PV227 GPU Rendering
Marek Vinkler
Department of Computer Graphics and Design
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Workflow
same operation exactly once for every
vertex/patch/primitive/fragment,
independent states, no communication,
program is for the entire pipeline,
data can be passed between shaders.
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Workflow – Shaders
Which shader to use for a given task?
depends on the modified data,
per vertex → vertex shader,
per patch → tessellation shader,
per primitive → geometry shader,
per fragment → fragment shader,
no idea → compute shader.
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Workflow – Shaders (cont.)
Which shader to use for a given task?
may depend on special properties of the processors:
cancel computation → fragment or geometry shader,
some build-in functions are defined only for certain
processors.
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Workflow – Properties
Shaders replace entire fixed pipeline.
If we want to modify the vertex transformation behaviour,
we also have to write code for lighting, texture generation,
. . .
This may be tedious when small changes are desired.
In bigger projects you usually rewrite it anyway ;-).
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Vertex Processor
Replaces the following fixed functionality:
Vertex transformation by modelview and projection matix.
Texture coordinates transformation by texture matrices.
Transformation of normals to eye coordinates.
Rescaling and normalization of normals.
Texture coordinate generation.
Per vertex lighting computations.
Color material computations.
Point size distance attenuation.
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Vertex Processor – Fixed Functionality
Fixed functionality applied to the result:
Perspective division on clip coordinates.
Viewport mapping.
Depth range scaling.
View frustum clipping.
Front face determination.
Culling.
Flat-shading.
Associated data clipping.
Final color processing.
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Vertex Processor – Input and Output
Figure: Scan from OpenGL Shading Language 3rd edition
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Input Data
vertex attributes (user-defined),
uniforms (built-in, user-defined),
textures,
special built-in variables (very few in core).
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Vertex Attributes
user-defined per vertex data,
consist of a number of indexed locations called current
vertex state,
limited number of attributes,
attributes are set with glVertexAttrib family of functions,
one indexed location can hold a quadruple,
matrix attributes are stored in column-major order in
succesive attribute locations,
the same value can be set for all vertices (that do not have
it otherwise specified).
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Vertex Attributes – Binding
void glBindAttribLocation(GLuint program, GLuint index, const GLchar ∗name);
program − the handler to the program.
index − index of the generic vertex attribute to be bound.
name − string containing the name of the vertex shader attribute variable to
which index is to be bound.
Used before linking to set the attribute name-index pairing.
Automatic assignment of index+1, [index+2, [index+3]] for matrix
name.
Reserved variables (gl_*) must not be bound this way.
May set the pairing of attributes from the same array for
different shaders consistently.
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Vertex Attributes – Binding (cont.)
GLint glGetAttribLocation(GLuint program, const GLchar ∗name);
program − the handler to the program.
name − string containing the name of the vertex shader attribute variable to
be queried.
Used after linking to get the attribute name-index pairing.
For matrix name the returned index is for the first column
(index+1, [index+2, [index+3]]).
For non-existent attributes or reserved variables (gl_*) −1 is
returned.
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Vertex Attributes – Enable
void glEnableVertexAttribArray(GLuint index);
void glDisableVertexAttribArray(GLuint index);
index − index of the generic vertex attribute to be enabled/disabled.
Enabled/disable vertex attributes for use in the draw calls.
By default all generic attributes are disabled.
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Vertex Attributes – Data
void glVertexAttribPointer (GLuint index, GLint size, GLenum type, GLboolean
normalized, GLsizei stride, const GLvoid ∗pointer);
void glVertexAttribIPointer (GLuint index, GLint size, GLenum type, GLsizei stride,
const GLvoid ∗ pointer);
index − index of the generic vertex attribute to be modified.
size − the number of components of the generic attribute (1|2|3|4) .
type − the type of each component.
normalized − whether fixed−point data should be normalized.
stride − byte offset between consecutive vertex attributes.
pointer − offset of the first attribute in the buffer bound to
GL_ARRAY_BUFFER target.
Specifies the location and format of vertex attributes.
The I variant passes integer attributes unchanged.
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Vertex Arrays and Buffers
All attributes are bound to a single vertex array object
(VAO).
