Colour space: sRGB and linear

A colour space tells the engine how to read the numbers in a texture. Colour textures are stored in sRGB, a curved encoding, while lighting maths runs in linear space, where twice the number means twice the light. GPU Gems states the rule: "Renderers, shaders, and compositors like to operate with linear data" (Gritz & d'Eon 2007).

For a texture artist this comes down to one checkbox per texture. With the correct setting, materials look the same in every engine. With the wrong one, a roughness map turns glossy or a base colour washes out, although the file itself looks normal.

#Why there are two spaces

Screens do not turn pixel values into light in a straight line. Gritz and d'Eon explain that "A typical gamma of 2.2 means that a pixel at 50 percent intensity emits less than a quarter of the light" (Gritz & d'Eon 2007). sRGB is the standard encoding built around this behaviour. HP and Microsoft proposed it in 1996 and it became the IEC standard 61966-2-1 in 1999 (Wikipedia: sRGB). Its curve combines "a linear function at low brightness values and a displaced power law for the rest of the range", and simplified implementations approximate it as plain gamma 2.2 (Wikipedia: sRGB).

Lighting, on the other hand, adds and multiplies amounts of light. If you light the stored sRGB numbers directly, GPU Gems warns, "the scene will appear dimmer than an accurate simulation" (Gritz & d'Eon 2007). So engines decode sRGB textures to linear values when they sample them, light the scene in linear, and encode the result for the screen at the end. Unity describes this process: it uses "your GPU's sRGB sampler by default to crossover from gamma to linear color space" (Unity Manual: Introduction to linear color space in Unity), and says linear rendering "gives more accurate rendering than working in gamma color space" (Unity Manual: Introduction to linear color space in Unity).

#Decoded values

The standard decoding, as written in the W3C's reference code, divides small values by 12.92 and raises the rest with ((value + 0.055) / 1.055)^2.4 (w3c/csswg-drafts: Sample code for color conversions). Worked through for 8-bit values:

#Which maps are colour and which are data

The rule: if a human judged the texture by eye as a colour, it is sRGB. If it stores a number the shader uses for maths, it is linear.

• sRGB: base colour and emissive. glTF requires both to be "encoded with the piecewise sRGB opto-electronic transfer function" (Khronos Group: glTF 2.0 Specification).
• Linear: normal, occlusion, roughness, metallic, height and masks. glTF says the normal and occlusion textures "MUST be encoded with the linear transfer function" (Khronos Group: glTF 2.0 Specification). GPU Gems notes that "Alpha channels, normal maps, displacement values (and so on) are almost certainly already linear" (Gritz & d'Eon 2007).

A channel-packed ORM texture holds three data maps, so it is always linear.

#Filtering and blending in linear space

The conversion matters for more than lighting. glTF asks that the transfer function be decoded "prior to linear interpolation" when a texture is filtered (Khronos Group: glTF 2.0 Specification), and notes that graphics APIs have "dedicated sRGB texture format enums" for this (Khronos Group: glTF 2.0 Specification). GPU Gems uses a two-pixel edge, "two pixels at 1.0 and two at 0.0", to show that averaging stored gamma values gives the wrong result (Gritz & d'Eon 2007). Worked through: averaging stored 0 and 255 gives 127.5, which decodes to about 0.21 of the light, not the 0.5 a real mix of black and white would give. This is why mipmaps of colour textures must be built from linear values.

#In different tools

• Unreal Engine: the sRGB checkbox on each texture; Masks, Normalmap and HDR compression switch it off (Unreal Engine documentation: Texture Compression Settings).
• Unity: the sRGB (Color Texture) checkbox, described as indicating "whether the texture is in gamma space" (Unity Manual: Default texture import settings reference). The project's colour space setting chooses linear or gamma rendering (Unity Manual: Introduction to linear color space in Unity).
• Godot: the importer keeps compressed formats sRGB-friendly by default (Godot documentation: Importing images). HDR files are linear; the HDR as sRGB option exists only for broken files and makes correct ones "look too dark" (Godot documentation: Importing images).
• glTF: colour space is fixed by the slot, and any ICC profile or gamma value in the image "MUST be ignored" (Khronos Group: glTF 2.0 Specification).

#Further reading and watching

Related: texture maps explained, texture formats, compression and mipmaps, physically based rendering, bit depth and banding, tonemapping.