DXT1UseAlpha 0 has no effect

[B-Interactive]

All attempts so far to produce a DXT1 with no alpha (0x83f0), have only resulted in a DXT1A with 1-bit alpha (0x83f1). The result is the same even when the source .png has had all alpha data stripped completely from it.

It's not clear what the DXT1UseAlpha option does.

$ magick atlas.png -background white -alpha remove -alpha off PNG24:atlas_rgb.png

$ compressonatorcli -fd DXT1 -DXT1UseAlpha 0 atlas_rgb.png atlas_dxt1.ktx
/usr/lib/compressonator-cli-bin/compressonatorcli
Processing source     : atlas_rgb.png
Processing destination: atlas_dxt1.ktx
Source data size      = 16777216 Bytes, width = 2048 px  height = 2048 px
Destination data size = 2097152 Bytes   Resulting compression ratio = 8.00:1

Done Processing

$ ktxinfo atlas_dxt1.ktx Header

identifier: «KTX 11»\r\n\x1A\n
endianness: 0x4030201
glType: 0
glTypeSize: 1
glFormat: 0
glInternalformat: 0x83f1
glBaseInternalformat: 0x1908
pixelWidth: 2048
pixelHeight: 2048
pixelDepth: 0
numberOfArrayElements: 0
numberOfFaces: 1
numberOfMipLevels: 1
bytesOfKeyValueData: 0

No Key/Value data.

Data Sizes (bytes)
------------------
Level 0: 2097152

Total: 2097152