This VAO consists of a number of buffers holding the
individual attributes.
The VAO holds all the information for the draw call e.g.
glDrawArrays or glDrawElements.
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Vertex Arrays and Buffers – Example
1 GLuint vao ;
2
3 / / Create the VAO
4 glGenVertexArrays (1 , &vao ) ;
5 glBindVertexArray ( vao ) ;
6
7 / / Create buffers f o r our vertex data
8 GLuint buffers [ 2 ] ;
9 glGenBuffers (2 , buffers ) ;
10
11 / / Vertex coordinates buffer
12 glBindBuffer (GL_ARRAY_BUFFER, buffers [ 0 ] ) ;
13 glBufferData (GL_ARRAY_BUFFER, sizeof ( ve rt ic es ) , vertices ,
GL_STATIC_DRAW) ;
14 glEnableVertexAttribArray (VERTEX_COORD_ATTRIB) ;
15 g l V e r t e x A t t r i b P o i n t e r (VERTEX_COORD_ATTRIB, 4 , GL_FLOAT, 0 ,0 ,0) ;
16
17 / / Index buffer
18 glBindBuffer (GL_ELEMENT_ARRAY_BUFFER, buffers [ 1 ] ) ;
19 glBufferData (GL_ELEMENT_ARRAY_BUFFER, sizeof ( faceIndex ) ,
faceIndex , GL_STATIC_DRAW) ;
20
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Vertex Arrays and Buffers – Example (cont.)
21 / / Unbind the VAO
22 glBindVertexArray (0) ;
23
24 . . .
25
26 / / Render VAO
27 glBindVertexArray ( vao ) ;
28 glDrawElements (GL_TRIANGLES, faceCount ∗3 , GL_UNSIGNED_INT, 0) ;
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Uniforms
user-defined: read-only in all shaders,
constant per draw call, changed per primitive at most (not
recommended for performance),
can be initialized inside the shader,
location indices are assigned during link,
limited number of uniforms (both build-in and user-defined),
uniforms can be grouped into named blocks.
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Uniforms – Blocks
all variables outside named block are in default block,
sampler variables must be in default block,
cannot be used for another program,
advantageous for variables tied to an individual
shader/program.
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Uniforms – Location
GLint glGetUniformLocation(GLuint program, const GLchar ∗name);
program − the handler to the program.
name − string containing the name of uniform variable to be queried.
Returns the memory location of a uniform variable.
Must be called after linking the program (location may
change with each link).
Not usable for structures, arrays, subcomponents of
vectors and matrices.
For structures and arrays, its elements can be set with “.”
and “[]”.
For non-existent uniforms or reserved names (gl_*)
−1 is returned.
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Uniforms – Lifetime
during link uniforms are set to 0,
their value can be modified only when their program is
used,
the values are preserved when the program is switched off
and on,
uniforms are set with glUniform family of functions.
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Uniforms – Data
void glUniform{1|2|3|4}{ f | i | ui }(GLint location , TYPE v);
location − the location of the uniform variable.
v − 1|2|3|4 component value of the uniform.
void glUniform{1|2|3|4}{ f | i | ui}v(GLint location , GLsizei count, const TYPE∗ v);
location − the location of the uniform variable.
count − number of array elements to be specified.
v − array of values to be loaded.
void glUniformMatrix{2|3|4|2x3|3x2|2x4|4x2|3x4|4x3}fv(GLint location, GLsizei count,
GLboolean transpose, const GLfloat∗ v);
location − the location of the uniform variable.
count − number of matrices to be specified.
transpose − load from row major order?
v − array of values to be loaded.
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Uniforms – Properties
types and sizes of the uniform variables must match the
functions,
locations for array elements and other variables cannot be
computed: loc("A[n]") != loc("A")+n.
