Lesson 4 – Deferred shading
PV227 – GPU Rendering
Jiˇrí Chmelík, Jan ˇCejka
Fakulta informatiky Masarykovy univerzity
08. 10. 2019
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Motivation: A loooot of lights
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Forward and deferred shading
Forward shading
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Forward and deferred shading
Deferred shading
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Task: Create and display G-buffer
Task 1
Complete notexture_deferred_fragment.glsl and
texture_deferred_fragment.glsl, output position, normal, and
albedo.
Look at the result. Note that positions are scaled to 0.02 (our scene
is large).
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Task: Create and display G-buffer
Positions Normals Albedo
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Task: Evaluate the lighting
Task 2
Evaluate the lighting in evaluate_quad_fragment.glsl.
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Task: Evaluate the lighting
Result
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Reducing the number of evaluated lights
View-frustum culling
Light volume
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Light volume implementation
Render one sphere per light, use instancing :-)
In vertex shader, place the sphere at the position of the light, and
scale it to the range of the light
In fragment shader, evaluate the light
Add all lights together using blending
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Light volume implementation
Use depth test
Don’t evaluate pixels if the light is behind something.
We need the depth texture of the scene, in addition to the color
buffer.
Only compare the depth with the scene, don’t render the spheres
into the depth buffer.
Render front faces only
Use blending to sum the lighting, configure the blend function
properly
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Task: Implement light volume algorithm
Task 3
Configurate the depth test, culling, and blending in C++ code
In evaluate_sphere_vertex.glsl, place the sphere to the position of
the light. Also, sphere’s radius is one, so enlarge it to the range of
the light (light_range constant).
In evaluate_sphere_fragment.glsl, do not evaluate the light yet,
output only the diffuse color of the light.
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Task: Implement light volume algorithm
Result
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Task: Implement light volume algorithm
Task 4
Evaluate the lighting in evaluate_sphere_fragment.glsl.
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Task: Implement light volume algorithm
Result
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Task: Implement light volume algorithm
Task 5
In evaluate_sphere_fragment.glsl, discard fragments that are too
far from the light.
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Task: Implement light volume algorithm
Result (when displaying only the diffuse color of the light)
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Pros and cons
Pros:
Lighting (one of the most expensive operations) is evaluated only
once per pixel.
Lighting is processed independently, reducing the number of
combinations of materials and lighting.
G-buffer is good for other postprocessing effects.
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Pros and cons
Cons:
Does not work with multisampling.
Transparency
G-buffer needs a lot of memory (ours: 3x16B/pixel,
100 MB FullHD)
Materials must not be too much complicated (the space of
G-buffer is limited)
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Things we used
OpenGL queries, very briefly:
glGenQueries generates a query object
glBeginQuery(GL_TIME_ELAPSED, obj) starts measuring of the
time, only one measuing may run at the same time.
glEndQuery(GL_TIME_ELAPSED) stops measuring.
GPU time is measured, not the CPU time.
glGetQueryObjectiv(obj, GL_QUERY_RESULT_AVAILABLE,
result) returns 1 if the result of the query is available.
glGetQueryObjectui64v(obj, GL_QUERY_RESULT, result) returns
the result of the query.
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Things we used
Interpolation of variables between vertex shader and fragment shader
Usage: flat int light_idx, in vertex and fragment shader.
smooth: default interpolation, interpolates the values between
vertices with perspective correction.
noperspective: linear interpolation, i.e. without perspective
correction.
flat: no interpolation, the value of the “provoking” vertex is used.
Necessary for integers.
Interpolation without and with perspective correction. Source
Wikipedia.
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Things we used
Shader storage buffer objects (SSBO):
Since OpenGL 4.3
Similar to uniform buffers, similar usage:
Use GL_SHADER_STORAGE_BUFFER instead of
GL_UNIFORM_BUFFER
Use buffer instead of uniform in shaders
Much larger than uniform buffers (at least 128 MB, but usually
limited by the memory of the GPU)
Shaders can write to them (preferably using atomic operations)
Try changing LIGHTS_MAX_COUNT and LIGHTS_STORAGE in the
project (6 places) to use shader storage buffers and 1000 lights. Don’t
use forward shading for this :-)
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Next lecture
Using G-buffer for:
Screen-space ambient occlusion
Depth of field
Screen-space ambient occlusion
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