When retouching pictures you should first address white balance and contrast. White balance is normally what one would 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 color of the illumination in the photo and normally has white as a goal. White balance apps attempt to retouch the hue of the illumination to neutral and to do that, the program normally needs some neutrals in the picture to calculate the correct filter tone from. For the whites one can use a piece of paper or a white wall or a dedicated white card. The grays are more difficult to find in real life, so one can use 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 images, one usually has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corected 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 applications can dispense with that, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Normally software only has a single slider for contrast that addresses all thre aspects at once. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. At best the software will have a control for luminance contrast and for color contrast.
The usual way to adjust contrast is simply by changing the difference between the individual R, G and B values and the middle 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 change the algorithm to: R=(R-RAverage)*contrast+RAverage where RAverage is the average red channel value of the image. And so on for G and B. Using 128 attempts the same and merely assumes the picture has a full range of brightness values, in which case the average will be 128.
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. Levels adjustment is meant for this type of correction. One can do this with Photoshop's levels adjustment like this: Convert the image to Lab. Select the L channel only and use Photoshop's levels adjustment on that channel only. Then convert back to RGB mode.
White balance deals with the color of the illumination in the photo and normally has white as a goal. White balance apps attempt to retouch the hue of the illumination to neutral and to do that, the program normally needs some neutrals in the picture to calculate the correct filter tone from. For the whites one can use a piece of paper or a white wall or a dedicated white card. The grays are more difficult to find in real life, so one can use 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 images, one usually has a temperature slider. Some RAW converters also have three color sliders for red, green and blue. Fluorescent and mixed light can be somewhat corected 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 applications can dispense with that, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. Normally software only has a single slider for contrast that addresses all thre aspects at once. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. At best the software will have a control for luminance contrast and for color contrast.
The usual way to adjust contrast is simply by changing the difference between the individual R, G and B values and the middle 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 change the algorithm to: R=(R-RAverage)*contrast+RAverage where RAverage is the average red channel value of the image. And so on for G and B. Using 128 attempts the same and merely assumes the picture has a full range of brightness values, in which case the average will be 128.
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. Levels adjustment is meant for this type of correction. One can do this with Photoshop's levels adjustment like this: Convert the image to Lab. Select the L channel only and use Photoshop's levels adjustment on that channel only. Then convert back to RGB mode.
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