Difference between revisions of "Gamma"
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| − | '''Gamma''' is the relation between pixel value | + | '''Gamma''' is the relation between pixel value respectively electric power and visible brightness on a specific output medium. |
Although for common uses the '''gamma''' value just controls the subjective brightness of the mid range | Although for common uses the '''gamma''' value just controls the subjective brightness of the mid range | ||
| − | of pixels, there are certain situations in a photo stitching workflow where it makes a | + | of pixels, there are certain situations in a photo stitching workflow where it makes a quantitative difference |
to the results: | to the results: | ||
* Pixel values are interpolated when correcting TCA (transverse [[chromatic aberration]]). | * Pixel values are interpolated when correcting TCA (transverse [[chromatic aberration]]). | ||
* Pixel intensity is scaled when correcting [[vignetting]]. | * Pixel intensity is scaled when correcting [[vignetting]]. | ||
| − | * Pixel values are interpolated when photos are | + | * Pixel values are interpolated when photos are re-projected into the output image. |
* Blending via feathering, [[enblend]] or [[smartblend]] averages pixel values between images. | * Blending via feathering, [[enblend]] or [[smartblend]] averages pixel values between images. | ||
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[[nona]], [[PTStitcher]] and [[PTmender]] all accept a '''g''' (gamma) parameter on the '''m''' line, most | [[nona]], [[PTStitcher]] and [[PTmender]] all accept a '''g''' (gamma) parameter on the '''m''' line, most | ||
[[GUI front-ends]] allow you to set this. | [[GUI front-ends]] allow you to set this. | ||
| + | |||
| + | [[enblend]] (including version 3.0) appears to assume that input data is already gamma-corrected, so if you are | ||
| + | working with linear data you probably need to change to gamma = 2.2 before running enblend. | ||
See Helmut Dersch's article on [http://www.path.unimelb.edu.au/~dersch/gamma/gamma.html Interpolation and Gamma Correction]. | See Helmut Dersch's article on [http://www.path.unimelb.edu.au/~dersch/gamma/gamma.html Interpolation and Gamma Correction]. | ||
Revision as of 23:09, 11 February 2007
Gamma is the relation between pixel value respectively electric power and visible brightness on a specific output medium.
Although for common uses the gamma value just controls the subjective brightness of the mid range of pixels, there are certain situations in a photo stitching workflow where it makes a quantitative difference to the results:
- Pixel values are interpolated when correcting TCA (transverse chromatic aberration).
- Pixel intensity is scaled when correcting vignetting.
- Pixel values are interpolated when photos are re-projected into the output image.
- Blending via feathering, enblend or smartblend averages pixel values between images.
In any of these situations, it may be necessary to work with linear data (gamma = 1.0) or to use tools that work internally with gamma corrected data like so:
When using fulla to correct chromatic aberration and vignetting, be sure to set the -i parameter to the gamma of your input images.
nona, PTStitcher and PTmender all accept a g (gamma) parameter on the m line, most GUI front-ends allow you to set this.
enblend (including version 3.0) appears to assume that input data is already gamma-corrected, so if you are working with linear data you probably need to change to gamma = 2.2 before running enblend.
See Helmut Dersch's article on Interpolation and Gamma Correction.
For details see the wikipedia gamma correctiona article and the Gamma FAQ.