When retouching pictures you should first address white balance and contrast. White balance is usually what you ought to consider first, then contrast.This order is important, because you can not set color contrast properly if the image has a color cast.
White balance deals with the hue or tone of the light within the image and normally has white as a goal. White balance apps attempt to adjust the color of the illumination to neutral and to do that, the app normally needs some whites or grays in the photo to find the suitable correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. The grays are ideally a dedicated gray card.
White balance software comes in two varieties: automatic and manual. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When converting RAW photos, one normally has a temperature slider. One can 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. There are a few programs that can dispense with the neutrals, but usually neutrals are needed.
There are three kinds of contrast: hue, saturation and brightness. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. At best the software will have a control for luminance contrast and for color contrast.
The usual 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 need to change the algorithm to use the average values of the image's R, G and B channels, 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 deals with the hue or tone of the light within the image and normally has white as a goal. White balance apps attempt to adjust the color of the illumination to neutral and to do that, the app normally needs some whites or grays in the photo to find the suitable correction tint from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. The grays are ideally a dedicated gray card.
White balance software comes in two varieties: automatic and manual. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When converting RAW photos, one normally has a temperature slider. One can 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. There are a few programs that can dispense with the neutrals, but usually neutrals are needed.
There are three kinds of contrast: hue, saturation and brightness. Very few applications have more than a single slider for contrast, that addresses all three kinds of contrast at once. It is not ideal with a single slider for all three, since the result usually suffers from over saturation and colorfulness. At best the software will have a control for luminance contrast and for color contrast.
The usual 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 need to change the algorithm to use the average values of the image's R, G and B channels, 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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