https://docs.unity3d.com/Manual/Lightmaps-TechnicalInformation.html
https://blog.uwa4d.com/archives/1882.html
http://ranaitian.com/2018/04/03/unity-lightmap- Mobile-ON /
https://zhuanlan.zhihu.com/p/35096536 this article describes unity_Lightmap_HDR
unity stores lightmaps with different compressions and encoding shemes, depending on the target platform and the compression setting in the lighting window.
unity projects can use two techniques to encoded baked light intensity ranges into low dynamic range textures when this is needed:
RGBM encoding. RGBM encoding stores a color in the RGB channels and a multiplier (M) in the alpha channel. 我一直纠结这个M是啥，原来是multiplier的首字母。the range of RGBM lightmaps goes from 0 to 34.49(5的2.2次方) in linear space, and from 0 to 5 in gamma space.
Double Low Dynamic Range (dLDR) encoding.
dLDR encoding is used on mobile platforms by simply mapping a range of [0, 2] to [0, 1].
Baked light intensities that are above a value of 2 will be clamped. The decoding value is computed by multiplying the value from the lightmap texture by 2 when gamma space is used, or 4.59482(2的2.2次方) when linear space is used. Some platforms store lightmaps as dLDR because their hardware compression produces poor-looking artifacts when using RGBM.
When Linear Color Space is used, the lightmap texture is marked as sRGB and the final value used by the shaders (after sampling and decoding) will be in Linear Color Space.
When Gamma Color Space is used, the final value will be in Gamma Color Space.
Note: When encoding is used, the values stored in the lightmaps (GPU texture memory) are always in Gamma Color Space.
The Decode Lightmap shader function from the UnityCG.cginc shader include file handles the decoding of lightmap values after the value is read from the lightmap texture in a shader.
HDR lightmap support
HDR lightmaps can be used on PC, Mac & Linux Standalone, Xbox OneMicrosoft’s eighth generation video game console.
See in Glossary, PlayStation 4Sony’s eighth generation video game console.
See in Glossary, iOSApple’s mobile operating system. More info
See in Glossary, tvOS and Android.
The Other Settings panel in the Player settings has a Lightmap Encoding option for these platforms, which controls the encoding/compression of the lightmaps.
choosing high quality will enabled HDR lightmap support, whereas normal quality will switch to using RGBM encoding.
low quality will switch to dLDR encoding on mobile platforms, on other platforms it is equivalent to Normal quality.
when lightmap compession is enabled in the lighting window,
the lightmaps will be compressed using the BC6H compression format on desktop and console platforms.
for mobile platforms, unity selects the HDR format according to the table below.
advantages of high quality (BC6H) lightmaps
HDR lightmaps do not use any encoding scheme to encode lightmap values, so the supported range is only limited by the 16-bit floating point texture format that goes from 0 to 65504.
BC6H format quality is superior to DXT5 + RGBM format encoding, ant it does not produce any of the color banding artifacts that RGBM encoding has.
shaders that need to sample HDR lightmaps are a few ALU instructions shorter because there is no need to decode the sampled values.
here is the list of encoding schemes and their texture compression formats per target platform:
*If the target is Android then you can override the default texture compression format from the Build Settings to one of the following formats: DXT1, PVRTC, ETC2, ASTC. The default format is ETC for RGB textures and ETC2 for RGBA textures.
Precomputed real-time Global Illumination (GI)
The inputs to the GI system have a different range and encoding to the output. Surface albedo is 8-bit unsigned integer RGB in gamma space and emission is 16-bit floating point RGB in linear space. For advice on providing custom inputs using a meta pass, see documentation on Meta pass.
The irradiance output texture is stored using the RGB9E5 shared exponent floating point format if the graphics hardware supports it, or RGBM with a range of 5 as fallback. The range of RGB9E5 lightmaps is [0, 65408]. For details on the RGB9E5 format, see Khronos.org: EXT_texture_shared_exponent.
