When retouching images one should first address white balance and contrast. White balance is normally the thing one ought to consider to begin with, then contrast.White balance should be addressed before contrast because color contrast can not be set properly if the image has a colorcast.
White balance is concerned with the color of the light within the image and sets white as an ideal. White balance applications attempt to retouch the tint of the illumination to neutral and in order to do that, the program needs some whites or grays in the picture to calculate the correct correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. Gray cards are manufactured for the purpose of adding a neutral gray to the photo.
White balance software usually has both a manual and an automatic mode. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When opening RAW images, one normally has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corrected with color sliders, but unfortunately color sliders usually tone the blacks and whites in an undesirable way. For automatic corrections, the software normally needs neutrals in the image, like a gray card and/or a white card. Some programs can dispense with that, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Software usually has a single slider for contrast adjustment, which addresses all three at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. Luminance contrast and color contrast should be treated separately.
The standard way to manipulate contrast is simply by altering the difference between the individual red, green and blue values and the average value (128); like this: R= (R-128) * contrast + 128; and likewise for green and blue. This method is only suitable for images that cover the entire brightness range. What if the image is very pale or very dark? In that case you can't use 128, but have to use the average of the individual channels in the image, like this: R=(R-RAverage)*contrast+RAverage. And so on for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.
If the darkest and brightest areas are not black and white a different situation arises. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one's software does not offer the option to expand brightness range, one can do it with Photoshop's levels adjustment like this: First convert the image to Lab mode, select the L channel only and run auto levels on that. Then convert back to RGB mode.
White balance is concerned with the color of the light within the image and sets white as an ideal. White balance applications attempt to retouch the tint of the illumination to neutral and in order to do that, the program needs some whites or grays in the picture to calculate the correct correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. Gray cards are manufactured for the purpose of adding a neutral gray to the photo.
White balance software usually has both a manual and an automatic mode. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When opening RAW images, one normally has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corrected with color sliders, but unfortunately color sliders usually tone the blacks and whites in an undesirable way. For automatic corrections, the software normally needs neutrals in the image, like a gray card and/or a white card. Some programs can dispense with that, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Software usually has a single slider for contrast adjustment, which addresses all three at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. Luminance contrast and color contrast should be treated separately.
The standard way to manipulate contrast is simply by altering the difference between the individual red, green and blue values and the average value (128); like this: R= (R-128) * contrast + 128; and likewise for green and blue. This method is only suitable for images that cover the entire brightness range. What if the image is very pale or very dark? In that case you can't use 128, but have to use the average of the individual channels in the image, like this: R=(R-RAverage)*contrast+RAverage. And so on for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.
If the darkest and brightest areas are not black and white a different situation arises. If that is the case, one should also be able to expand the brightness range to reach black and white. This is essentially what levels adjustment does. If one's software does not offer the option to expand brightness range, one can do it with Photoshop's levels adjustment like this: First convert the image to Lab mode, select the L channel only and run auto levels on that. Then convert back to RGB mode.
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