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mv348

OpenGL
Shadow mapping advice

9 posts in this topic

So I believe the next step in m engine is to add shadows. After doing some research, shadow mapping seems to be a popular and fast method.

The theory makes sense to me, but I find it slightly surprising that it uses a full scene render per light-source. I've noticed that modern games seem to use a pretty high number of light sources! Is this technique advisible for a large number of lights?

Moreover, the method works by rendering from the lamps point of view, so the light is limited to a frustum pointing in one specific direction. I presume you would have to do multiple renders for a light spreading out in all directions?

I read some hardware offers 'shadow mapping support'. What kind of support is available? I'm using OpenGL/GLSL
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[quote name='mv348' timestamp='1346482494' post='4975351']
I find it slightly surprising that it uses a full scene render per light-source. I've noticed that modern games seem to use a pretty high number of light sources[/quote]Yes, some new games do use as many as [i]a thousand[/i] dynamic lights, however, only a small number of them will actually be shadow-casting lights.

Also, in specific scenes / specific scenarios, you don't need a "full scene" render per light.
e.g. I once worked on a racing game where all of the environment shadows used static light-map techniques, so only the dynamic cars themselves needed shadow mapping. We also used "[url="http://mynameismjp.wordpress.com/samples-tutorials-tools/deferred-shadow-maps-sample/"]deferred shadow maps[/url]" where you accumulate the shadow information into a full-screen texture. We would render each car individually into it's own shadow map ([i]which makes good use of low resolution shadow textures[/i]), and then composited each single shadow into the full-screen texture. This meant we had to render ([i]at most[/i]) an extra [font=courier new,courier,monospace]numCars * numLights[/font] for shadows -- not [font=courier new,courier,monospace]wholeScene * numLights[/font].
You should also perform frustum culling ([i]and full visibility culling if your scene supports it[/i]) on the light, so that you're only rendering objects that are visible from it's viewpoint. If no cars were visible from a particular light, then no extra rendering was performed for it.
[quote]Moreover, the method works by rendering from the lamps point of view, so the light is limited to a frustum pointing in one specific direction. I presume you would have to do multiple renders for a light spreading out in all directions?[/quote]Yes, the most obvious solution is 6 different 90º frustums rendered to a "cube-shadow-map". There's also hemisphere-based solutions that use fewer frustums ([i]Dual-Paraboloid SM or Dual Sphere-Unfolding SM[/i]), or complex distortion methods like [url="http://www.gamedev.net/topic/630491-camera-space-shadow-mapping-paper/"]CSSM[/url], but these alternatives generally exhibit more distortion than the basic solution. Edited by Hodgman
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The "hardware support" for shadow maps amounts to "free" depth comparison and 2x2 PCF filtering when sampling the shadow map. Usually it's implemented by rendering your shadow map depth to a depth buffer, then sampling that depth buffer with a special instruction and/or sampler state in the shader. Unfortunately I'm not familiar with how it's done in GLSL so I couldn't tell you offhand.
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For OpenGL / GLSL, you need to set up your depth texture to do a comparison upon lookup using for instance:

[CODE]glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE, GL_COMPARE_REF_TO_TEXTURE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL);[/CODE]

Then, in your shader, you should give your shadowmap uniform the type [font=courier new,courier,monospace]sampler2DShadow[/font]. This will allow you to call the regular [font=courier new,courier,monospace]texture[/font] function, or one of its variants, with a 3 dimensional texture coordinate. The third coordinate is the depth value used for comparison.
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Thanks for all the input guys! I [url="http://www.youtube.com/watch?v=NCY2QFg4fGk&feature=plcp"]just got shadow-mapping working[/url]with a single light, for a single model. So now I am moving towards multiple lights, and multiple models.

To avoid excessive extra-rendering, I am planning on enclosing each lamp in a bounding volume (i.e. an Axis Aligned Bounding Box) and checking for intersections with each models bounding volume. I admit I haven't read up much on this just yet, there's something called 'Cascaded Shadows' that I need to look into, but I wanted to get your input on the algorithm I came up with today.

Here's the psuedo-code. Should be pretty self explanatory:

[code]

Algorithm:

PositionModels()
PositionsLights()
/////////////////
// SHADOW PASS //
/////////////////



for light in AllLights
{
// determine which models are hit by the light
for model in AllModels:
{
if (collision(light.volume, model.volume)
{
light.modelsInView.add(model);
// give model access to light data, including its shadow map
model.shadowCasters.add(light);
}
}

// if any models were hit by the light
if (light.modelsInView.Count > 0)
{
prepareShadowRender(light);

// render all models in this light's view to the light's shadowmap
for model in light.modelsInView
{
model.render();
}

}

}

//////////////////
// FINAL RENDER //
//////////////////

for model in models:
{
for light in model.shadowCasters:
{
shaderSetupShadowMap(light)
}
model.render();

}

[/code]


Obviously the relevant lists need to be cleared after each rendered. Further optimizations can obviously be made, such as caching shadows for stable objects. But I think the general structure is captured here, wouldn't you say?

Any input/feedback would be appreciated. Edited by mv348
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Your approach seems correct form my view, I have something similar, but not in OpenGL.

[CODE]
//Generating shadows
if(shadows)
{
foreach light
{
foreach object
{
if(distance(object,light)<30
{
render object to shadowmap for this light
}
}
}
}
//Rendering lights with shadows
foreach light
{
if(shadows && cameradistance<***)
{
render light with shadow map
}
else
{
render light
}
}
[/CODE] Edited by RetroIP
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Thanks! Glad I'm on the right track!

I have a question though. The algorithm I outlined should work fine provided I am using spot-lamps with a relatively small area of effect. However, using directional lights, such as sunlight, poses a bit of a problem. I would have to render the entire scene from a sufficient distance to capture everything. A single shadow-map spread out over an entire scene is going to look awful pixelated for large, outdoor environments.

How can I address this issue?
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Cascaded shadow maps is the most common solution for directional lights, and I'd recommend it as a starting point.
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Thanks so much, MJP!

So I started reading through [url="http://developer.download.nvidia.com/SDK/10.5/opengl/src/cascaded_shadow_maps/doc/cascaded_shadow_maps.pdf"]this [/url]article by Nvidia. I'm making some headway with it, but its a bit difficult to follow at times. Some of the details are posted in the references, but require a subscription to read.

Any particular tutorial or paper suggestions would be welcome.
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[quote name='mv348' timestamp='1347772331' post='4980555']
To avoid excessive extra-rendering, I am planning on enclosing each lamp in a bounding volume (i.e. an Axis Aligned Bounding Box) and checking for intersections with each models bounding volume.
[/quote]
For directional lights, I have written a tutorial on how to “capture” only objects that are inside the camera frustum or could cast shadows into the camera’s frustum, tightly excluding all other objects.

[url="http://lspiroengine.com/?p=153"]Tutorial: Tightly Culling Shadow Casters for Directional Lights (Part 1)[/url]
[url="http://lspiroengine.com/?p=187"]Tutorial: Tightly Culling Shadow Casters for Directional Lights (Part 2)[/url]


L. Spiro
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