From BlenderWiki

Photo Enhancement is a term that generally means performing a series of operations on an image to make it appear better-looking. As previously mentioned, during post processing, the director may wish to take a shot that was filmed during the day, and make it appear as though it was nighttime.

This section covers a few of the common uses for nodes in processing images. I wanted to call this page "Making bad photos taken by crappy digital cameras under poor lighting conditions look Good", but the editors wouldn't let me, but you get the idea.

Color Correction

Depending on lighting and materials, you may find that after hours of rendering, the image is composed perfectly, but the colors just don't look right. Rather than make some adjustments and re-render, you can use compositing nodes to quickly correct an image or a series of images. Blender is blindingly fast at performing these operations on images; much faster than re-rendering.

Consider the amazingly poor quality picture of a beautiful wife and two charming children to the right. You may click on the expand icon to see it in its full dismal glory. This is an actual unretouched photo taken by the author at night using a crappy digital camera. Some of the things you notice right away are:

• oversaturated reds and blues
• blurry
• lots of dead space on the left side
• nighttime shot probably has other issues with color balance.

Using the compositing node editor, you can add an Image input node to bring in this single image for post-processing. Immediately, the Saturation needs to be turned down, so add a Hue Saturation color node and reduce the saturation. Connecting that output to a viewer shows good results with a Saturation of around 0.90. Reducing saturation too much results in a black and white image, or an image where the colors look washed out. In that case, use a saturation higher than 1.00 to enhance colors.

Next up is focus; the image is blurry as the photographer had a hard time focusing properly because it was night and he was in a hurry to use a crappy camera. Connect a Sharpen filter node to an RGB Curve node so that we can tweak the individual color channels. Reducing Red and Blue, and actually decreasing overal contrast using the Composite channel on the RGB Curve node brings out flesh tones, and reveals the soft green color of the grass. The Compositing node map is shown below:

In the node map, the RGB Curves node was replicated to show you each of the Red, Green, and Blue curves. Use the shape of the Composite curve shown to reduce contrast. Usually, when you reduce color in all channels, you have reduced brightness, and you should reduce contrast accordingly.

Saving this .blend file means that you can re-use this rig for every other bad photo you took at night with your crappy little camera. Simply plug in that image, and Blender will instantly perform all these operations. Using Blender in this manner saves you quite a bit of time over using other non-open-source photo editing software.

Finally, use the soften filter to blend it back together, and the translate node to perform rudimetary cropping. The final result is shown; a significant improvement!

If this was a movie clip, and the clip was shot under similar lighting conditions, simply using the Image input node controls to bring in the image sequence automatically. Blender will perform these operations on every image.