Programmable
Graphics Hardware
David Luebke
1
7/27/2016
Recap: How are current
GPU’s different from CPU?
1. GPU is a stream processor
■
Multiple programmable processing units
■ Connected by data flows
2
Fragment
Processor
Textures
Framebuffer
Vertex
Processor
Framebuffer
Operations
David Luebke
Assembly &
Rasterization
Application
7/27/2016
Cg uses separate vertex
and fragment programs
Fragment
Processor
Framebuffer
Vertex
Processor
Framebuffer
Operations
Assembly &
Rasterization
Application
Textures
Program
Program
David Luebke
3
7/27/2016
Cg programs have two
kinds of inputs
● Varying inputs (streaming data)
■ e.g. normal vector – comes with each vertex
■ This is the default kind of input
● Uniform inputs (a.k.a. graphics state)
■ e.g. modelview matrix
● Note: Outputs are always varying
vout MyVertexProgram(float4 normal,
uniform float4x4 modelview)
{
…
David Luebke
4
7/27/2016
Two ways to bind
VP outputs to FP inputs
a)
Let compiler do it
■
■
■
Define a single structure
Use it for vertex-program output
Use it for fragment-program input
struct vout {
float4 color;
float4 texcoord;
…
};
David Luebke
5
7/27/2016
Two ways to bind
VP outputs to FP inputs
Do it yourself
b)
■
■
■
■
Specify register bindings for VP outputs
Specify register bindings for FP inputs
May introduce HW dependence
Necessary for mixing Cg with assembly
struct vout {
float4 color : TEX3 ;
float4 texcoord : TEX5;
…
};
David Luebke
6
7/27/2016
Some inputs and
outputs are special
● e.g. the position output from vert prog
■ This output drives the rasterizer
■ It must be marked
struct vout {
float4 color;
float4 texcoord;
float4 position : HPOS;
};
David Luebke
7
7/27/2016
How are current GPU’s
different from CPU?
2.
Greater variation in basic capabilities
■
■
■
•
•
•
•
Most processors don’t yet support branching
Vertex processors don’t support texture mapping
Some processors support additional data types
Compiler can’t hide these differences
Least-common-denominator is too restrictive
We expose differences via language profiles
(list of capabilities and data types)
Over time, profiles will converge
David Luebke
8
7/27/2016
How are current GPU’s
different from CPU?
3.
Optimized for 4-vector arithmetic
■
■
•
•
•
•
Useful for graphics – colors, vectors, texcoords
Easy way to get high performance/cost
C philosophy says: expose these HW data types
Cg has vector data types and operations
e.g. float2, float3, float4
Makes it obvious how to get high performance
Cg also has matrix data types
e.g. float3x3, float3x4, float4x4
David Luebke
9
7/27/2016
Some vector operations
//
// Clamp components of 3-vector to [minval,maxval] range
//
float3 clamp(float3 a, float minval, float maxval) {
a = (a < minval.xxx) ? minval.xxx : a;
a = (a > maxval.xxx) ? maxval.xxx : a;
return a;
}
? : is per-component
for vectors
Swizzle – replicate and/or
rearrange components.
Comparisons between vectors
are per-component, and
produce vector result
David Luebke
10
7/27/2016
Cg has arrays too
● Declared just as in C
● But, arrays are distinct from
built-in vector types:
float4 != float[4]
● Language profiles may restrict array usage
vout MyVertexProgram(float3 lightcolor[10],
…) {
…
David Luebke
11
7/27/2016
How are current GPU’s
different from CPU?
4.
No support for pointers
■
5.
Arrays are first-class data types in Cg
No integer data type
■
■
David Luebke
Cg adds “bool” data type for boolean operations
This change isn’t obvious except when declaring vars
12
7/27/2016
Cg basic data types
● All profiles:
■ float
■ bool
● All profiles with texture lookups:
■ sampler1D, sampler2D, sampler3D,
samplerCUBE
● NV_fragment_program profile:
■ half -- half-precision float
■ fixed -- fixed point [-2,2)
David Luebke
13
7/27/2016
Other Cg capabilities
● Function overloading
● Function parameters are value/result
■ Use “out” modifier to declare return value
void foo (float a, out float b) {
b = a;
}
● “discard” statement – fragment kill
if (a > b)
discard;
David Luebke
14
7/27/2016
Cg Built-in functions
● Texture mapping (in fragment profiles)
● Math
■ Dot product
■ Matrix multiply
■ Sin/cos/etc.
