially UInt8.
   2. FixedPointNumbers.jl v0.8 does not support unsigned Fixed.
4. Alpha component should be normalized in the range [0, 1] like ARGB.

YUVColorType.jl uses the following strategies to solve or mitigate the above problems.

1. Use julia's parametric types and CICP (Coding-independent code points).
2. Add some abstract types such as AbstractYUV.
3. Make the range depend on bit depth instead of normalizing into [0, 1].
4. When the component type is an integer type, encode alpha implicitly as Normed.

YUV type

The actual type definition underlying YUVColorTypes is as follows:

struct YUV{T,P,U,R,D} <: AbstractYUV{T}
    y::T
    u::T
    v::T
end

It is worth noting that while RGB has a single type parameter T which specifies the type of the component, YUV has four additional type parameters.

Be prepared that this type may not work well because some of the downstream packages do not assume color types with type parameters other than the component type T.

In any case, the end-users do not need to be aware of the details of the type implementation, and it is undesirable to write codes relying on the internal implementation, since it may change in the future.

YUVColorTypes provides several type aliases. For BT.601 625-line, the following type aliases are defined for opaque, i.e., without alpha, YCbCr.

• YCbCrBT601_625
• YCbCr24BT601_625
• YCbCr30BT601_625
• YCbCr36BT601_625
• YCbCr48BT601_625

The first alias is the basic type for which no bit depth is specified. Subsequent aliases are specified with component bit depths of 8, 10, 12, and 16 bits.

The bit depth specification changes the ranges of possible values for the components. The following five colors are different representations of the same color.

using YUVColorTypes

[
    YCbCrBT601_625(100, 80, 100),
    YCbCr24BT601_625(100, 80, 100),
    YCbCr30BT601_625(400, 320, 400),
    YCbCr36BT601_625(1600, 1280, 1600),
    YCbCr48BT601_625(25600, 20480, 25600),
]

Note that unlike RGB24, the types with bit depths are still parametric.

using FixedPointNumbers
[
    YCbCr24BT601_625{UInt8}(100, 200, 100), # `UInt8` is the default component type
    YCbCr24BT601_625{Int}(100, 200, 100),
    YCbCr24BT601_625{Q11f4}(100.5, 200.5, 100.5),
    YCbCr24BT601_625{Float32}(100.5, 200.5, 100.5),
]

Then, what is the difference between YCbCrBT601_625 without specifying bit depth and YCbCr24BT601_625? In fact, they behave almost identically. The difference is the way the component type is determined in the conversion. YCbCrBT601_625 attempts to maintain the precision. On the other hand, types with bit depth do not care about the fractional part or out-of-range values. (FIXME)

julia> YCbCrBT601_625(YCbCr30BT601_625{UInt16}(401, 322, 403))YCbCrBT601_625{Float32}(100.25, 80.5, 100.75)
julia> YCbCr24BT601_625(YCbCr30BT601_625{UInt16}(401, 322, 403))YCbCr24BT601_625{Float32}(100.25, 80.5, 100.75)