Talk:Gamma correction

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[edit] Power Law Relationship

is the formula given here the same power law relationship described in Gamma characteristic?

Yes.


How about a link in the text here instead of a footnote? Or do we in fact need a seperate page for Gamma characteristic -- could we merge that into here & leave a redirect? -- Tarquin

Regarding the Linear intensity vs. Linear encoding table at the top, with IE 6.0, the text is black, making a portion of the left half of the table impossible to read.

I think we should unite this page with Gamma correction. Wikipedia is not a dictionary, so it would make no sense to have 2 articles dealing with the same issue. --Uri

I agree (I suggested the same thing on the other page ;-). Merge the text here into the other & leave a redirect. (or the other way round. I'm not sure which has precedence) -- Tarquin 07:22 Aug 18, 2002 (PDT)

Perhaps move them both into Gamma (computer graphics)? --Uri
I'd rather not, that's a title that no-one will ever remember. I suspect Gamma correction is most likely to be linked to.
Well, I suppose you're right. I'll do the merge. --Uri

[edit] limited signal bandwidth

The two images shown while speaking about monitors with an analogue input are great, since it shows that alternating black and white on one row does not work, due to monitor limitatios. This could be improved, however, in several ways:

1. It should state *which* one is correct (the horizontal lines).

2. It should state *how* to percieve the images (move back really far, if possible, since this is better than squinting, since you are not affecting your eyesight whatsoever by merely stepping back). This also goes for the gamma correction chart on the side (which someone has mentioned below, as they don't 'get it').

3. It should state that if you see two different shades, that you can have an affect on the shade that is wrong. Place the monitor in a lower resolution mode and / or lower refresh rate, and you may notice the shades become the same, as you would be reducing this artifact of analogue inputs.

4. It should explain why pages like this are *wrong*: http://freespace.virgin.net/hugo.elias/graphics/x_gamma.htm (because, even though it is a checkerboard pattern, it it the same thing as verticle lines, if you realize the monitor 'draws' the image one row at a time, completely independent of other rows). *Many* pages use this type of incorrect image. For my CRT, I get 1.3 gamma from these images, when it is actually 2.5! Wow.

137.186.22.215 15:04, 6 May 2005 (UTC)

[edit] (Non)Displayable symbols

Hello

With Microsoft IE under Windows (2YK Pro, but not limited to this brand), the operator "proportion" (mark up ∝ ∝"), as other math symbols from the iso-10646 char set, is simply displayed as the default white square("∝" should show a white square if you are using IE under Windows), used for non displayable symbols within the iso-8859-1 latin char set.

Is there a solution with some special font to be downloaded?

I suppose this is a general problem for all pages using iso-10646 symbols.

Jean Paul (gerard.jph@wanadoo.fr) 20040315

I've no idea about Windows, but I replaced the equations with TeX versions, to be rendered as images. Unfortunately, due to a bug in the Wiki software, it won't render a "proportional" symbol, so I used the "similar to" symbol. -- Hankwang 08:54, 16 Mar 2004 (UTC)
You need to install a font that has all the needed symbols, for IE and Microsoft Word that would be Arial Unicode, or Verdana Unicode. You can obtain those fonts from the Word setup. I don't know if Firefox supports special unicode fonts when freshly installed.

[edit] What this image supposed to be ?

Q: I don't get what the check-your-gamma-value graphic on the right side of the article is supposed to be. when i look at the image, i see lines on the left side, and blank fields with slightly different grey values on the right side. I can't tell what field has the same grey value as the line fild, as my brain doesn't fool me. am i supposed to blur the image (look unsharply at it)? thanks, --Abdull 19:28, 17 Jan 2005 (UTC)

A: As explained above in 'limited signal bandwidth', you are supposed to step back until your eye percieves the horizontal stripes as a solid color. Then, you can compare the left side and the right side. The squares that appear as one solid color, rather than two different colored squares next to each other, shows the gamma correction of your display. You'll notice your eye is very good at noticing even slight differences in colors. The page should explain this. 137.186.22.215 15:07, 6 May 2005 (UTC)

I edited Cyp's latest addition involving black and white stripes. The first image ("gammaaargh") has nothing to do with gamma correction, because it is composed entirely of black and white pixels and therefore says nothing about the monitor's linearity. The difference in the apparent brightnesses of the two squares is just a function of the monitor's analogue bandwidth. I left it in anyway, for the time being, because it's interesting, but it really belongs in some other article. The second image ("gammatest") is closer to a true gamma test, as it allows you to compare an apparent 50% luminance generated by alternating black and white pixels with solid areas of varying luminance. It says nothing about the linearity at values other than 50%, however. -- Heron 12:29, 30 May 2004 (UTC)

The use of horizontal lines for the images mean that they flicker badly on interlaced displays.

