User Guide

324 Chapter 15: Publishing
6. If you selected 8-bit for Bit Depth in step 4, select a Dither option to specify how pixels of
available colors are mixed to simulate colors not available in the current palette. Dithering can
improve color quality, but it increases file size. Select from the following options:
None turns off dithering and replaces colors not in the basic color table with the solid color
from the table that most closely approximates the specified color. Turning dithering off can
produce smaller files but unsatisfactory colors.
Ordered provides good-quality dithering with the smallest increase in file size.
Diffusion provides the best-quality dithering but increases file size and processing time. It also
works only with the web 216 color palette selected.
7. Select one of the following Palette Types to define the color palette for the PNG image:
Web 216 uses the standard 216-color, browser-safe palette to create the PNG image, for good
image quality and the fastest processing on the server.
Adaptive analyzes the colors in the image and creates a unique color table for the selected
PNG file. This option is best for systems showing thousands or millions of colors; it creates the
most accurate color for the image but results in a file size larger than a PNG created with the
web 216 palette.
Web Snap Adaptive is the same as the Adaptive palette option except that it converts very
similar colors to the web 216 color palette. The resulting color palette is optimized for the
image, but when possible, Flash uses colors from web 216. This produces better colors for the
image when the web 216 palette is active on a 256-color system.
To reduce the size of a PNG file with an adaptive palette, use the Max Colors option to
decrease the number of palette colors, as described in the next step.
Custom specifies a palette that you have optimized for the selected image. The custom palette
is processed at the same speed as the web 216 palette. To use this option, you should know how
to create and use custom palettes. To select a custom palette, click the Ellipsis (...) button to the
right of the Palette box at the bottom of the dialog box and select a palette file. Flash supports
palettes saved in the ACT format, exported by Macromedia Fireworks and other leading
graphics applications; for more information, see “Importing and exporting color palettes”
on page 83.
8. If you selected the Adaptive or Web Snap Adaptive palette in step 7, enter a value for Max
Colors to set the number of colors used in the PNG image. Selecting a smaller number of colors
can produce a smaller file but might degrade the colors in the image.
9. Select one of the following Filter Options to select a line-by-line filtering method to make the
PNG file more compressible, and experiment with the different options for a particular image:
None turns off filtering.
Sub transmits the difference between each byte and the value of the corresponding byte of the
prior pixel.
Up transmits the difference between each byte and the value of the corresponding byte of the
pixel immediately above.
Average uses the average of the two neighboring pixels (left and above) to predict the
value of a pixel.