Lightmap RGBA Channels breakdown?

Does anyone know what each channel represents within the Lightmap RGBA? I assume that RGB is used for GI luminosity and Color but what about the Alpha channel? Is that AO? How does Lightmass use these channels?

I have enclosed two views of the test mesh level, the original (uncompressed) HG_Lightmap and its RED channel and ALPHA channel.

@DanielW would you be so kind as to explain this to us and what the banding portion in the Alpha is use for?

I’d still like to hear from Staff in regards to the Lightmap channel breakdown.

Hi there,

I was also wondering about this and I made a question on the Developer network.
someone from the staff replied so it might be useful to check this out!

Thanks for the response. Unfortunately, I am unable to gain access to the link because I am not part of technology developers program. Could you cut and paste the information from that link response to here? That would be most helpful. Thanks!


For lightmaps the engine uses 2 samples from the same texture:

Sample0.a and Sample1.a encodes the precomputed lighting HDR luminance.

Sample0.rgb encodes the precomputed lighting color.

Sample1.rgb encodes the directional scale factors.

The HDR luminance data is only encoded when using HQ lightmaps (see “r.HighQualityLightMaps”).
This doubles the lightmap sizes but allows smooth gradients and wider range support.

You can look at how the channels are used in LightmapCommon.usf if you want the details.

For shadowmaps, the engine encodes 1 light shadow attenuation per channel, supporting up to 4 precomputed shadows per mesh.