Adobe 29180155 - Photoshop Elements 4.0 Tutorial page 395

GIF image with selective color (left), and GIF image with web color (right)
You can choose the number of colors in a GIF image and control how colors dither in a browser. GIF supports background transparency or
background matting, by which you blend the edges of the image with a web page background color.
About the PNG-8 format
The PNG-8 format uses 8-bit color. Like the GIF format, PNG-8 efficiently compresses areas of solid color while preserving sharp detail like those
in line art, logos, or type.
Because PNG-8 is not supported by all browsers, you may want to avoid this format when you are distributing the image to a wide audience.
The PNG-8 format uses more advanced compression schemes than GIF does, and a PNG-8 file can be 10% to 30% smaller than a GIF file of the
same image, depending on the image's color patterns. Although PNG-8 compression is lossless, optimizing an original 24-bit image as an 8-bit
PNG file can subtract colors from the image.
Note: With certain images, especially those with simple patterns and few colors, GIF compression can create a smaller file than PNG-8
compression. View optimized images in GIF and PNG-8 format to compare file size.
As with the GIF format, you can choose the number of colors in an image and control how colors dither in a browser. The PNG-8 format supports
background transparency and background matting, by which you blend the edges of the image with a web page background color.
PNG-8 with 256 colors and no dither (left), and PNG-8 with 16 colors and dithering (right)
About the PNG-24 format
The PNG-24 format supports 24-bit color. Like the JPEG format, PNG-24 preserves the subtle variations in brightness and hue found in
photographs. Like the GIF and PNG-8 formats, PNG-24 preserves sharp details like those in line art, logos, or type.
The PNG-24 format uses the same lossless compression method as the PNG-8 format. For that reason, PNG-24 files are usually larger than
JPEG files of the same image. You may want to avoid PNG-24 format when you are distributing your image to a wide audience.
In addition to supporting background transparency and background matting, the PNG-24 format supports multilevel transparency. You can have up
to 256 degrees of transparency from opaque to completely transparent, so you can blend the edges of an image smoothly with any background
color. However, not all browsers support multilevel transparency.
About animated GIFs
Animated GIFs create the illusion of movement by displaying a sequence of images, or frames, over time. Photoshop Elements provides a
powerful, easy way to create animated GIFs from a multiple-layer image.
To create an animated GIF file, you work with layers. Each layer becomes a frame when Photoshop Elements generates an optimized animation.
The animated GIFs are compiled into a RGB color document.
You can open an existing animated GIF file using the Open command. For each frame in the file, Photoshop Elements creates a layer. To view a
specific frame in the document window, make the layer for that frame visible in the Layers panel, and hide other layers.