Retouching White Balance and Contrast
by Michael Henriksen
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 concerns the hue or tone of the light in the image and sets white as a goal. White balance applications attempt to retouch the hue of the light to white and to do that, the software normally needs some neutrals in the image to find the suitable correction tint 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. Manual correction comes as a temperature slider, which is fine for incandescent light, but not for fluorescent light or mixed light. When converting RAW pictures, one normally has a temperature slider. Apart from the 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. Automatic white balance correction usually needs neutrals in the image, like a gray card or white card or both. There are a few apps that can dispense with the neutrals, 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. A single slider usually results in an over saturated image and gaudy colors. The best software has two contrast sliders: one for luminance contrast and one 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 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.
What if the darkest and brightest areas are not black and white? In that case one should be able to expand the brightest 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: First convert the image to Lab mode, select the L channel only and run autolevels on that. Then convert back to RGB mode.
This article is based on <a href="http://www.articlerich.com/Article/White-balance-issues/2621472">White balance</a> and <a href="http://www.articlerich.com/Article/Contrast-in-Photo-Retouching/2626369">Contrast</a>
---------------------------------------------------
You are receiving this because you signed up for it on 2011-08-23 from IP
To fine-tune your selection of which articles to receive, just login here:
http://www.uniquearticlewizard.com/bloggers/
using your username:
To unsubscribe please use the following link:
http://www.uniquearticlewizard.com/unsubscribe.php?mail=kidloveme.huyen@blogger.com&code=b83952f8ca040e326c958cbc4b8fd96e
---------------------------------------------------
---------------------------------
New Unique Article!
Title: Retouching White Balance and Contrast
Author: Michael Henriksen
Email: jan@janesmann.com
Keywords: white balance, contrast,photo retouching, image editing,digital photography
Word Count: 481
Category: Hobbies
---------------------------------
No comments:
Post a Comment