[webkit-dev] WebKit runtime image compression

[David Hyatt]

On Dec 14, 2008, at 1:12 AM, Paul Pedriana wrote:

> For a lot of web pages, images are the primary source of memory  
> usage. These images seem to be typically stored as RGB or ARGB data  
> in the BitmapImage class.

This isn't always true though.  For example, the ports that use CG  
have CGImageRefs and not raw ARGB data.

> It seems to me that applications that wish or need to save runtime  
> memory could benefit from runtime image compression whereby images  
> are stored in some compressed form instead of RGB or ARGB formats. I  
> am currently looking into this possibility and I'm wondering if  
> anybody has previously looked into it.
>
> My current idea is to support two kinds of compressed images: RLE  
> and YCoCg-DXT5. The former could be for any images that are simple  
> enough, and the latter could be for general images and give a fixed  
> 4:1 compression ratio. RGB transforms to YCoCg with better  
> decorrelation (breakdown of RGB to YCC) than YCbCr and many images  
> can get PSNR values over 40 (i.e. hard to tell from original). If  
> the computer has decent vector processing or a programmable GPU, the  
> performance could be decent, otherwise it depends on the CPU. (A)RGB  
> could of course remain as a fallback.
>
> I'm wondering if anybody has looked into runtime image compression  
> with WebKit and considered options such as this.
>

This could in theory be done as a separate library that is then just  
used by the specific ports of WebKit that might need it.  I'm not  
convinced it would need to be part of WebKit itself.

dave