--David Woolley 15:06, 16 October 2005 (UTC)

[edit] Why is there gamma correction

What is the reason for gamma correction? Does it have something to do with the Weber-Fechner law (or better: Stevens' power law), which deal with the fact that we freaky humans perceive stimuli logarithmically? --Abdull 11:52, 23 Mar 2005 (UTC)

I believe this is simply because TV cameras tend to produce current proportional to luninous intensity, but CRTs tend to produce a response proportional to that to about the power 2.2. For TVs it was much cheaper to correct at the transmitter than every receiver. For PCs, "IBM PCs" use cheap hardware that simply converts colour numbers to tube drive voltages. Macs try to correct to produce as a subjectively linear scale.

--David Woolley 15:03, 16 October 2005 (UTC)

Wrong Answer: The correct answer: there is gamma correction because every monitor is different. Usually you can adjust your monitor using the controls for gamma correction. Alternatively, you can adjust your video card output for gamma correction.

Wrong data in the article: The hexadecimal values for the boxes are equally spaced in the bottom row, but are not equally spaced in the top row. Maybe this goes back to the problem of engineers counting from "0" instead of "1" as the first number.  :)

[edit] Some curiosities

"The gamma function, or its inverse, has a slope of infinity at zero."

This http://en.wikipedia.org/wiki/Gamma_function gamma function? Since it's probably referring to the I ~ V^gamma instead, how do you get a slope of infinity at zero for that? To me the slope seems to approach zero at zero.

Another thing that confuses me is why is any gamma correction applied at all? The top chart with linear intensity and linear encoding clearly shows linear intensity looks wrong to the human eye. Besides, usually gamma is only corrected from 2.5 to 2.2 so even after correcting it's still not linear.

I believe the 2.5 is bogus. Gamma normally gets corrected from 1.0 (that of photo-sensors) to 2.2 (that of CRTs). The reason for correcting is that CRTs are non-linear and it used to be cheaper to correct at source.
Gamma, here, has nothing to do with gamma functions. --David Woolley 22:17, 22 November 2005 (UTC)

[edit] Wikipedia or Links, who's got it right ?

Following the gamma correction bar at the right of this article, I get a gamma value of about 2.8. This results in a *very* bright image on my screen, which doesn't look good at all. However, following the instructions at the Links calibration page -- http://links.twibright.com/calibration.html -- I get a gamma value of around 1,4 (set using nvidia-settings in Linux). This looks much better, but which one is actually correct ?

[edit] Optimum subjective gamma

My understanding is that the optimum gamma to obtain equal subjective brightness steps is rather lower than 2.2 or 2.5. I seem to remember it to be about 1.6, and I believe it is the correction applied by Macs for non-Web images.

Also, my undertsnding is that the typical CRT has a gamma of 2.2 rather than 2.5. 2.2 is chosen because it is typical of CRTs, not because it is physiologically optimum.

--David Woolley 15:12, 16 October 2005 (UTC)

This caries depending on platform. There's display gamma, video hardware gamma correction and net gamma response of a system displaying an image. There is also gamma correction built into a system.

Monitors have a gamma response of about 2.2-2.5 (I thing the 2.5 is a actually a boost given to compensate for ambient light and non zero black levels)

So when you display on a PC there is typically no gamma correction in hardware, the display typically has a 2.2 response and so to display an image correctly it has to have built in correction or software correction of about 2.2 to 2.5, This is what's built into the sRGB colorspace standard.

The net gamma response of a PC is 2.2 (ish)

On a Mac the net gamma response if 1.8, the display still has a response of about 2.2 but the hardware does gamma correction to make the net response 2.2. Images that look OK on a PC will look too bright on a mac unless the display software compensates for it. This is wht file formats like PNG have a built in gamma factor, so that a PC and mac can display images from either platform corrctly.

Other platforms are different, e.g. SGI have built in default gamma corrections of 1.6 but you can change this using the command line gamma function.

