Basics of Color

Color Depth

Color Depth or bit depth is the number of colors a pixel (and its image) can display. A 1-bit image can display 2 colors, black and white (or colors defined). A 4-bit image displays 16 colors (2 to the 4th power), 8-bit image displays 256 colors, 16-bit = 65,536 colors, and 24-bit = 16.7 million colors. Grayscaled images are 8-bit images that display black, white, and 254 other shades of gray (total 256 shades). Before you can apply many effects in PSP, the color depth must be 24-bit. The image is allowed no more than one raster layer if it is less than 24-bit, and is allowed no adjustment layers at all.

1-bit 8-bit
24-bit


Decreasing Color Depth: When you decrease an image's color depth, PSP must compensate for the missing colors. It uses one of three color reduction methods and then creates a reduced color palette (*.pal files).

Color Reduction Method
  • Nearest Color Match replaces the missing color with a color in the new palette that is the closest RGB match. This results in less dithering and a sharper and more contrasting image.
  • Error Diffusion chooses the closest match then applies an error or statistical difference to the surrounding pixels before it selects the nearest color to the missing color.
  • Ordered Dither Method adjusts the surrounding pixels to give the illusion of a third color, resulting in distinct patterns of light and dark. PSP uses this method when reducing to 8-bit or lower when using Window's colors or Web-safe colors.
Original Nearest Color Error Diffusion Ordered Dither
Note: color depth palettes created with Optimized Median Cut, except Ordered Dither.



Color Reduction Palette Generation

  • Optimized Median Cut uses the occurance of colors as weighting and ranks. Weighting sets colors closer to black or white before the reduction and results in less dithering and sharper edges. This method is accurate to 5 bits per channel and may not represent each color exactly.
  • Optimized Octree palettes are created quicker and are accurate to 8 bits per channel, but are not as good at weighting color importance.
  • Standard/Web Safe Palette uses a balanced number of colors and produces images that can be viewed on different monitors with very little color distortion.

Original

Optimizied Octree
Nearest Color Error Reduction 		Standard/Web Safe Palette
Nearest Color Error Reduction