Examples: Fix alignment issue in LUTCubeLoader

[kchapelier]

This patch sets to 1 the unpackAlignment of the DataTexture3D generated by LUTCubeLoader.

There was an issue when LUTCubeLoader is used to load valid 3D .cube files with specific table sizes, the call to gl.texImage3D would fail with the following error: WebGL: INVALID_OPERATION: texImage3D: ArrayBufferView not big enough for request. I specifically noticed the issue with 2x2x2 and 3x3x3 LUT tables.

It should be noted that the issue didn't happen when using the DataTexture (2D) generated by LUTCubeLoader because that class has a default unpackAlignment of 1, unlike DataTexture3D which has a default unpackAlignment of 4. For the sake of clarity, I explicitely set the unpackAlignment to 1 for both the DataTexture and the DataTexture3D in this patch.

/fyi @gkjohnson

Thanks to twitter user KimiruHamiru for the help btw.

Example of a valid 3D .cube file which would trigger the issue :

# Header
TITLE "WhiteToYellow"

LUT_3D_SIZE 3

DOMAIN_MIN 0.0 0.0 0.0
DOMAIN_MAX 1.0 1.0 1.0

# Data table
0 0 0
0.5 0 0
1 0 0

0 0.5 0
0.5 0.5 0
1 0.5 0

0 1 0
0.5 1 0
1 1 0

0 0 0.5
0.5 0 0.5
1 0 0.5

0 0.5 0.5
0.5 0.5 0.5
1 0.5 0.5

0 1 0.5
0.5 1 0.5
1 1 0.5

0 0 1
0.5 0 1
1 0 1

0 0.5 1
0.5 0.5 1
1 0.5 1

0 1 1
0.5 1 1
1 1 0

[gkjohnson]

I think it makes more sense to to default the DataTexture3D.unpackAlignment to 1 especially if that's what DataTexture.unpackAlignment defaults to. I don't think there's a reason the two should default to different values and 1 is definitely the more user-friendly value from my understanding of the setting. I imagine it was an simple oversight when DataTexture3D was created.

@mrdoob @Mugen87 is there a reason DataTexture3D.unpackAlignment isn't initialized to 1?

[mrdoob]

Yeah, I think setting unpackAlignment to 1 in DataTexture3D (and DataTexture2DArray too while we're at it) makes sense.

@DavidPeicho Sounds good?

[kchapelier]

@gkjohnson Yes, the difference of unpackAlignment between DataTexture and DataTexture3D caught me off-guard, but I assumed there was a reason for it, either for performance or something else.

[DavidPeicho]

Sounds good to me!

[mrdoob]

Done: #21633

[mrdoob]

@kchapelier Thanks for raising the issue! 🙏