GLB Quality Issue with pnquant 8-bit Compressed Textures

[ShantanuSriv]

@toji FYI, this PR broke model-viewer.

I've fixed it by passing { premultiplyAlpha: 'none' } to createImageBitmap by default: 7e5d148

Originally posted by @mrdoob in #19518 (comment)

I am generating glTF + bin + textures from FBX using Facebook’s FBX2glTF. After that I am compressing textures using pngquant and re-packaging it as a GLB file using gltf-import-export. Now when I try to load the asset on ThreeJS editor, glTF file loads properly – without any quality issues – but its corresponding GLB file is having quality issues.

The same file loads up fine on donmccurdy GLTF viewer (uses r116) and many other platforms except threeJS editor (using r120) and Google model-viewer (using r120).

I am not sure what has happened between these releases because I remember it used to work fine with older threeJS versions. If anyone knows what has changed in between and how can this be resolved, please help out.

Attaching screenshots for reference.

glTF File on ThreeJS Editor

GLB File on ThreeJS Editor

GLB File on DonMccurdy's Viewer

[mrdoob]

@toji FYI, this PR broke model-viewer.

What PR?

[toji]

That's actually your comment, from #19518 (comment) 😄

Looks like the actual issue here is what's posted after Originally posted by @mrdoob

[mrdoob]

That's actually your comment, from #19518 (comment) 😄

That was my 20-years-younger-self 👴

[ShantanuSriv]

Closing this PR since this is not the root cause of the problem I am facing.

Main reason for the above issue is -

three.js/examples/js/loaders/GLTFLoader.js

Line 2263 in 7830fe3

hasAlpha = new DataView( bufferView, 25, 1 ).getUint8( 0, false ) === 6;

- this check was previously not there for the older verisons, I guess 6 months before. After this check got added, pngquant compressed textures in the bufferView field for GLBs seems to not adhere to the above conditon and the hasAlpha field gets set to False with the compressed textures in the latest ThreeJS releases.

pngquant converts the 24-bit and 32-bit PNG files to 8-bit files – is that the main reason why it is not adhering to the above conditon for alpha channel is something I am not completely sure of, as of now.

If anyone knows what this condition is actually checking against and can let me know then it would be really helpful. Also, if there is any suggestion for compressing PNG textures better than pngquant and tinypng, do let me know.

Thanks

[mrdoob]

That condition is checking if the texture has an alpha channel.

[ShantanuSriv]

That condition is checking if the texture has an alpha channel.

The file which is coming out of pngquant is holding the alpha channel for the texture but still it's failing this check and returning false. The same file seems to be working properly with the older verisons of three.js. Only when this condition got added, hasAlpha flag is getting set to false. My understanding is that pngquant adheres completely with PNG specifications and https://sandbox.babylonjs.com/ also seems to be rendering with the alpha channel for compressed-textures.

Compressed Image Info:

  Filename:image.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: DirectClass
  Geometry: 2048x2048+0+0
  Units: Undefined
  Colorspace: sRGB
  Type: PaletteAlpha
  Base type: Undefined
  Endianness: Undefined
  Depth: 8-bit
  Channel depth:
    Red: 8-bit
    Green: 8-bit
    Blue: 8-bit
    Alpha: 8-bit
  Channel statistics:
    Pixels: 4194304
    Red:
      min: 0  (0)
      max: 255 (1)
      mean: 58.3066 (0.228653)
      standard deviation: 37.2582 (0.14611)
      kurtosis: 9.23021
      skewness: 1.80823
      entropy: 0.607401
    Green:
      min: 0  (0)
      max: 255 (1)
      mean: 81.4999 (0.319607)
      standard deviation: 43.5139 (0.170643)
      kurtosis: 1.5865
      skewness: -0.00149309
      entropy: 0.620068
    Blue:
      min: 0  (0)
      max: 255 (1)
      mean: 58.2928 (0.228599)
      standard deviation: 37.3246 (0.146371)
      kurtosis: 5.75047
      skewness: 0.96205
      entropy: 0.655589
    Alpha:
      min: 0  (0)
      max: 255 (1)
      mean: 192.342 (0.754281)
      standard deviation: 109.63 (0.429922)
      kurtosis: -0.599153
      skewness: -1.18255
      entropy: 0.272346
  Image statistics:
    Overall:
      min: 0  (0)
      max: 255 (1)
      mean: 97.6103 (0.382785)
      standard deviation: 56.9317 (0.223262)
      kurtosis: -0.377448
      skewness: 0.932973
      entropy: 0.538851
  Alpha: srgba(71,112,76,0)   #47704C00
  Colors: 256
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</details>

[mrdoob]

Ah I see. So is it the case that the hasAlpha check only works when the png is 32-bit?

#18843

/cc @donmccurdy

[donmccurdy]

I guess we'll have to check alpha in a different way for some types of .png files. @ShantanuSriv can you share an example .png file that is not working?

[ShantanuSriv]

I guess we'll have to check alpha in a different way for some types of .png files. @ShantanuSriv can you share an example .png file that is not working?

Sample 8-bit PNG File

[donmccurdy]

Fix coming in #20376, thanks for the report!