Posterize

This transformation lets you render an image using a limited palette of colors -- a little like reproducing an image as a silk-screened poster. Once you specify a palette of colors you want to use, Picture Window scans the image and converts each pixel to the closest color.

You can add new colors to the palette in either of two ways:

1) Increase the number of colors and click on the palette color to bring up a color picker. Use the color picker to select a color.

2) Click on the input image to add the color at the point where you clicked to the end of the palette.

To change an existing color, click on the palette color you want to change and use the color picker to adjust the color.

Amount

The amount control lets you control how much of the posterize transformation is applied to the input image. You can apply a percentage of the transformation to the entire image, or you can specify an amount mask to restrict the effects of the transformation to only part of the input image.

Number of Colors

This control lets you specify the number of colors that will be used to represent the input image. You can specify any number from 2 to 64.

Remap Colors

Turning on the Remap Colors option adds an extra level of options to the Posterize transformation. If the box is checked, then Picture Window displays an alternate color square next to each of the palette selected colors, and an extra step is introduced into the computation of the result image. As before, each pixel in the input image is scanned and the closest matching palette color is determined. Then, instead of making the output pixel the palette color, the alternate color is substituted instead.

Antialias

If you check the Antialias box, Picture Window smooths out the sometimes jagged-edged, pixelized result of posterization by very precisely recomputing the result image at the boundaries between areas of different colors. This involves making a second pass over the data that generally takes somewhat longer than the first pass, but the resulting image has much smoother edges. To speed up previewing, you may want to leave antialiasing disabled until the very end.

Auto Palette

If you click the Auto Palette button, Picture Window scans the input image and automatically selects a palette of colors that can represent the image reasonably well. It then sets the color controls (see below) for the color palette and the remap colors to the colors it selected. After selecting an auto palette, you can go on to modify any of the palette or remap colors to vary the results. After an auto palette, all remapped colors are reset to the palette colors.

Refine Palette

When you click Refine Palette, Picture Window performs one pass on the input image and makes an incremental adjustment to the existing palette colors using a mathematical technique called k-means clustering. After refining the palette, all remapped colors are reset to the palette colors.

Colors

At the bottom of the Posterize dialog box Picture Window displays a small color square for each palette color. These color squares are arranged in as many rows as necessary. If Remap Colors (see above) is checked, a second, alternate color square is displayed next to each palette color square. To select or change a primary or alternate color, click on the corresponding color square to bring up a color picker window and use this to choose a color.

Settings Menu

Reset All

This command resets to palette to just two colors - black and white, with no remapped colors and no antialiasing. It is useful if you have a default settings file and just want to start over.

Reset Remapped Colors

This command resets all remapped colors back to the corresponding palette colors.

Probe Size

Use these options to set the probe size. A larger probe is less sensitive to noise; a small probe is better for picking out fine detail.

Tips

Auto Palette does not work very well unless the number of colors is fairly large.

Depending on the image, it may benefit from multiple Refine Palette operations, although the improvement diminishes rapidly. Palette refinement adjusts existing colors a will not select new colors that are significantly different, so if you want a specific color to appear in the final image you need to add it to the palette manually.