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 hue or tone of the light in the picture and normally has white as a goal. White balance applications attempt to retouch the color of the light to white and in order to do that, the program needs some whites or grays in the image to find the suitable correction tint from. For the whites one can use a piece of paper or a white wall or a dedicated white card. Gray cards are manufactured for the purpose of adding a neutral gray to the picture.
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 converting RAW images, 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. There are a few apps that can dispense with the neutrals, 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. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. The best software has two contrast sliders: one for luminance contrast and one for color contrast.
The standard way to manipulate contrast is simply by changing the difference between the individual red, green and blue values and the middle value (128); like this: R= (R-128) * contrast + 128; and likewise for green and blue. This method is not suitable for very dark or very pale images. 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. 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. 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 addresses the hue or tone of the light in the picture and normally has white as a goal. White balance applications attempt to retouch the color of the light to white and in order to do that, the program needs some whites or grays in the image to find the suitable correction tint from. For the whites one can use a piece of paper or a white wall or a dedicated white card. Gray cards are manufactured for the purpose of adding a neutral gray to the picture.
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 converting RAW images, 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. There are a few apps that can dispense with the neutrals, 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. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. The best software has two contrast sliders: one for luminance contrast and one for color contrast.
The standard way to manipulate contrast is simply by changing the difference between the individual red, green and blue values and the middle value (128); like this: R= (R-128) * contrast + 128; and likewise for green and blue. This method is not suitable for very dark or very pale images. 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. 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. 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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