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Uniforms – Example
1 uniform s t r u c t
2 {
3 s t r u c t
4 {
5 f l o a t a ;
6 f l o a t b [ 1 0 ] ;
7 } c [ 2 ] ;
8 vec2 d ;
9 } e ;
1 loc1 = glGetUniformLocation ( prog , "e . d" ) ; / / v a l i d : vec2
2 loc2 = glGetUniformLocation ( prog , "e . c [ 0 ] " ) ; / / i n v a l i d : s t r u c t
3 loc3 = glGetUniformLocation ( prog , "e . c [ 0 ] . b" ) ; / / v a l i d : array
4 loc4 = glGetUniformLocation ( prog , "e . c [ 0 ] . b [ 2 ] " ) ; / / v a l i d :
array element
5
6 glUniform2f ( loc1 , 1.0 f , 2.0 f ) ; / / v a l i d : vec2
7 glUniform2i ( loc1 , 1 , 2) ; / / i n v a l i d : not ivec2
8 glUniform2f ( loc1 , 1.0 f ) ; / / i n v a l i d : not f l o a t
9 glUniform2fv ( loc3 , 10 , &f ) ; / / v a l i d : b [ 0 ] (+10)
10 glUniform2fv ( loc4 , 10 , &f ) ; / / i n v a l i d : out of range
11 glUniform2fv ( loc4 , 8 , &f ) ; / / v a l i d : b [ 2 ] (+8)
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Uniforms – Samplers
only glUniform1i and glUniform1iv can be used to load
samplers,
the loaded value is the index of the texture unit to be used,
the same unit cannot be loaded into samplers of different
types.
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Special Built-in Variables
gl_VertexID – implicit vertex index passed by e.g.
DrawArrays,
gl_InstanceID – implicit primitive index passed by
instanced draw calls e.g. glDrawArraysInstanced,
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Output Data
special built-in variables (very few in core),
varying variables (user-defined),
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Special Built-in Variables
in vec4 gl_Position;
homogeneous position in clip space (modelview,
projection),
must be set, used by the rest of the pipeline,
in float gl_PointSize;
size of the rasterized points,
must be set if points are rasterized,
in float gl_ClipDistance [];
array of distances to user clipping planes,
must be set if user clipping is enabled.
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Varying Variables
passed from vertex processor to rasterizer,
anything can be passed,
more variables can be outputed than used by follow-up
shader,
interpolation type can be set,
limited number of interpolated values.
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Geometry Processor
Optional (no fixed pipeline equivalent).
Receives assembled primitives, outputs zero (culling) or
more primitives.
May receive adjacency information.
The type of input and output primitives need not match
(triangles → points).
Designed for moderate geometry amplification, not
tessellation.
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Geometry Processor – Primitives
Input primitives:
points,
lines,
lines_adjacency,
triangles,
triangles_adjacency.
Output primitives:
points,
line_strip,
triangles_strip.
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Input Data
varying variables (built-in, user-defined),
uniforms (built-in, user-defined),
textures,
special built-in variables (very few in core).
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Varying Variables
build-in and user-defined varying variables for each vertex,
in the form of array of structures (user-defined or
gl_PerVertex),
definition must match vertex shader.
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Uniforms
defined the same way as for vertex shader,
can be the same set of variables as in vertex shader,
no need to setup uniforms for each shader,
limited number of uniforms (both build-in and user-defined).
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Output Data
same output as the vertex shader,
definition of primitives,
special built-in variables (very few in core),
varying variables (user-defined).
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Fragment Processor
Replaces the following fixed functionality:
Texture environments and texture functions.
Texture application.
Color sum.
Fog.
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Fragment Processor – Fixed Functionality
Fragment shader does not change the following operations:
Texture image specification.
Alternate texture image specification.
Compressed texture image specification.
Texture parameters that behave as specified even when a
texture is accessed from within a fragment shader.
Texture state and proxy state.
Texture object specification.
Texture comparison modes.
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Fragment Processor – Input and Output
Figure: Scan from OpenGL Shading Language 3rd edition
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Input Data
interpolated varying variables (built-in, user-defined),
uniforms (built-in, user-defined),
textures,
special built-in variables (very few in core).
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Varying Variables
in vec4 gl_FragCoord;
window coordinate position (xy), fragment depth (z),
in bool gl_FrontFacing;
whether the fragment originated from front facing primitive,
in vec2 gl_PointCoord;
position of the fragment (only for point primitives),
user defined varying variables,
definition must match vertex/geometry shader.
