Lesson 2 – Workflow, Tools
PV227 – GPU Rendering
Jiˇrí Chmelík, Jan ˇCejka
Fakulta informatiky Masarykovy univerzity
29. 9. 2015
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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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Workflow
The 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 type of 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 type of shader to use for a given task?
May also depend on special properties of the processors:
need of 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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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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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 – Example
Figure: Taken from: cs.utah.edu Figure: Taken from: cs.utah.edu
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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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Vertex Processor – Inputs
Types of Input Data
uniforms (built-in, user-defined),
vertex attributes:
user-defined (mostly),
built-in variables (very few in core),
textures.
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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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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
Sampler represents a single texture of a particular texture type.
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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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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Vertex Attributes
user-defined per vertex data,
consist of a number of indexed locations called current vertex
state,
number of attributes is limited (by HW),
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
Binding: associates the name of the variable with an index.
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.
For example:
glBindAttribLocation(ProgramObject, 10, "myAttrib")
would bind the attribute "myAttrib" to index 10.
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Vertex Attributes – Binding (cont.)
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 I
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 ( v e 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 II
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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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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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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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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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,
values of varying variable will be interpolated,
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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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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Geometry Processor
everything geometry shaders can do can be accomplished in
other ways
Has ability to generate geometry from a small amount of input
data, that allows you to reduce CPU → GPU bandwidth usage.
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Geometry Processor – example
Single draw call:
glDrawArrays(GL_POINTS, 0, 4);
can produce (without geometry shader):
Figure: Four red dots. Taken from: https://open.gl/geometry
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Geometry Processor – example
Single draw call:
glDrawArrays(GL_POINTS, 0, 4);
or this (using geometry shader):
Figure: Taken from: https://open.gl/geometry
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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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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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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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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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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 or newer,
use what is already there,
syntax highlighting, IntelliSense.
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Recommended Setup
create folder H:\PV227 (not Desktop, Documents, . . . ),
unzip the Assignments-setup.zip into PV227 folder,
unzip given assignment into Assignments folder,
launch the projects with Ctrl-F5 (keeps the console open).
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Manual setup – 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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Manual setup – GLEW
library for accessing OpenGL core and extension functionality,
download built libraries at http://glew.sourceforge.net/,
Recommended version (2015): 1.13.0.
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Manual setup – Visual Studio Paths
Project properties → Set All Configurations:
VC++ Directories,
Include Directories: <path>\freeglut\include;<path>\glew\include;
Library Directories:
<path>\freeglut\lib;<path>\glew\lib\Release\Win32;
Debugging,
Environment:
PATH=<path>\freeglut\bin;<path>\glew\bin\Release\Win32;
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Visual Studio Editor
Improving of syntax highlighting:
1 Tools → Options,
2 Text Editor → File Extension,
3 add ‘ vert’, ‘geom’, ‘frag’ with Microsoft Visual C++
syntax,
4 update usertype.dat in the VS2013 directory: C:\Program
Files (x86)\Microsoft Visual Studio
12.0\Common7\IDE.
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Outline
1 Workflow
2 Vertex Processor
Uniforms
Vertex Attributes
Built-in Variables
Output Data
3 Geometry Processor
4 Fragment Processor
5 Tools & Set-up
6 Assignments
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Assignment – Intro\Triangle
TODO:
Complete the CPU calls.
Vertex Shader: Project the triangle!
Fragment Shader: Shade triangle!
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“Advanced” Assignment
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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