Comments on: Colorspaces in XSI

By: Digital Photography For Dummies | Gary Zimm's Photo

Digital Photography For Dummies | Gary Zimm's Photo — Fri, 30 Apr 2010 01:54:21 +0000

[...] Softimage Blog » Blog Archive » Colorspaces in XSI [...]

By: Harry Bardak

Harry Bardak — Sun, 09 Aug 2009 10:18:41 +0000

it s all fixed now

By: James Rowell

James Rowell — Sat, 08 Aug 2009 06:20:13 +0000

Ok – fourth time is a charm? Corrected shake nodes as per Jeromee’s points above.

linRGB_to_sRGB_1 = ColorX(0,
r<=0.0031308 ? r*12.92 : (1.055 * pow(r, 1.0/2.4)) – 0.055,
g<=0.0031308 ? g*12.92 : (1.055 * pow(g, 1.0/2.4)) – 0.055,
b<=0.0031308 ? b*12.92 : (1.055 * pow(b, 1.0/2.4)) – 0.055,
a, z);
sRGB_to_linRGB_1 = ColorX(0,
r<=0.04045? r/12.92 : pow((r+0.055)/1.055, 2.4),
g<=0.04045? g/12.92 : pow((g+0.055)/1.055, 2.4),
b<=0.04045? b/12.92 : pow((b+0.055)/1.055, 2.4),
a, z);

By: James Rowell

James Rowell — Fri, 07 Aug 2009 21:26:21 +0000

Another mistake in my little post – when posting a comment here the blog translates the ascii double quote character into something a little fancier. When you paste the code above into shake – it breaks the “resize” node. Just be sure to select “filter” type “impulse” to see what I’m talking about. Thanks – and sorry for the multiple posts in a row.

By: James Rowell

James Rowell — Fri, 07 Aug 2009 20:05:01 +0000

ooops – that’s an old copy of my sRGB_to_linRGB, and visa-versa, nodes. If anyone is thinking of using my code there (for use elsewhere), please change the colorx alpha channel to just be “a”, i.e.; NO correction on alpha. Sorry! :-)

By: James Rowell

James Rowell — Fri, 07 Aug 2009 19:59:52 +0000

Adrian – I was just jumping on this site to make a comment about the same thing. I think Harry has it wrong in the “sRGB Example: Gray Gradient” section. There are a few confusing red-herrings across the web regarding linear colorspace workflow, and this was one that took me a while to straighten out for myself as I was trying to learn this stuff. Since so many people link to this site, it’s worth fixing.

So Harry – if one is looking at a zero to one ramp in shake, with NO LUT on the viewer AND assuming that you are using a gamma 2.2 monitor, THEN the ramp on the left should look correct to your eye. You are wrong to think that your example on the left is too bright. Here’s how you can convince yourself – cut and paste the following nodes into shake.

Ramp1 = Ramp(21, 486, 1, 1, 0.5, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0);
Bytes1 = Bytes(Ramp1, 4);
Resize1 = Resize(Bytes1, 720, height, “impulse”, 0);
linRGB_to_sRGB_1 = ColorX(Resize1, r<=0.0031308 ? r*12.92 : pow(r*1.055, 1.0/2.4) – 0.055,
g<=0.0031308 ? g*12.92 : pow(g*1.055, 1.0/2.4) – 0.055, b<=0.0031308 ? b*12.92 : pow(b*1.055, 1.0/2.4) – 0.055,
a<=0.0031308 ? a*12.92 : pow(a*1.055, 1.0/2.4) – 0.055, z);
sRGB_to_linRGB_1 = ColorX(Resize1, r<=0.04045? r/12.92 : pow((r+0.055)/1.055, 2.4),
g<=0.04045? g/12.92 : pow((g+0.055)/1.055, 2.4), b<=0.04045? b/12.92 : pow((b+0.055)/1.055, 2.4),
a<=0.04045? a/12.92 : pow((a+0.055)/1.055, 2.4), z);

So – if you examine the output of my “resize” node – the ramp looks perfect on a shake viewer with NO LUT on a gamma 2.2 monitor. That is part of the whole point of gamma monitors – that equal steps in value look like equal steps up in value to our eye. If you look at the sRGB_to_linRGB node output, it’s FAR too dark in the blacks, i.e.; the steps do NOT look like equal steps up the scale. (Clearly previwing the linRGB_to_sRGB is way off.)

Hope you can fix your post Harry – it’s a little confusing to the newly indoctrinated to sort this out. I hope my commnt helps! Feel free to use my shake snippet if it helps you rework the writeup.

Thanks for posting this though – it is very helpful.

By: Adrian P

Adrian P — Mon, 20 Apr 2009 15:34:42 +0000

I don’t think your images in “sRGB Example: Gray gradient” are correct.

If you take the linear-light ramp image on the left and pass it through the linear-light to sRGB function, the image should appear lighter, not darker like the image on the right.

By: Mako

Mako — Thu, 27 Nov 2008 16:19:30 +0000

Hey Harry,

We just had lunch earlier today and I just realize now that I had read your explanations while I was doing my tests using the spheron camera.

Well done. I’d be happy to discuss even longer about all this with you one day.

By: Harry Bardak

Harry Bardak — Wed, 30 Jul 2008 16:27:16 +0000

Obviously with XSI 7 you don’t need to bother with theses shaders. But the concept stay the same.

By: harry bardak

harry bardak — Sat, 21 Jun 2008 10:06:03 +0000

Use a gamma 1/2.2 node instead and the simple_tone_mapping shader.
That would do the trick.

There is no point to change that now.