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Uniforms
defined the same way as for vertex/geometry shader,
can be the same set of variables as in vertex/geometry
shader,
no need to setup uniforms for each shader,
limited number of uniforms (both build-in and user-defined).
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Output Data
special built-in variables (very few in core),
user-defined output,
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Special Built-in Variables
out float gl_FragDepth;
replaces fragment depth (can be also discarded),
fragments x,y position cannot be changed,
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User-defined Output
output color or discard fragment,
multiple buffers may be updated.
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User-defined Output – Rendering Targets
void glDrawBuffers(GLsizei n, const GLenum ∗bufs);
n − number of render targets.
bufs − array of output buffers .
sets the output rendering targets,
void glBindFragDataLocation(GLuint program, GLuint colorNum, const char ∗name);
program − the handler to the program.
colorNum − the color number to bind the user−defined varying out variable to.
name − the name of the varying out variable whose binding to modify.
the index of the target as specified in glDrawBuffers,
also possible to set from shader code.
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Tools
shaders are just strings → any editor you desire,
RenderMonkey (http://developer.amd.com/
resources/archive/archived-tools/
gpu-tools-archive/rendermonkey-toolsuite/),
FX Composer
(https://developer.nvidia.com/fx-composer),
OpenGL Shader Designer (http:
//www.opengl.org/sdk/tools/ShaderDesigner/),
and many more, mostly discontinued,
shader programming got diverse, only IDEs for specialized
tasks.
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Debuggers & Profilers
NVIDIA NSight,
for Registered developers,
AMD CodeXL,
directly downloadable,
gDEBugger (http://www.gremedy.com/),
directly downloadable, up to OpenGL 3.2
VS2010,
use what is already there,
syntax highlighting, IntelliSense.
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Project Setup
create folder H:\PV227 (not Desktop, Documents, . . . ),
create subfolders Templates and Final,
unzip the libraries into the Templates folder (optionally also
to the Final),
unzip the source codes into these folders,
launch the projects with Ctrl-F5 (keeps the console open).
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GLUT
multiplatform windowing system for OpenGL,
not updated, alternatives exist: FreeGLUT
(http://freeglut.sourceforge.net/),
download built libraries at
http://www.transmissionzero.co.uk/software/
freeglut-devel/.
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GLEW
library for accessing OpenGL core and extension
functionality,
download built libraries at
http://glew.sourceforge.net/,
use the older version 1.10.0.
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Visual Studio Paths
Project properties → Set All Configurations:
VC++ Directories,
Include Directories:
<path>\freeglut\include;<path>\glew-1.10.0\include;
Library Directories:
<path>\freeglut\lib;<path>\glew-1.10.0\lib\Release\Win32;
Debugging,
Environment: PATH=<path>\freeglut\bin;<path>\glew-
1.10.0\bin\Release\Win32;
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Visual Studio Editor
Syntax highlighting:
Tools → Options,
Text Editor → File Extension,
add vert, geom, frag with Microsoft Visual C++ syntax,
update usertype.dat in the VS2010 directory C:\Program
Files\Microsoft Visual Studio 10.0\Common7\IDE.
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Example
Complete the CPU calls.
Vertex Shader: Project the triangle!
Fragment Shader: Shade triangle!
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“Advanced” example
Rotate triangle on the CPU.
Vertex Shader: Rotate triangle, set varying attribute (color).
Fragment Shader: Draw inverse color.
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Build-in Constants
values accessible from OpenGL API by glGet,
give minumum value for OpenGL conforming
implementation.
1 const i n t gl_MaxVertexAttribs = 16;
2 const i n t gl_MaxVertexUniformComponents = 1024;
3 const i n t gl_MaxFragmentUniformComponents = 1024;
4 . . .
glGetIntegerv(GL_MAX_{VERTEX|GEOMETRY|FRAGMENT}
_UNIFORM_COMPONENTS, &nComponents);
glGetIntegerv(GL_MAX_VARYING_FLOATS, &nFloats);
glGetIntegerv(GL_MAX_VERTEX_ATTRIBS, &nAttribs);
glGetIntegerv(GL_MAX_DRAW_BUFFERS, &nBuffers);
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