[x3d-public] texture transform ordering

[Andreas Plesch]

Thanks. These are quite helpful and at least X_ITE and x3dom agree on
how these complex cases should be rendered.

What I was looking for is really more an explanation of why the
ordering needs to be reversed. It does produce the expected result for
combined rotation and scaling, not the shearing I initially expected.
The center field for me is still best understood as an inverted
center.

Andreas


On Wed, Aug 8, 2018 at 3:37 PM Nicholas Polys <npolys at vt.edu> wrote:
>
>
> Some examples for TextureTransform:
>
> http://www.web3d.org/x3d/content/examples/Vrml2Sourcebook/#Chapter18TextureMapping
>
> http://www.web3d.org/x3d/content/examples/ConformanceNist/Appearance/TextureTransform/index.html
>
>
> On Wed, Aug 8, 2018 at 8:42 AM Andreas Plesch <andreasplesch at gmail.com> wrote:
>>
>> Mirroring a texture works by scaling S by -1, eg. a texturetransform
>> with a '-1 1' scale field. This requires repeatS='true' since S
>> coordinates would be cut off, clamped at 0 otherwise.
>> To allow repeatS='false' the S coordinates have to be also shifted to
>> become positive. Since translation is applied _before_ scaling a
>> translation of '-1 0' is needed. This is unintuitive but clear from
>> the spec.
>> Another way would be to have a center of scaling in the middle. This
>> is accomplished by a '-0.5 -0.5' center field if the matrix
>> multiplication formula in the spec. is applied literally which is what
>> x3dom does.
>> This center value is very hard to understand since the spec. explains
>> the center point as the point about which scaling and rotation should
>> be performed. The strange value is probably a result of the inverted
>> ordering of transformation.
>> There is a spec. note:
>> NOTE  This transformation order is the reverse of the Transform node
>> transformation order since the texture coordinates, not the texture,
>> are being transformed (i.e., the texture coordinate system).
>> I do not quite understand the reasoning here. I understand that
>> scaling by a factor of 2 results in a smaller image since then the
>> image margin at uv=1,1 is now at the position where uv=0.5,0.5 was. I
>> also understand that shifting u by plus 0.1 results in the image
>> shifted to the left since the margin at u=1 is now where u=0.9 was.
>> But I cannot find an argument why the _ordering_ should be reversed.
>> Is it an opengl convention somehow ?
>> The only way I can make sense of the center field is by mentally
>> applying it as a translation before the scaling/rotation and then
>> unapplying it afterwards, as the formula demands, not actually as the
>> center of scaling/rotation. This explains why the center value has to
>> be inverted from what it is expected (by me).
>> It gets harder if you want to apply both a scale and a rotation. Since
>> the rotation is applied before the scale, the scale will then deform
>> the texture typically then into a sheared shape which is not what is
>> typically wanted as an outcome.
>>
>> Any insights or references very welcome,
>>
>> -Andreas
>> --
>> Andreas Plesch
>> Waltham, MA 02453
>>
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>
>
>
> --
> Nicholas F. Polys, Ph.D.
>
> Director of Visual Computing
> Virginia Tech Research Computing
>
> Affiliate Professor
> Virginia Tech Department of Computer Science



-- 
Andreas Plesch
Waltham, MA 02453