imread (repost)

[Daniel J Sebald]

soren at hauberg.org wrote:
> Quoting Daniel J Sebald <daniel.sebald at ieee.org>:
> 
>> 100% transparent would be not visible, I guess.  The applications  
>> would be special effects or masking off a rectangular image to make  
>> it appear of a different shape.
> 
> 
> But what does 'not visible' mean? Should we just show the background  
> color of the figure?

Yes, whatever is behind the image (up to the point of plotting the image) gets blended with the image.  Wherever the image is 100% transparent the contents behind it is completely visible, i.e., unaltered.

> If so, I can easily imagine myself being confused  
> whenever I see an all gray image -- am I looking at an all gray image  
> or a transparent one?

Depends on the contents of the background and on the contents of the alpha channel.  A gray background (as opposed to white or black) and potentially single shaded images isn't a good combination in any circumstance.

Dan