Output image looks blurry if using XYB and chroma subsampling other than 4:4:4

[TransparentLC]

Describe the bug
I tried to compress a couple of images with XYB colorspace and different chroma subsampling settings, but found them all to be very blurry except for 4:4:4, and it seems that Luma is subsampled as well. Without using XYB there is no such problem.

Is this a bug? Or it is not recommended to use XYB and chroma subsampling together?

To Reproduce
Compress an image using jpegli with args given below.

Expected behavior
The output image should be about as crisp as the input image.

Screenshots
Test image: https://r0k.us/graphics/kodak/kodak/kodim21.png

cjpegli kodim21.png kodim21_444.jpg --xyb --chroma_subsampling=444 -q 50

cjpegli kodim21.png kodim21_420.jpg --xyb --chroma_subsampling=420 -q 50 looks blurry compared with 4:4:4.

cjpegli kodim21.png kodim21_422.jpg --xyb --chroma_subsampling=422 -q 50 looks blurry and stretched horizontally.

cjpegli kodim21.png kodim21_440.jpg --xyb --chroma_subsampling=440 -q 50 looks blurry and stretched vertically.

Environment

• OS: Windows
• Compiler version: Not applicable. I downloaded the encoder from libjxl's latest CI artifacts https://github.com/libjxl/libjxl/actions/runs/14097851256
• CPU type: x86_64
• cjxl/djxl version string: JPEG XL encoder v0.12.0 b1842b9 [AVX2,SSE4,SSSE3,SSE2]

[wwinniww]

As I can understand it color subsampling is for YUV / YCbCr color modell only,
XYB should be used with no color subsmapling, aka 444, only.

The result of XYB + 42x subsampling reduce the luminance resolution. I have no idea why this is an allowed option in the cjpegli encoder.

[TransparentLC]

So if chroma subsampling doesn't apply to XYB JPEG, I guess there should be a warning or an error.

Some other observations:

• cjpegli kodim21.png kodim21_444.jpg --xyb --chroma_subsampling=444 -q 50 kodim21_444.jpg
• cjpegli kodim21.png kodim21_.jpg --xyb -q 50 kodim21_.jpg

Both looks OK but they are different images. But as far as I know the conversion between RGB and YUV 4:4:4 should be lossless?

Besides, MediaInfo says the colorspace is RGB instead of XYB...

General
Complete name                            : kodim21_444.jpg
Format                                   : JPEG
File size                                : 36.7 KiB

Image
Format                                   : JPEG
Width                                    : 768 pixels
Height                                   : 512 pixels
Color space                              : RGB
Bit depth                                : 8 bits
Compression mode                         : Lossy
Stream size                              : 36.7 KiB (100%) / 36.7 KiB (100%)
ColorSpace_ICC                           : RGB

General
Complete name                            : kodim21_.jpg
Format                                   : JPEG
File size                                : 35.4 KiB

Image
Format                                   : JPEG
Width                                    : 768 pixels
Height                                   : 512 pixels
Color space                              : RGB
Bit depth                                : 8 bits
Compression mode                         : Lossy
Stream size                              : 35.4 KiB (100%) / 35.4 KiB (100%)
ColorSpace_ICC                           : RGB

[wwinniww]

jpegli is still under development. The --XYB option has little to no failsafes right now.
Users of jpegli should know what they do or choose options that makes sense.