Shader Programming

Introduzione

History

• We have seen ever-increasing graphics performance in PCs since the release of the first 3dfx Voodoo cards in 1995. Although this performance increase has allowed PCs to run graphics faster, it arguably has not allowed graphics to run much better.

History• he fundamental limitation thus far in PC

graphics accelerators has been that they are mostly fixed-function. Fixed-function means that the silicon designers have hard-coded specific graphics algorithms into the graphics chips, and as a result the game and application developers have been limited to using these specific fixed algorithms.

Now• Now, for the first time, low-cost consumer

hardware has reached the point where it can begin implementing the basics of programmable shading similar to the RenderMan graphics language with real-time performance. (Pixar’s shading language )

• The principal 3D APIs (DirectX and OpenGL) have evolved alongside graphics hardware. One of the most important new features in DirectX Graphics is the addition of a programmable pipeline that provides an assembly language interface to the transformation and lighting hardware (vertex shader) and the pixel pipeline (pixel shader).

Why use Vertex Shaders?• If you use Vertex Shaders, you bypass

the fixed-function pipeline or T&L pipeline.

• Why would you want to skip them?• The fixed-function pipeline doesn't give

the developer the freedom he need to develop unique and revolutionary graphical effects

What can u get?• Procedural Geometry (cloth simulation, soap

bubble) • Advanced Vertex Blending for Skinning and

Vertex Morphing (tweening)• Texture Generation• Advanced Keyframe Interpolation (complex

facial expression and speech) • Particle System Rendering • Real-Time Modifications of the Perspective

View (lens effects, underwater effect) • Advanced Lighting Models• First Steps to Displacement Mapping

What can u get?

• In addition to opening up creative possibilities for developers and artists, shaders also attack the problem of constrained video memory bandwidth by executing on-chip on shader-capable hardware.

Why use Pixel Shaders

• The final output of any 3D graphics hardware consists of pixels. Depending on the resolution, in excess of 2 million pixels may need to be rendered, lit, shaded and colored. Prior to DirectX 8.0, Direct3D used a fixed-function for pixel processing. The effects possible with this approach were very limited on the implementation of the graphics card device driver and the specific underlying hardware. A programmer was restricted on the graphic algorithms implemented by these

Why use Pixel Shaders

• Pixel shaders are small programs that are executed on individual pixels.

• a whole new programming universe can be explored by game/demo coders

Why use Pixel Shaders• Single pass, per-pixel lighting• True phong shading• Anisotropic lighting• Non-Photorealistic-Rendering: cartoon

shading, hatching, gooch lighting, image space techniques

• Volumetric effects• Advanced bump mapping (self-shadowing

bump maps (also known as Horizon Mapping) • Procedural textures and texture perturbation• Bidirectional reflectance distribution functions

The architecture

• Vertex

• Pixel

Vertex Shader Architecture

• All data are in a vertex shader is represented by 128-bit quad-floats (4 x 32-bit):

Vertex Shader Architecture• A hardware vertex shader can be seen as a

typical SIMD (Single Instruction Multiple Data) processor for you are applying one instruction and affecting a set of up to four 32-bit variables. This data format is very useful, because most of the transformation and lighting calculations are performed using 4x4 matrices or quaternions. The instructions are very simple and easy to understand. The vertex shader at their introduction doesn’t allow any loops, jumps or conditional branches, which means that it executes the program linearly - one instruction after the other.

Costants• The constant registers (Constant Memory) are loaded

by the CPU, before the vertex shader starts executing parameters defined by the programmer. The vertex shader is not able to write to the constant registers. They are used to store parameters such as light position, matrices, procedural data for special animation effects, vertex interpolation data for morphing/key frame interpolation and more. The constants can be applied within the program and they can even be addressed indirectly with the help of the address register a0.x, but only one constant can be used per instruction. If an instruction needs more than one constant, it must be loaded into on e of the temporary regsiters before it its required. The names of the constant registers are c0 - c95

Instructions

MIPS InstructionsOpName dest, [-]s1 [,[-]s2 [,[-]s3]]Examples..• Add dest, src1, src2• Mov dest, sr• Mul dest, src1, src2

Pixel Shaders Architecture

• Before an image that is draw by the Vertexes become available for the Pixel Shader several step happen.

