Thanks a lot that helps a lot.
haggi
Am 03.02.2022 um 21:44 schrieb Larry Gritz:
PNG files are by specification "unassociated alpha". The OIIO
convention is to convert all input to associated alpha automatically
upon being read.
Unassociated alpha (the color channels are not premultiplied by the
alpha) is bad for a number of reasons, especially inside a renderer.
Associated alpha (aka premultiplied colors) are more like how light
and cameras work, so the computations in any renderer are making this
assumption, unless they are bending over backwards to accommodate
unassociated alpha textures. Another reason that associated is
preferred is that a lot of useful images have no way to be represented
as unassociated -- the common example is an image that's a
"transparent glow" -- high color value, but low alpha value. You just
can't represent that correctly an an associated alpha PNG.
Exr files are by specification associated alpha. TIFF can go either
way -- there is a field in the header that's supposed to say which it
is -- but it's almost always associated. So that's why you're not
seeing the same behavior for tif or exr.
I think that you can keep the png data unassociated as you convert it
to a texture by using oiiotool instead of maketx:
oiiotool -iconfig oiio:UnassociatedAlpha 1 in.png -otex out.tx
But in the renderer itself, you're going to have to account for the
fact that this particular texture file came from an unassociated alpha
source image. The math you need to do if you're using the texture will
need to be different, you'll need to multiply the colors by alpha in
the shader.
-- lg
On Feb 2, 2022, at 5:56 AM, haggi <ha...@haggi.de> wrote:
Hi,
we use recently started to use PNG files for texturing for Unity
games and want to use the same texturing for Arnold and Unity. for
effective texturing we use alpha channels with PNG files and had to
discover that the maketx creates an color channel premultiplied with
the alpha. This results in darker textures for our Arnold renderings
compared to the ones we see in Unity or Maya OpenGL viewport. This
behaviour only occurs with PNG files, not with tif or exr.
So my question is: What is the reason that the color channel is
premultiplied for tx files?
haggi
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--
Larry Gritz
l...@larrygritz.com
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