gamma and computer displays

Just a brief collection of bits and pieces. For a much more in-depth study, see the resources cited in the JPEG FAQ, specifically Poynton's gamma FAQ.

gamma adjustment

Some software offers configuration adjustments to compensate for the different gamma properties of different displays. Take care! Some expects to be configured with an actual gamma value, whereas others expect a factor which they apply to their nominal (default) assumption about computer screens. A widely-used default assumption for PC screens seems to be 2.2. With Macs you have to take account of the fact that the software applies its own contribution to the equation already, although the physical properties of the display itself are similar (again, see Poynton's FAQ for more detail on this).

WWW image formats and gamma adjustment

When making images, some nominal gamma value needs to be assumed. The PNG image format makes a point of providing for the nominal gamma to be stated in the image data, so that software can be able to apply suitable compensation and produce a normalised display for every reader. More about this at the PNG Home Site, specifically PNG Intro: Gamma Correction and PNG Gamma tutorial.

gamma adjustment in vic (MBone video)

The MBone video application, vic, came with a GIF image, gamma.gif, intended for image calibration. This snippet from the man page shows how to use it:

You can choose a proper gamma using the test image, gamma.gif, included in the vic distribution. View the image from several feet away and choose the bar which appears to have a uniform gray level. The number printed below this bar is the gamma of your display. Take this number, divide it by 2.2 (the gamma correction built into an analog video signal), and use this result for vic's gamma correction (i.e., Vic.gamma).

A copy of the image is inlined here.

[vic gamma.gif]

Povray's gamma adjustment

The Povray site offers an image and an accompanying plain-text explanation. I've also in-lined the image here.

[Povray gamma.gif]


Just for the record, based on my own PC display at the time of writing, I got a value of about 2.2 using both of the procedures mentioned above.

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