When editing pictures you should first address white balance and contrast. White balance is normally the thing one would 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 color of the illumination in the photograph and normally has white as an ideal. White balance apps attempt to adjust the hue of the light to neutral and to do that, the application normally needs some whites or grays in the photo to calculate the correct filter hue from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. 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.
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. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. Luminance contrast and color contrast should be treated separately.
The usual way to adjust contrast is simply by changing the difference between the individual R, G and B values and the average value (128); like this: R= (R-128) * contrast + 128; and similar for the green and blue channel. 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. 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 is concerned with the color of the illumination in the photograph and normally has white as an ideal. White balance apps attempt to adjust the hue of the light to neutral and to do that, the application normally needs some whites or grays in the photo to calculate the correct filter hue from. There are dedicated white cards, but one can also do with a sheet of white paper or a white wall. 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.
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. However, a single slider for all three is unsatisfactory since the result is usually over saturated and gaudy. Luminance contrast and color contrast should be treated separately.
The usual way to adjust contrast is simply by changing the difference between the individual R, G and B values and the average value (128); like this: R= (R-128) * contrast + 128; and similar for the green and blue channel. 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. 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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