Hi,
I have a SpotLight with angle 40 degrees, SpotExponent 10 and ConstantAttenuation 0.01.
My scene consists of a simple Quad.
I see that not onlt the Front Face of the quad is illuminated, but also the backface, even if I set the
glLightModeli(GL_LIGHT_MODEL_TWO_SIDE, GL_FALSE);
Shouldn't it be dark?
You shouldn't have any two sided lighting in the first place as it is turned off by default.
Ok,
but I set it to false just to be sure...
It seems that if I don't set the attenuation factor it works fine...
If memory serves, LIGHT_MODEL_TWO_SIDE just flips the normal for the back side (for polygons where both sides represent the "outside" of the object).Originally Posted by Devdept2
Did you set the material ambient to 0,0,0,0, the material emission to 0,0,0,0. If not, your light-space back faces (on lit geometry) may not be black.
The attenuation is not a problem here as it is only relevant if lighting calculattions are actually done.It seems that if I don't set the attenuation factor it works fine...
Attenuation is a factor by which radiance (or luminance depending on your view) is decreasing as the source of light increases distance to the object it illuminates. This is because the solid angle the light source subtends as seen from the object being illuminated becomes smaller.
In physics, the attenuation factor is simply
1 / d^2
where d is the distance between sender and receiver.
In fixed-function OpenGL you have a sum of three factors defined as the sum of
1 / (A_constant + A_linear + A_quadratic)
which is physically completely inaccurate but serves the purpose of fine-controlling the appearance of lit geometry in the rendered image.
To make it physically correct you'd set A_constant and A_linear to 0. However, if you play around with the values a bit, you'll notice that physical correctness and pleasant visual appearance aren't necessarily the same.
This is the illumination from the top
and this is the illumination from the bottom:
The material ambient is set to 0.01 for both front and back face.
I never set the emission so it should be 0.
So is it wrong to assign the ConstantAttenuation a value between 0 and 1?
That would make the attenuation factor > 1...
First of all, let me correct myself. The factor is actually
1 / (A_constant + A_linear * d + A_quadratic * d^2)
I was reminded when checking the documentation. The factors are actually weights to control the impact of the distance of vertex and light source. A_i can be any non-negative value. So yes, with the right factors you might actually produce higher radiance - or a brighter appearing object. As long as the sum in the denominator isn't getting < 1 you'll decrease radiance. Otherwise it will go up - which isn't the purpose of attenuating. In my opinion the denominator should be clamped to [1, MAX_VALUE].
Also, know that this is not applied to directional light, only positional, i.e. point and spot lights. This makes sense since directional light are conceptually light sources at infinity.
I see that even with ConstantAttenuation = 1, in the back face there is a little illumination of the spot (but very very dim), so if constantAttenuation < 1, it would go to increase that dim illumination giving this illumination even to the back face.
This is more visible with my shader (that computes phong illumination):
top:
bottom:
But also without the shader and fixed function OpenGL:
top no shader:
bottom no shader:
So we're back to the original question: why is there this little illumination on the back face? (that can become very intense in the ConstantAttenuation factor is << 1)
Wait a second. Are you doing lighting using shaders or fixed function? If "no shader" is fixed then the GL behaves correctly. If you mess it up using a shader, please show us the shader code. I'm confused.
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