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.
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.