Dithering & Decreasing Color Depth

Dithering is a process whereby missing colors are represented by intermingling pixels of two or more colors in the images palette . If the missing color is too unlike the colors in the palette, the result of dithering will leave a grainy or mottled appearance, as shown in image 2 below. Error diffusion dithering produces image quality that is superior to that achieved by non-error adjusted dithering.

This is not to say you should use dithering and 8 bit gifs for your webpages, but if the need arises, it is an option. In this case, the 8 bit image with error diffusion and reduced color bleeding is 10kb larger than the original jpg. On the other hand, if the image had a transparent background this method will give better results than simply reducing the colors with a standard palette to the nearest color.

Reduce Color Bleeding Option

Error diffusion dithering causes colors to bleed from left to right, most noticeable in images with hard vertical edges. This is because the edges are softened as the colors 'bleed' into each other. To counter-act this effect, check the Reduce Color Bleed option in the Decrease Color Depth window, Options box.

Where Do I Find All This Stuff in PSP?

Click on Colors on the menu bar
Click on Decrease Color Depth
Click on one of the reduced color options to open the Decrease Color Depth Option box.

Consider the four images below. We've decided we want to put a transparent background on it. To simply save it as an 8 bit gif gives us a file size of 25kb... unacceptable. But, to save it as a 4 bit gif, while reducing the file size to 16 kb, gives a poor quality. There's one option left, we can choose to decrease the color depth using the Xcolors option.

image 4

Original jpg.

image 5

4 bit gif, inferior quality
File size: 16kb

image 6

Color depth decreased using Xcolors, with the color option set to 50
File size: 16kb

image 7

A regular 8 bit, max. 256 color gif. File size: 25kb

Notice while not as clear as the jpg, image 6 is far superior to image 5,
and more desirable than image 7 due to file size.

To decrease the color depth by XColors:

Click on Colors
Click on Decrease Color Depth
Click on XColors to open the Decrease by XColors window.
Take a wild guess and put a number in the palette:number of colors box
Click ok and see how the image looks
If you don't care for the results, or if it's too dithered repeat the process, increasing the number of colors you type into the box and 'testing' until you're satisfied with the results.