[x3d-public] (FlipY) Texture coordinates for material textures.

[Brutzman, Donald (Don) (CIV)]

Michalis, thanks for noting that problems emerged in prior glTF conversions
for the online examples for Web3D.

If someone is able to help us correct and update them satisfactorily, that
will be welcome.  Likely interesting too since we can then use subversion
diff to look at differences.

all the best, Don
-- 
Don Brutzman  Naval Postgraduate School, Code USW/Br        brutzman at nps.edu
Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA    +1.831.656.2149
X3D graphics, virtual worlds, Navy robotics https://
faculty.nps.edu/brutzman

-----Original Message-----
From: x3d-public <x3d-public-bounces at web3d.org> On Behalf Of Michalis
Kamburelis
Sent: Sunday, April 10, 2022 9:00 AM
To: Holger Seelig <holger.seelig at yahoo.de>
Cc: X3D <x3d-public at web3d.org>
Subject: Re: [x3d-public] (FlipY) Texture coordinates for material textures.

The examples on
*
https://x3dgraphics.com/examples/X3dForAdvancedModeling/GltfSampleModels/ind
ex.html
are incorrect.

You can instead just use
https://github.com/KhronosGroup/glTF-Sample-Models/ examples, and convert
them to X3D yourself using latest view3dscene with "--no-x3d-extensions"
command-line parameter.

Explanation:

Unfortunately glTF has different treatment of texture coordinates than X3D,
assuming that Y=0 is "up" and Y=1 is "bottom of the image". This is
consistent with WebGL, but inconsistent with OpenGL, OpenGLES, X3D.
(Yes, it sucks that WebGL is inconsistent with OpenGL/OpenGLES in this
case.)

OTOH: X3D, OpenGL, OpenGLES, all say that Y=0 is "bottom" and Y=1 is "up of
the texure image".

There are various possible solutions to this:

1. CGE/view3scene by default places "flipVertically=true" in texture nodes,
see 
*
https://castle-engine.io/x3d_implementation_texturing_extensions.php#section
_flip_vertically
But this is non-standard CGE extension.

2.  If you perform conversion using CGE/view3scene->glTF with
"--no-x3d-extensions" command-line parameter, than we don't place
"flipVertically" in file, instead we process TextureCoordinates contents to
flip them.

3. Finally, one can also use TextureTransform as you say.

"""
      textureTransform TextureTransform {
        translation 0 -1
        scale 1 -1
      }
"""

achieves what is necessary.

AD 3 is also what i would advise most X3D browsers. (Because AD 2 requires
per-vertex processing, which should be avoided in general. AD
1 is CGE specific, and probably most X3D viewers don't care about this extra
optimization that "avoids doing TextureTransform at runtime".)

The examples on
https://x3dgraphics.com/examples/X3dForAdvancedModeling/GltfSampleModels/ind
ex.html
are just wrong. Someone used CGE/view3dcene without "--no-x3d-extensions" to
generate glTF outout, and then removed flipVertically=true, thus making
texture coords incorrect. I recall we talked about it with Don,
CGE/view3dscene didn't have "--no-x3d-extensions" initially. Looks like
noone had time to finalize this properly. The examples should be recreated
with "--no-x3d-extensions".

Regards,
Michalis

niedz., 10 kwi 2022 o 17:05 Holger Seelig <holger.seelig at yahoo.de>
napisał(a):
>
> I recently tested the new glTF examples from
https://x3dgraphics.com/examples/X3dForAdvancedModeling/GltfSampleModels/ind
ex.html with X_ITE alpha version and recognized that the textures
coordinated are y-flipped. I also tested View3DScene and it also happens
there. I know that glTF does no y-flip with textures  as X3D does, thus a
conversion from glTF to X3D should handle this and add a TextureTransform
with translation 0 1 and scale 1 -1. This would mean that this
TextureTransform must be added to the examples. Is this correct, or are the
textures from material nodes somewhere handled specially?
>
> Best regards,
> Holger
>
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