• A vertex leaves the Vertex Shader as a transformed and colored vertex.

Pixel Pipeline• Backface Culling removes all triangles, that

are facing away from the viewer or camera. • User Clip Planes can be set by the developer

to clip triangles • Homogenous or perspective Divide

happens. This means that the x-, y- and z-coordinates of each vertex of the homogenous coordinates are divided by w. The perspective divide makes nearer objects larger, and further objects smaller as you would expect when viewing a scene in reality.

Pixel Shaders Rise• Now comes the Triangle Setup, where

the life of the vertices end and the life of the pixel begins. It computes triangle for triangle the parameters required for the rasterization of the triangles.it defines the first and the last pixel of the triangle scan line by scan line:

Pixel Pipeline• Rasterizer interpolates color and depth

values for each pixel from the color and depth values of the vertices.

• These values are interpolated using a weighted average of the color and depth values of the edge's vertex values, where the color and depth data of edge pixels closer to a given vertex more closely approximate values for that vertex. Then the rasterizer fills in pixels for each line.

Pixel Shader

• The Pixel Shader is not involved on the sub-pixel level. It gets the already multisampled pixels along with z, color values and texture information

After the Pixel Shader

• Alpha Blending stage blends the pixel's data with the pixel data already in the Render Target. The blending happens with the following formula:

• FinalColor = SourcePixelColor * SourceBlendFactor + DestPixelColor * DestBlendFactor

After the Pixel Shaders• Dithering tries to fool the eye into seeing

more colors than are actually present by placing different colored pixels next to one another to create a composite color that the eye will see

• Dithering tries to fool the eye into seeing more colors than are actually present by placing different colored pixels next to one another to create a composite color that the eye will see

Pixel Shaders Registers

• The Color Registers stream iterated vertex color data from a vertex shader or a fixed-function vertex pipeline to a pixel shader.

• Constant Registers provide constants to the shader, that are loaded by program or in the pixel shader with the def instruction.

• Temporary Registers rn are able to store temporary data. The r0 register also serves as the Output register of the pixel shader.

Pixel Shaders Instructions

• As for the Vertex Shaders the instructions resemble a MIPS SIMD execution unit.

• The there are different instruction that provide texture functions.

“We come One”

• We can easly see the architecture and the structure of Vertex and Pixel Shaders are very strictly correlated and very close to each other.

• Not only the datastructure used in both Shaders is equal, also the instructions are close..

“We come one”• The big difference, is more where this 2

kind of Shaders are used!• The vertex shader create a new 3D

model from the starting one• Pass it to pixel Shaders• Pixel Shaders Elaborate the world and

change the appearance of single pixel

“We come one”• In future GPU that are coming out we can expect

to see more and more Unified design shaders.• Example: ATI’s Xenos Xbox 360• “This GPU also features 48 unified shaders, which

can be used as either vertex or pixel units.“ www.TomsHardware.com

• Windows Graphics Foundation 2, the version of DirectX that will ship with Longhorn, will be designed around the idea that the graphics card will have unified vertex and pixel pipelines

ATI Point of view“Rather than separate pixel and vertex pipelines, we’ve created a single unified pipeline that can do both.Providing developers throw instructions at our architecture in the right way, Xenos can run at 100% efficiency all the time, rather than having some pipeline instructions waiting for others. For comparison, most high-end PC chips run at 50-60% typical efficiency.The super cool point is that ‘in the right way’ just means ‘give us plenty of work to do’. The hardware manages itself.”

• ATI European Developer Relations Manager , Richard Huddy