Gamma conversion factor between PC an SGI

3D/2D CGI and the tools used in their creation (Maya, Photoshop, Blender, GIMP, etc.).
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squeen
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Unread postby squeen » Mon Nov 06, 2006 1:19 pm

nekonoko wrote:I set my IRIX machines to 1.0 myself.


Everything looks so oddly dark to me on the Linux boxes with gamma=1.0. How can you stand it Neko?

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Unread postby unixmuseum » Mon Nov 06, 2006 3:23 pm

squeen wrote:
nekonoko wrote:I set my IRIX machines to 1.0 myself.


Everything looks so oddly dark to me on the Linux boxes with gamma=1.0. How can you stand it Neko?
I guess I'm even worse: I set mine at 0.85 if the machines are hooked up to LCDs...

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DDT
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Re: Gamma conversion factor between PC an SGI

Unread postby DDT » Mon Nov 13, 2006 4:21 pm

This gamma problem is a real PITA.
The worst problem is with web pages, since HTML, JPG, GIF do not embed gamma information, and they look bad or suboptimal on a platform they were not designed for (e.g. Mac).

The solutions are:
1) Add gamma/colorspace info to HTML and image formats.
2) Switch all platforms to a common gamma (which is going to change as soon as new display devices have greater range and capabilities).

I'm for the 1st.. sooo likely ;)

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Unread postby fieldframe » Sun Dec 10, 2006 11:07 am

Remember the monitor is always doing gamma ~2.2 - it's a function of the electron gun.

So you do a gamma 1.0 - which mkaes the graphics LUT linear and the monitor is the only device in the display path doing a gamma operation.

Now, SGI's implementation of gamma is a correction, ie 1.0/gamma which is the opposite of what xgamma does on a Linux box (and other gamma tools on other platforms). IMO, SGI got it completely wrong. When SGI gamma is set to 1.7, the effective system gamma is ~1.3

A default Linux/Windows system gamma would be 2.2 as the gamma lut in the graphics card will be linear unless tweaked otherwise. This would make the image darker than the default SGI setting.

Does this clarify things?

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Gamma Standards

Unread postby abagail07 » Mon Dec 11, 2006 1:33 am

A previous poster stated that

"irix' default is 1.7, pc is 1.0 and mac is 1.3.
since macs were the de facto standard for dtp 1.3 was/is the reference for any kind of 'arts'."

The only part of that statement that is correct is the first four words.

Default Gamma on a PC, and sRGB which is the de-facto colour space for PCs is 2.2. Mac's have traditionally been 1.8, which was the gamma of the first Laserwriter printer, but this has changed over the last few years and now "most pre-press and retouching professionals use D65 G2.2.

:-)

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Re: Gamma conversion factor between PC an SGI

Unread postby Frapazoid » Tue Jun 12, 2007 8:58 am

I dealt with this problem recently.

I have three computers - a PC, a Mac and an SGI - all sharing one monitor right now. I have to unplug stuff to change compute - PITA. Anyway I wanted them to all show the same 'standard PC' 2.2 gamma.

The PC was easiest - I put the monitor on "sRGB" mode and left it at defaults. I used color calibration guides to confirm I was getting a result 2.2 gamma. That's in both Windows and Linux.

The Mac was almost the same - I just went through the calibration utility to confirm the monitor's natural gamma and etc. and have Mac set it to 2.2 instead of 1.8.

The SGI was the trickiest for one main reason that everything was way brighter - the blackpoint was way too high when I had sRGB mode. So when I hook up the SGI, I turn the sRGB mode off, and turn the brightness all the way down and the contrast all the way up getting the black point and white point to good spots.

Then I just used a gamma calibration guide (the simple thing with the grey and the black+white alternating lines) to tweak it to 2.2 gamma.

I think the setting I used was "gamma 0.875" or something in that area to set it permanently.

Before I did this, whenever I'd draw something on the SGI darks would be way darker and invisible on normal computers but now it all looks fantastical.

I did this beforehand and when I saw it on a PC everything in the background was black except the lava - but that seems to be the corrected version I uploaded there. I later did the next page on the SGI with the gamma and everything set and it looked totally right on the Mac and PC the first try - I didn't have to load up Gimp and change the curve.
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Re: Gamma Standards

Unread postby lewis » Sat Jun 23, 2007 3:15 pm

Default Gamma on a PC, and sRGB which is the de-facto colour space for PCs is 2.2. Mac's have traditionally been 1.8, which was the gamma of the first Laserwriter printer, but this has changed over the last few years and now "most pre-press and retouching professionals use D65 G2.2.


Default gamma as you mean it there on a PC is 2.5, ie no gamma correction since the CRT has a response of gamma=2.5. Gamma 2.2 is a compromise introduced with sRGB, and from the video world where that's what cameras do. People with calibrated screens may opt for D65/2.2 but the majority of Mac owners are still sat at 1.8. 2.2 makes the OS X UI look nasty to me, and the less linear working space makes a lot of image processing tasks look slightly worse than the closer-to-linear 1.8 old standby.


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