Game Programming 12 - Shaders

Game Programming
Shaders
Nick Prühs

Objectives
• To get an overview of the graphics pipeline of
modern GPUs
• To understand the basics of shader development
• To learn how to write your own advanced Cg
shaders
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Shaders
• Control the shape, appearance, and motion of
objects drawn using programmable graphics
hardware
• Very specialized
• Incredibly fast
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CPU vs. GPU
• CPUs are, by design, general purpose
• GPUs are, due to their specialized design, much
faster at graphics tasks such as vector math
• add, multiply, multiply-add
• minimum and maximum
• component-wise swizzling (pos.xz)
• negation, subtraction
• dot and cross products
• vector-by-scalar division and vector normalization
• exponential, logarithmic, and trigonometric
approximations
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Performance Requirements
• Desired frame rate of 60 or more frames per
second
• A million or more pixels that require redrawing
• GPU typically processes every pixel on the screen
many times
• to account for how objects occlude each other
• to improve the appearance of each pixel
• Thus, real-time 3D applications can require
hundreds of millions of pixel updates per second
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Shader Languages
• Based on a data-flow computational model
• Operate on vertices and fragments (think "pixels"
for now as we do not know yet what a fragment is)
that are processed when rendering an image
• Execute in relative isolation
• Well suited for hardware execution by highly pipelined
and parallel hardware
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Cg
• "C for graphics”
• Developed by Nvidia in close collaboration with the
Microsoft Corporation
• Compatible with both the OpenGL API and
Microsoft's High-Level Shading Language (HLSL) for
DirectX 9.0
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Cg Language Features
• Arrays
• Structures
• Vectors
• Matrices
• Loops
• Conditionals
• Function Calls
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Cg Limitations
• No classes
• No pointers
• No memory allocation
• No file I/O
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Limited Execution
• GPUs are specialized
• Our program not only must be correct, but it also
must limit itself to the restrictions imposed by the
particular profile used to compile our Cg program
• GPU architecture
• graphics API
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Vertex Data
• Position
• Color
• Specular color
• Texture coordinate sets
• Normal vector
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Vertex Transformation
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• Performs a sequence of math operations on each
vertex
• transforming the vertex position into a screen position
for use by the rasterizer
• generating texture coordinates for texturing
• lighting the vertex to determine its color

Primitive Assembly
• Assembling vertices into geometric primitives
based on the geometric primitive batching
information that accompanies the sequence of
vertices
• sequence of triangles, lines, or points
• Clipping to the view frustum
• Discarding polygons based on whether they face
forward or backward (culling)
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Rasterization
• Determining the set of pixels covered by a
geometric primitive
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Pixel vs. Fragment
• Pixels represent the contents of the frame buffer at
specific locations
• Color
• Depth
• Fragments are the states required potentially to
update particular pixels (“potential pixel”)
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Fragment Data
• Pixel location
• Depth value
• Set of interpolated parameters derived from the
transformed vertices that make up the particular
geometric primitive used to generate the fragments
• Color
• Specular color
• One or more texture coordinate sets
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Fragment Texturing &
Coloring
• Determines a final color for each fragment
• Determines a new depth or may even discard the
fragment to avoid updating the frame buffer's
corresponding pixel
• Emits one or zero colored fragments for every input
fragment it receives
• Texturing operations enable the processor to access
a texture image using a set of texture coordinates
and then to return a filtered sample of the texture
image
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Raster Operations
1. Pixel Ownership Test (owned by DirectX or
OpenGL?)
2. Scissor Test (within window?)
3. Alpha Test (correct alpha value?)
4. Stencil Test (comparison with stencil buffer)
5. Depth Test (comparison with depth buffer)
6. Blending (combines fragment with frame buffer)
7. Dithering (select between two color values)
8. Logic Op (between fragment and frame buffer)
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Hello World
ShaderLab (Unity)
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// Shader name as shown in Unity.
Shader "Tutorial/Green"
{
SubShader
{
Pass
{
// Use Cg shader language.
CGPROGRAM
// Define shader entry points.
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"
// ...
ENDCG
}
}
}

Hello World
Cg
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// Define vertex shader output format.
struct v_Output
{
float4 position : POSITION;
float4 color : COLOR;
};
v_Output vert(float4 position : POSITION)
{
v_Output OUT;
// Transform object coordinates to clip coordinates.
OUT.position = mul(UNITY_MATRIX_MVP, position);
// Set vertex color to RGBA green.
OUT.color = float4(0, 1, 0, 1);
return OUT;
}

