Re: Issue with transparency and lighting

[James W. W. <os...@jw...>]

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.


> 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.


> The specular component is just added on, not being affected by the
> alpha.



I was omitting specular for simplicity.