Today most PCs have adjustable gamma correction available through desktop settings. So they could gamma correct to look like a Mac for example but sRGB is now the standard on that platform for all content (and gamma has beneficial effects w.r.t. presision and human contrast sensitivity).

[edit] Display appearance

I am using Firefox 1.6a on Windows XP, and the linear intensity scale looks about right, while the linear encoding scale looks black until .04 with .05 le being equivalent to .02 li, In IE it's the same way, except the numbers are all black, while in Firefox they are all gray. Also GammaAaargh.png looks nowhere near uniformly bright. GammaTest, the left side is a constant color that is brighter than any square on the right, also in both browsers, though in IE, the first square on the right is darker. Don't know for sure the differences between browsers, but the differences are slight.

The linear intensity version should have the mid level to the left of centre, and does to me, because the eye doesn't have a gamma of 1. The backgrounds should be exactly the same on all graphical browsers using the same hardware because it is part ot the specificatin of the web as how they should appear. However, if you have colour profiles for your devices, it is possible that one browser isn't correcting for the difference between the display gamma and the sRGB curve and the other one is. --David Woolley 22:42, 22 November 2005 (UTC)

In summary, some web browsers apply their own gamma correction and this varies from browser to browser and platform to platform. It's a thorny issue, complicated by PNG for example which has built in gamma, however this makes it look the same on all platforms because the net gamma is applied to take the image back to a consistent display linear space. In summary it's bloody difficult to show an image in a browser that definitively characterises the gamma response on all systems, doubly so on systems which do 'the right thing' and impossible using PNGs with any browser that is written 'correctly' to account for gamma.

[edit] The main purpose of gamma correction

"The main purpose of gamma correction in video, desktop graphics, prepress, JPEG, and MPEG is to code luminance or tristimulus values (proportional to intensity) into a perceptually-uniform domain, so as optimize perceptual performance of a limited number of bits in each RGB (or CMYK ) component." Excerpted from The rehabilitation of gamma. This article needs more emphasis on the real purpose of gamma correcion. -- Shotgunlee 00:14, 1 July 2006 (UTC)

[edit] incorrect GammaTest.png

GammaTest.png is not correct.

If you compare the image at the right side of the article (GammaTest.png) to the similar image at

http://www.normankoren.com/makingfineprints1A.html

and to the gamma correction images here:

http://www.photoscientia.co.uk/Gamma.htm

you will see that they do not agree. And I think these people know what they are talking about.

Simastrick 17:35, 20 July 2006 (UTC)

Interesting. The RGB values in GammaTest.png are correct. According to your links and Gammatest.png displayed in GIMP or gThumb, my monitor is 2.2 (as it should be). However, displayed in Opera, Firefox, and my old XV image viewer, it gives the likely incorrect value of about 1.9-2.0. I'm not sure what the various PNG libraries assume for the gamma value of the display. Han-Kwang 21:15, 22 July 2006 (UTC)

[edit] {0..1} or [0, 1]?

I think that it would be more precise to write in the first paragraph that V_in and V_out belongs to the range [0, 1], not {0..1}. In mathematics we use curly braces to denote elements of a set. Maybe it would be more clear to non-mathematics and precise at the same time if we write:

   0 <= V_in, V_out <= 1

[edit] xgamma mention

Part of the article mentions that Linux operating systems can adjust the gamma using the xgamma command as root. I see several problems with this. First, it's not Linux specific---it's a command any operating system using the X Window System has. Second, you don't need to be root to use it. Finally, is this even appropriate for Wikipedia? I'd correct the text myself, but I don't want to bother if the paragraph is better just stricken completely. 129.110.241.254 15:08, 30 September 2006 (UTC)

[edit] Safari comment

The test pattern section was changed to suggest that it "should not be used when viewed using the Safari Browser on Apple operating systems, as it is the only browser that does not assume the image to be sRGB."

This is not true on either point. Many browsers make no attempt to deal with colorspace, sRGB or otherwise. They treat images as if in monitor space; that is, they send the 8b color data to the screen without modification. On a Mac as on any other platform, adjusting for this test pattern will therefore make the monitor space approximately appropriate for sRGB. Of course, if you adjust the hardware, your monitor profile should change, but this will not affect the display in non-profile-aware apps beyond the hardware effect that you've adjusted.

Dicklyon 23:46, 7 November 2006 (UTC)

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