Shader Semantics
• Indicate how the variables preceding them connect
to the rest of the graphics pipeline
• The POSITION semantic (in this case, in an output
structure used by a Cg vertex program) is the clip-space
position for the transformed vertex
• Later graphics pipeline stages will use the output vector
associated with this semantic as the post-transform,
clip-space position of the vertex for primitive assembly,
clipping, and rasterization
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Shader Semantics
• Indicate how the variables preceding them connect
to the rest of the graphics pipeline
• The COLOR semantic in this context is what Direct3D
calls the "diffuse vertex color" and OpenGL calls the
"primary vertex color.“
• Color interpolation for a triangle or other geometric
primitive during rasterization depends on the primitive's
per-vertex colors.
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Hello World
Cg
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// Define fragment shader output format.
struct f_Output
{
};
f_Output frag(v_Output v)
{
// Pass-through.
f_Output OUT;
OUT.color = v.color;
return OUT;
}

Shader Parameters
ShaderLab (Unity)
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Shader "Tutorial/Color Parameter"
{
// Properties exposed to the Unity inspector.
Properties
{
_Color ("Main Color", Color) = (1, 1, 1, 0.5)
}
SubShader
{
Pass
{
CGPROGRAM
#pragma vertex vert
// ...
ENDCG
}
}
}

Gotcha!
Usually, when a Cg program declares a
variable as uniform, it conveys that the
variable's initial value comes from an
environment that is external to the specified
Cg program.
In Unity (and RenderMan), the uniform
storage modifier indicates variables whose
values are constant over a shaded surface.
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Shader Parameters
Cg
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struct v_Output
{
};
v_Output vert(float4 position : POSITION, const uniform float4 _Color)
{
v_Output OUT;
// Use passed vertex color.
OUT.color = _Color;
return OUT;
}

Texture Sampling
ShaderLab (Unity)
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Shader "Tutorial/Texture Sampling"
{
Properties
{
_Texture ("Texture", 2D) = "white" {}
}
SubShader
{
Pass
{
CGPROGRAM
#pragma vertex vert
// ...
ENDCG
}
}
}

Texture Sampling
Cg
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struct v_Output
{
float2 texCoord : TEXCOORD0;
};
v_Output vert(float4 position : POSITION, float4 color : COLOR, float2 texCoord : TEXCOORD0)
{
v_Output OUT;
// Use passed vertex color and texture coordinates.
OUT.color = color;
OUT.texCoord = texCoord;
return OUT;
}

Texture Sampling
Cg
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struct f_Output
{
};
f_Output frag(v_Output v, sampler2D _Texture)
{
// Texture lookup.
f_Output OUT;
OUT.color = tex2D(_Texture, v.texCoord);
return OUT;
}

Texture Sampling
ShaderLab (Unity)
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Shader "Tutorial/Double Vision"
{
Properties
{
_Texture ("Texture", 2D) = "white" {}
_LeftSeparation ("Left Separation", Vector) = (-0.1, 0, 0, 0)
_RightSeparation ("Right Separation", Vector) = (0.1, 0, 0, 0)
}
SubShader
{
Pass
{
CGPROGRAM
#pragma vertex vert
// ...
ENDCG
}
}
}

Texture Sampling
Cg
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struct v_Output
{
float2 leftTexCoord : TEXCOORD0;
float2 rightTexCoord : TEXCOORD1;
};
v_Output vert(float4 position : POSITION, float4 color : COLOR, float2 texCoord :
TEXCOORD0, uniform float2 _LeftSeparation, uniform float2 _RightSeparation)
{
v_Output OUT;
// Use passed vertex color.
OUT.color = color;
// Modify texture coordinates.
OUT.leftTexCoord = texCoord + _LeftSeparation;
OUT.rightTexCoord = texCoord + _RightSeparation;
return OUT;
}

Texture Sampling
Cg
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struct f_Output
{
};
f_Output frag(v_Output v, sampler2D _Texture)
{
f_Output OUT;
// Texture lookup.
float4 leftColor = tex2D(_Texture, v.leftTexCoord);
float4 rightColor = tex2D(_Texture, v.rightTexCoord);
// Linearly interpolate texture values.
OUT.color = lerp(leftColor, rightColor, 0.5);
return OUT;
}

