Add support for R11F_G11F_B10F, and/or fix IBL instructions in Sandcastle

[emackey]

In the Image-Based Lighting Sandcastle Demo, there are comments recommending how to create a .ktx file for use as an HDRI lighting environment. The instructions recommend downloading Google Filament and using their cmgen tool (from the bin folder).

So I tried that with Filament 1.8.1 on a free HDRI called Symmetrical Garden (from HDRI Haven), and the result is this KTX file:

symmetrical_garden_1k_ibl.zip

The KTX file came out of Filament with a pixel format of R11F_G11F_B10F (35898), which is not supported in Cesium.

Can support for this pixel format be added? (I tried naively adding the format to the supported list, but clearly it takes a little more than that to get it working).

[emackey]

Worth noting that the old cmgen, at the time this demo was first published, had this note in the command-line help message:

   KTX files are always encoded with 4-channel RGBM data

But the latest cmgen has this note instead:

   KTX files are always encoded with 3-channel RGB_10_11_11_REV data

[lilleyse]

@YoussefV you ran into something similar right? I know there were a few open threads about Filament, the KTX1->KTX2 converter, and CesiumJS. Where did we settle on those?

CC #9040

[YVin3D]

Yep, so for R11F_G11F_B10F pixel formats, you can use the WebGLConstants.R11F_G11F_B10F as the internal format of the image in your compressedTextureBuffer. The IBL sandcastle instructions didn't work for me either when i tried converting the kiara environment map to .ktx2, so I had to basically rewrite the original .ktx binary.