Re: [AGG] Blending a partially transparent color with a fully transparent color

[Peter <jun...@dp...>]

On Wednesday 22 September 2010 6:48:12 pm Jim Barry wrote:
> On 2010-09-22 23:23, Peter wrote:
> > How as AGG designed to handle the situation where you are blending a
> > partially transparent color with a fully transparent color? I would
> > assume that if it were working with a pixel format that included an
> > alpha channel, it would just overwrite the fully transparent color
> > with the partially transparent, since a fully transparent color would
> > have absolutely zero impact on the final pixel values when blended
> > with anything.
> 
> Yes. However...
> 
> > But when using AGG, it appears to blend partially transparent colors
> > with fully transparent colors, leading to some very strange results
> > where the pixels that you are drawing are actually blended with a
> > non-existant color!
> 
> This is because you are using a pixel format (pixfmt_bgra32) that blends 
> to a premultiplied destination format. In premultiplied space, the RGB 
> values are scaled by the alpha value, in order to make blending 
> operations more efficient.
> 
> If any RGB value is larger than the alpha value then the blending 
> algorithm produces "weird" results such as you are seeing.
> 
> Also, the sample application is not going to do anything with the alpha 
> channel when drawing to the screen - it just assumes that all pixels are 
> fully opaque.
> 
> > I'm not sure if I'm describing this correctly, so I've attached a
> > patch of lion.cpp example that demonstrates this behaviour that I'm
> > talking about. It sets the background color to green but fully
> > transparent. Normally, you'd expect the RGB channels to have
> > absolutly zero effect when the alpha channel is transparent, but in
> > this example, it somehow blends the lion with the green in the
> > background even though the background is fully transparent, making
> > the lion have a green tint!
> 
> In premultiplied space, there is no such thing as "fully transparent 
> green". Since the RGB values must not exceed the alpha value, the only 
> valid fully transparent colour is all-zeros.
> 
> If you really need to work in non-premultiplied space, you can use the 
> pixfmt_bgra32_plain format.
> 
> - Jim

Thank you for your patient and  thorough explanation. I obviously have a lot to learn about AGG and even graphics in general. :)

The pixel blender used in the pixfmt_bgra32_plain format does exactly what I was wanting it to do. I guess I must have missed that bit of information when I was trying to figure out the library.

Since I made this mistake because I didn't understand the library, is there any documentation or source code that would be good for me to get familiar with so I could avoid making another mistake like this in the future? 

Peter