Re: Issue with transparency and lighting

[Roger H. <rog...@mi...>]

On 22 Aug 2009, at 17:25, James W. Walker wrote:

>
> On Aug 22, 2009, at 1:48 AM, Roger Holmes wrote:
>
>>
>> On 22 Aug 2009, at 01:25, James Walker wrote:
>>
>>> I thought I was correctly handling transparency with multiple
>>> lighting
>>> passes in QORenderer::Renderer::RenderTransparent, but it turns out
>>> that
>>> in some cases it can be visibly incorrect.  I'll try to explain a
>>> simplified scenario.
>>>
>>> Suppose we have an nearly-opaque object with color A, and in front  
>>> of
>>> that there is a transparent object with color B.  There are 2
>>> lighting
>>> passes, contributing brightnesses L and M respectively.  Then the
>>> resulting color value ought to be B*(L+M) + (1-alpha)*A*(L+M).
>>
>> For the diffuse component of the lighting, shouldn't that be
>> alpha*B*(L
>> +M) + (1-alpha)*A*(L+M) ?
>
>
> I was assuming premultiplied alpha.

I see. I  always try to keep the alpha channel separate. The saving in  
time is rather small in 3D.

>
>
>> As my renderers are pure software ones, I'm not familiar with
>> glBlendFunc so cannot comment on that but maybe it supports my  
>> version
>> of the formula.
>
>
> Maybe your renderer doesn't have a limit of 8 lights, so you don't
> need multiple passes.

No limit, but to store all the shadow depth buffers I do have to use  
multiple passes when there is insufficient memory to store them. I  
also sometimes have to band the image when high resolution/large image/ 
anti-aliasing. Separate scan lines can be done in separate processors  
if available. Must do a 64 bit version sometime.

>
>
>> The specular component is just added on, not being affected by the
>> alpha.
>
>
>
> I was omitting specular for simplicity.

OK.

>
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