This document describes how color is used in the ttauri library.

Usage of color spaces

Usage	Space	depth	alpha
draw API	tsRGB	float32	straight
vertices	tsRGB	float16	straight
images	tsRGB	float16	pre-multiplied
fragment shaders	tsRGB	float32	pre-multiplied
frame buffers	tsRGB	float16	pre-multiplied
swap chain	sRGB*	uint8*	pre-multiplied

PNG decoding

TTauri's PNG decoder will create an image of float16 RGBA values in the tsRGB color space. The rgb values in the resulting image are pre-multiplied by the alpha.

The PNG decoder will use the color space and transfer function information from the PNG data to do the conversion of pixel values to the tsRGB color space.

sRGB color space and transfer function is implied if color space information is not available in the PNG data.

Theme files

The colors in ttauri's theme file may be edited by users, there are three different ways of specifying colors.

Format	Color space
'#' 6 or 8 hex digits	sRGB
3 or 4 integers	sRGB
3 or 4 floating point numbers	tsRGB

Swap chain

The swap chain images are the ones that are presented to the desktop compositor.

On windows 10 this will be 8-bit sRGB. On Windows 10 HDR/high gamut formats are only available for full screen applications.

On MacOS the swap chain will be in tsRGB float16, where the desktop compositor is able to handle HDR/high gamut in the tsRGB format.

tsRGB color space

The tsRGB color space is actually very common but never explicitly named. Here are examples of very simular color spaces:

scRGB, specified for 16 bit biased and scaled integers with a range between -0.5 and 7.5. Often used outside of this spec as float16 or float32 with extended range.
Extended-sRGB, specified with a non-linear transfer function that is mirrored for negative values. Often used outside of this spec using a linear transfer function.
The unnamed color space of float16 linear RGB swap-chain images.

The tsRGB color space has the same color primaries and white-point as the sRGB/BT.709 color spaces.

	x	y	Luminosity contribution
Red	0.6400	0.3300	0.2126
Green	0.3000	0.6000	0.7152
Blue	0.1500	0.0600	0.0722
White D65	0.3127	0.3290

The RGB values have a linearly transfer function where RGB(0.0, 0.0, 0.0) is black and RGB(1.0, 1.0, 1.0) is white with a luminance 80 cd/m2. Values above 1.0 will produce higher luminance values available on HDR monitors.

When all RGB values are within 0.0 and 1.0 the tsRGB color space is fully compatible with linear sRGB.

Negative color component values will allow colors outside of the sRGB gamut triangle, which may be rendered on high gamut monitors. However the luminosity of the color must be greater or equal to zero.

tYUV/tLUV related color space

The tLUV color space is used for mixing background and foreground colors for anti-aliasing text, where the perceived line width is an important attribute. For more detailed information see: The trouble with anti-aliasing

Conversion from tRGB to tYUV/tLUV:

Y = R * 0.2126 + G * 0.7152 + B * 0.0722
L = sqrt(Y)
U = B - Y
V = R - Y

Conversion from tYUV/tLUB to tRGB:

Y = L * L
R = V + Y
B = U + Y
G = Y - V * 0.2973 - Y * 0.1001

The color types

color

The color type is a 32 bit floating point 3D RGB vector + alpha scalar.

The RGB values are linear and may comprise the complete floating point range, including values above 1.0 and value below zero.

The alpha value is a linear value between 0.0 (transparent) and 1.0 (opaque).

Depending on where the colors are used the RGB values are pre-multiplied by the alpha value, for example: inside fragment shaders, images and frame buffers.

matrix3

A color conversion matrix can be stored in the matrix3 type. And a color value can be transformed by a matrix3. This transformation ignores and preserves the alpha value.

color storage

Color format types are used in vertex arrays and images when communicating with the GPU.

The types have the following syntax: numeric-type _ color-components [ _pack ]

Numeric type

Type	Description
norm	Signed integer mapped to the floating point range between -1.0 and 1.0.
unorm	Unsigned integer mapped to the floating point range between 0.0 and 1.0.
int	Signed integer.
uint	Unsigned integer.
float	Floating point number.
srgb	Non-linear sRGB format for the RGB component, the alpha remains linear.

Color components

The color components are the lower case letters: r, g, b & a. The ordering of the letters describe the order of the color components in memory. A number describes the number of bits of each component before it.

Here are a few examples of components:

Combination	Description
rgba32	32 bits per component red, green, blue & alpha.
rgba16	16 bits per component red, green, blue & alpha.
rgba8	8 bits per component red, green, blue & alpha.
rgb8	8 bits per component red, green, blue.
rg8	8 bits per component red, green.
r8	8 bits per component red.
abgr8	8 bits per component alpha, blue, green & red.
a2bgr10	2 bit alpha, 10 bits per component blue, green & red

Packing

If the format is non-packed, then each each color component must be 8, 16, 32 or 64 bits in size. Each color component is stored in memory in native-byte-order, and the components are ordered with in increasing memory addresses.

If the format is packed, then the color components are packed together in a single integer. The color components are packed inside the integer from msb to lsb. The integer are 8, 16, 32, 64 or 128 bits in size. The integer is stored in memory in native-byte-order.