Creating hard-edged transparency in GIF and PNG-8 images

When you apply background matting to images that include anti-aliasing, a halo effect can result when the matte color is different from the original background color of the image. Anti-aliasing creates partially transparent pixels around the edges of the image to blend the image with the original background color.

When working with GIF or PNG-8 files, you can create hard-edged transparency, in which all pixels that are more than 50% transparent in the original image are fully transparent in the optimized image, and all pixels that are more than 50% opaque in the original image are fully opaque in the optimized image. Hard-edged transparency prevents the halo effect. However, hard-edged transparency can cause jagged edges in the image. To prevent jagged edges, apply background matting to the image. See Creating background matting in GIF and PNG images.

To create hard-edged transparency in a GIF or PNG-8 image:

1 Import a Smart Photoshop image that contains transparency or select a Smart Photoshop object already on a Web page and click Settings in the Smart Image Inspector.

2 In the Optimize panel in the Save For Web dialog box, select GIF or PNG-8 from the format menu.

3 Select Transparency.

4 Select None from the Matte pop-up menu to make all pixels with greater than 50% transparency into fully transparent pixels, and all pixels with 50% or less transparency into fully opaque pixels.

Images and Page Design > Making transparent and matted Photoshop images > Creating hard-edged transparency in GIF and PNG-8 images