Image Optimization, Part 5: AlphaImageLoader

Note: the following is a translation of the next article “Image Optimization, Part 5: AlphaImageLoader” from the YUI blog. Stoyan Stefanov this time talks about the intricacies of applying the AlphaImageLoader filter for IE. My comments further in italics.

This is the fifth part of a series of articles about image optimization. The previous parts can be found at:

• Image Optimization Part 1: The Importance of Images
• Image Optimization Part 2: Selecting the Right File Format
• 4 steps to reduce file size
• Sequential JPEGs - To be or not to be?

This article in the image optimization series talks about the technique available only in IE, the AlphaImageLoader CSS filter, which developers use to solve transparency problems for full-color PNG images in IE. The main problem with AlphaImageLoader is that it affects page performance, and thereby degrades user perception. I argue that you should avoid using the AlphaImageLoader in all possible cases.

Small excursion

As mentioned in one of the previous articles , PNGs can be of several types, which can be divided into 2 main ones:

• Indexed (palette), also called PNG8, you can use up to 256 colors.
• Full-color PNGs, also called PNG32 or PNG24.

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