■ Normalize
● Misc
■ Partial derivative (when supported)
● See spec for more details
David Luebke
15
7/27/2016
New vector operators
● Swizzle – replicate/rearrange elements
a = b.xxyy;
● Write mask – selectively over-write
a.w = 1.0;
● Vector constructor builds vector
a = float4(1.0, 0.0, 0.0, 1.0);
David Luebke
16
7/27/2016
Change to constant-typing
mechanism
● In C, it’s easy to accidentally use high precision
half x, y;
x = y * 2.0;
// Double-precision multiply!
● Not in Cg
x = y * 2.0;
// Half-precision multiply
● Unless you want to
x = y * 2.0f; // Float-precision multiply
David Luebke
17
7/27/2016
Dot product,
Matrix multiply
● Dot product
■ dot(v1,v2);
// returns a scalar
● Matrix multiplications:
■ matrix-vector: mul(M, v); // returns a vector
■ vector-matrix: mul(v, M); // returns a vector
■ matrix-matrix: mul(M, N); // returns a matrix
David Luebke
18
7/27/2016
Demos and Examples
● Show Assn 4a skeleton code
● Show Cg effects browser (
■ Fresnel
■ Simple lighting
David Luebke
19
7/27/2016
Cg runtime API helps
applications use Cg
● Compile a program
● Select active programs for rendering
● Pass “uniform” parameters to program
● Pass “varying” (per-vertex) parameters
● Load vertex-program constants
● Other housekeeping
David Luebke
20
7/27/2016
Runtime is split into three
libraries
● API-independent layer – cg.lib
■ Compilation
■ Query information about object code
● API-dependent layer –
cgGL.lib and cgD3D.lib
■ Bind to compiled program
■ Specify parameter values
■ etc.
● NB: New API introduced since the following slide
■ See user’s manual for details
■ Pay attention to the basic idea here
David Luebke
21
7/27/2016
Runtime API for OpenGL
// Create cgContext to hold vertex-profile code
VertexContext = cgCreateContext();
// Add vertex-program source text to vertex-profile context
// This is where compilation currently occurs
cgAddProgram(VertexContext, CGVertProg, cgVertexProfile, NULL);
// Get handle to 'main' vertex program
VertexProgramIter = cgProgramByName(VertexContext, "main");
cgGLLoadProgram(VertexProgramIter, ProgId);
VertKdBind = cgGetBindByName(VertexProgramIter, "Kd");
TestColorBind = cgGetBindByName(VertexProgramIter, "I.TestColor");
texcoordBind = cgGetBindByName(VertexProgramIter, "I.texcoord");
David Luebke
22
7/27/2016
Runtime API for OpenGL
//
// Bind uniform parameters
//
cgGLBindUniform4f(VertexProgramIter, VertKdBind, 1.0, 1.0, 0.0, 1.0);
…
// Prepare to render
cgGLEnableProgramType(cgVertexProfile);
cgGLEnableProgramType(cgFragmentProfile);
…
// Immediate-mode vertex
glNormal3fv(&CubeNormals[i][0]);
cgGLBindVarying2f(VertexProgramIter, texcoordBind, 0.0, 0.0);
cgGLBindVarying3f(VertexProgramIter, TestColorBind, 1.0, 0.0, 0.0);
glVertex3fv(&CubeVertices[CubeFaces[i][0]][0]);
David Luebke
23
7/27/2016
Cg Summary
● C-like language
● With capabilities for GPU’s
● Compatible with Microsoft’s HLSL
● Use with OpenGL or DirectX
● NV20/DX8 and beyond
● NV30 + Cg =
You control the graphics pipeline
David Luebke
24
7/27/2016
More Information
● NVIDIA’s “Learn About Cg” page:
■ http://developer.nvidia.com/view.asp?IO=cg_about
● For information, inspiration, and examples, a web
forum on writing Cg shaders:
■ http://www.cgshaders.org
● The Cg user’s manual:
■ http://developer.nvidia.com/view.asp?IO=cg_users_manual
● Cg downloads—compiler, plug-ins, etc.
■ http://developer.nvidia.com/view.asp?IO=cg_toolkit
David Luebke
25
7/27/2016