PNG8 Palette-Based Alpha Transparency
Web designers use alpha channel to create various effects (like translucent surfaces, shadow and glow effects) and for edge anti-aliasing. Starting from the version 2.4.3, JPEG Imager has an ability to emulate alpha channel semi-transparency for palette-based PNG files.
PNG format specification allows defining more than one palette entry as fully or partially transparent. This feature of the format makes it possible to convert a 32-bit true color image to 8-bit indexed palette while preserving transparency information. Converting images to indexed color format may help to optimize page loading time since converted files are usually much smaller.
Modern web browsers, such as Internet Explorer 7-9, Mozilla Firefox, Opera 6+ and Safari have full alpha support (including palette alpha). But earlier versions of Internet Explorer (5.5-6.x/Win32), which are still in wide use today, will handle true alpha channel incorrectly, rendering non-palette images fully opaque with a solid gray background by default.
The indexed alpha images, however, are displayed differently in those older browsers, as if the image has a single transparent color (binary transparency, similar to GIF). Such limited transparency often produces a better effect (compared to absolute opaqueness) with acceptable degradation of visual quality.
| Examples, 200% zoom | |
|---|---|
PNG32 rendered in Firefox
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PNG32 rendered in IE6
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PNG8 rendered in Firefox |
PNG8 rendered in IE6 |
This unique property of PNG8 makes it more "compatible" with crippled old browsers, yet enables full power of alpha transparency in those browsers that support it.
Most of the image-editing tools are unable to create palette-based images with variable transparency. You can use JPEG Imager to convert your graphics to PNG8 format. JPEG Imager implements high quality color quantization algorithm with real-time preview. It efficiently reduces the number of colors in the image taking into account alpha values. See a sample picture at the bottom of this page. You also have an option to create images with old-fashioned binary transparency with dithered alpha.
Transparency Demo
click on the pictures below to change their background (java-script required)
| Original image | Converted image |
|---|---|
| RGBA PNG 32bit, 200x300 pix, 79 KB | Palette PNG 8bit, 256 colors, 22 KB |
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