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 hue or tone of the light within the picture and normally has white as a goal. White balance software attempts to correct the hue of the illumination to white and in order to do that, the app normally needs some whites or grays in the image to calibrate the correct filter tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are ideally a dedicated gray card.
White balance software comes in two varieties: automatic and manual. The manual mode usually consist of a single temperature slider for adjusting the light cool or warm. This is OK for incandescent light, but not for fluorescent light or mixed light. When opening RAW photos, one usually has a temperature slider. One can also have three color sliders for red, green and blue. Color sliders can somewhat correct fluorescent light and mixed light, but the problem with using color sliders is that the black and the whites usually get a bad tone. Software with an auto option usually need neutrals in the image to work well, such as a gray card or white card. Some software can dispense with that, but usually neutrals are needed.
There are three kinds of contrast: hue, saturation and brightness. Normally software only has a single slider for contrast that addresses all three aspects 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 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. In that case one should be able to expand the brightest range to reach black and white. This is essentially what levels adjustment does. One can do this 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 hue or tone of the light within the picture and normally has white as a goal. White balance software attempts to correct the hue of the illumination to white and in order to do that, the app normally needs some whites or grays in the image to calibrate the correct filter tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. The grays are ideally a dedicated gray card.
White balance software comes in two varieties: automatic and manual. The manual mode usually consist of a single temperature slider for adjusting the light cool or warm. This is OK for incandescent light, but not for fluorescent light or mixed light. When opening RAW photos, one usually has a temperature slider. One can also have three color sliders for red, green and blue. Color sliders can somewhat correct fluorescent light and mixed light, but the problem with using color sliders is that the black and the whites usually get a bad tone. Software with an auto option usually need neutrals in the image to work well, such as a gray card or white card. Some software can dispense with that, but usually neutrals are needed.
There are three kinds of contrast: hue, saturation and brightness. Normally software only has a single slider for contrast that addresses all three aspects 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 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. In that case one should be able to expand the brightest range to reach black and white. This is essentially what levels adjustment does. One can do this 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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