When editing images you ought to first address white balance and contrast. White balance is usually what you would look at first, then contrast.The reason for correcting white balance first of all is that you can't correct color contrast if the image has a color cast.
White balance addresses the color of the illumination within the picture and sets white as a goal. White balance software attempts to adjust the tint of the illumination to white and to do that, the software normally needs some neutrals in the picture to calculate the correct filter tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. Gray cards are manufactured for the purpose of adding a neutral gray to the photograph.
White balance software usually has both a manual and an automatic mode. 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 converting RAW pictures, one normally has a temperature slider. Some RAW converters 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. Automatic white balance correction usually needs neutrals in the image, like a gray-card or white-card or both. Some applications can dispense with that, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. 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 changing the difference between the individual R, G and B values and the middle value (128); like this: R= (R-128) * contrast + 128; and similar for green and blue. If the image is very dark or very pale this method will not do. 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 similarly for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.
Another problem with contrast adjustment is that not only may the average value not be 128, but the darkest and brightest areas may not be black and white. 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: 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 addresses the color of the illumination within the picture and sets white as a goal. White balance software attempts to adjust the tint of the illumination to white and to do that, the software normally needs some neutrals in the picture to calculate the correct filter tone from. The whites can for example be a white wall or a sheet of paper or a dedicated white card. Gray cards are manufactured for the purpose of adding a neutral gray to the photograph.
White balance software usually has both a manual and an automatic mode. 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 converting RAW pictures, one normally has a temperature slider. Some RAW converters 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. Automatic white balance correction usually needs neutrals in the image, like a gray-card or white-card or both. Some applications can dispense with that, but usually neutrals are needed.
Contrast comes in three varieties: contrast of hue, brightness and saturation. 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 changing the difference between the individual R, G and B values and the middle value (128); like this: R= (R-128) * contrast + 128; and similar for green and blue. If the image is very dark or very pale this method will not do. 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 similarly for G and B. The algorithms are essentially the same since a full brightness range image will have 128 as an average value.
Another problem with contrast adjustment is that not only may the average value not be 128, but the darkest and brightest areas may not be black and white. 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: 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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