How to draw from the inside of the light geometry in deferred shading

Question

I'm trying to implement a deferred shader with OpenGL and GLSL and I'm having trouble with the light geometry. These are the steps I'm taking:

Bind multitarget framebuffer
Render color, position, normal and depth
Unbind framebuffer
Enable blend
Disable depth testing
Render every light
Enable depth testing
Disable blend
Render to screen

But since I'm only rendering the front face, when I'm inside a light it disappears completely, rendering the back face does not work, since I would get double the light power (And when inside, half [or the normal amount]).

How can I render the same light value from inside and outside the light geometry?

@NicolBolas I... have no idea... it wasn't working before without that, I guess I wasn't using blending before. I'll remove from the question. — Luke B., Jan 19 '13 at 10:18
you probably should include stencil test to exclude areas from the light volume where no geometry is rendered. — fscan, Jan 19 '13 at 21:15
set stencil to 1 when rendering geometry, blit depth/stencil to light buffer, render lightvolumes only where stencil is 1. — fscan, Jan 20 '13 at 02:20
Nowhere-01's is right, you can do depth testing with GEQUAL, to render only back faces behind geometry. — fscan, Jan 20 '13 at 02:35

score 3 · Answer 1 · answered Jan 19 '13 at 22:48

well in my case, i do it like that:

Bind gbuffer framebuffer
Render color, position, normal
Unbind framebuffer

Enable blend
Enable depth testing

glDepthMask(0);
glCullFace(GL_FRONT);   //to render only backfaces
glDepthFunc(GL_GEQUAL); //to test if light fragment is "behind geometry", or it shouldn't affect it
Bind light framebuffer
Blit depth from gbuffer to light framebuffer //so you can depth-test light volumes against geometry
Render every light

fscan · Accepted Answer · 2013-02-01T18:09:50.777

2

If i remember correctly, in my deferred renderer i just render only the backfaces of the light volume. ~~The drawback is you cannot depth test, you will only know if a light is behind a geometry after the light calculation is done and discard the pixel.~~ As another answer explained, you can do depth testing. Test for greater or equal to see if the backface is behind or on a geometry, therefore intersects with the surface of the geometry.

Alternatively you could check if you are inside the light volume when rendering and switch front faces accordingly.

edited Feb 01 '13 at 18:09

answered Jan 19 '13 at 21:06

fscan

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Rendering only the backface worked, but after a number of lights if I'm inside the light geometry my fps drop to half, is this related? – Luke B. Jan 20 '13 at 01:20
try the stencil trick, if your inside the light geometry you are rendering the whole screen otherwise. – fscan Jan 20 '13 at 02:25

How to draw from the inside of the light geometry in deferred shading

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