PNG with No Alpha Channel?

Trevor DeVore lists at mangomultimedia.com
Wed Jun 11 10:01:19 EDT 2008


On Jun 11, 2008, at 9:50 AM, Luis wrote:

> Ok, if you look at a 32 bit png in ascii near the start of the file  
> then you'll spot the bKGD chunk, this defines the background and it  
> looks like it also contains the alpha value. On a 24 bit png (no  
> alpha) this value is not present...
> The bKGD chunk may not be honoured in all viewers.
> I don't know how this change may reflect on the final image but it  
> might be worth a shot!

The bKGD chunk can be useful for decoders that ignore transparency so  
that they will display an appropriate background color. A 24-bit PNG  
would have no need of setting this property since all pixels have a  
known color and no transparency. I imagine this is why you don't see  
it in the 24-bit PNG.

Regards,

-- 
Trevor DeVore
Blue Mango Learning Systems
www.bluemangolearning.com    -    www.screensteps.com



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