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Transformations
• Graphics pipeline takes geometric data
representing an object or scene in three
dimensions and creates a two-dimensional image
from it.
• Our application supplies the geometric data as a
collection of vertices that form polygons, lines, and
points.
• The resulting image typically represents what an
observer or camera would see from a particular
point of view.
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Object Space
• Applications specify vertex positions in a coordinate
system known as object space (also called model
space).
• When an artist creates a 3D model of an object, the
artist selects a convenient orientation, scale, and
position with which to place the model's
constituent vertices.
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World Space
• Provides some absolute reference for all the
objects in our scene
• How a world-space coordinate system is
established is arbitrary
• For example, we may decide that the origin of world
space is the center of our room.
• Objects in the room are then positioned relative to the
center of the room and some notion of scale and
orientation
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Eye Space
• We want to look at our scene from a particular
viewpoint (the "eye")
• In the coordinate system known as eye
space (or view space), the eye is located at the
origin of the coordinate system.
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Clip Space
• Axis-aligned cube containing the region of eye
space where objects are viewable
• Also known as the view frustum
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Normalized Device Space
• All the visible geometric data lies in a cube with
positions
• between <-1, -1, -1> and <1, 1, 1> in OpenGL
• between <-1, -1, 0> and <1, 1, 1> in Direct3D
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Window Space
• Final coordinate system that is measured in pixels
for x and y
• Determines the final image
• Scales the z value of the vertices into the range of
the depth buffer for use in depth buffering
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Basic Per-Vertex Lighting
(Phong)
• Color is computed as the sum of four lighting
contributions:
• Emissive light represents light emitted or given off by a
surface.
• Ambient light accounts for light that has bounced
around so much in the scene that it seems to come from
everywhere.
• Diffuse light accounts for directed light reflected off a
surface equally in all directions.
• Specular light represents light scattered from a surface
predominantly around the mirror direction.
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Emissive Light
• RGB value that indicates the color of the emitted
light
• Contribution is independent of all light sources
• If we were to view an emissive material in a completely
dark room, it would appear to be this color
• Same all over the object
𝑒𝑚𝑖𝑠𝑠𝑖𝑣𝑒 = 𝐾𝑒
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Ambient Light
• Does not appear to come from any particular
direction
• Does not depend on the light source position
• Affected by the global ambient lighting
𝑎𝑚𝑏𝑖𝑒𝑛𝑡 = 𝐾𝑎 × 𝑔𝑙𝑜𝑏𝑎𝑙𝐴𝑚𝑏𝑖𝑒𝑛𝑡
62 / 77

Diffuse Light
• Light that bounces off in all directions
• Amount of light reflected is proportional to the angle of
incidence of the light striking the surface
• With N as the normalized surface normal, and L as the
normalized vector toward the light source, N dot L is a
measure of how much light the surface will receive
• The smaller the angle between the vectors, the greater the
dot-product value will be, and the more incident light the
surface will receive
• Surfaces that face away from the light will produce negative
dot-product values
𝑑𝑖𝑓𝑓𝑢𝑠𝑒 = 𝐾 𝑑 × 𝑙𝑖𝑔ℎ𝑡𝐶𝑜𝑙𝑜𝑟 × max(𝑁 ⋅ 𝐿, 0)
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Specular Light
• Most prominent on very smooth and shiny surfaces
• Depends on the location of the viewer
• Shinier materials have smaller, tighter highlights,
whereas less shiny materials have highlights that are
more spread out
• With facing = 1 if N · L is greater than 0, and 0
otherwise, V as the normalized vector toward the
viewpoint, L as the normalized vector toward the light
source, and H as the normalized vector that is halfway
between V and L
𝑠𝑝𝑒𝑐𝑢𝑙𝑎𝑟 = 𝐾𝑆 × 𝑙𝑖𝑔ℎ𝑡𝐶𝑜𝑙𝑜𝑟 × 𝑓𝑎𝑐𝑖𝑛𝑔 × max 𝑁 ⋅ 𝐻, 0 𝑠ℎ𝑖𝑛𝑖𝑛𝑒𝑠𝑠
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(Phong)
ShaderLab (Unity)
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Shader "Tutorial/Lighting“ {
Properties {
_Ke ("Ke", Color) = (1, 1, 1, 1)
_Ka ("Ka", Color) = (1, 1, 1, 1)
_Kd ("Kd", Color) = (1, 1, 1, 1)
_Ks ("Ks", Color) = (1, 1, 1, 1)
_Shininess ("_Shininess", Float) = 5.0
}
SubShader {
Pass {
Tags { "LightMode" = "ForwardBase“ }
CGPROGRAM
#pragma vertex vert
// ...
ENDCG
}
}
}

(Phong)
Cg
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v_Output vert(float4 position : POSITION,
float3 normal : NORMAL,
const uniform float4 _Ke,
const uniform float4 _Ka,
const uniform float4 _Kd,
const uniform float4 _Ks,
const uniform float _Shininess,
const uniform float4 _LightColor0)
{
v_Output OUT;
// Compute the emissive term.
float4 emissive = _Ke;
// Compute the ambient term.
float4 ambient = _Ka * UNITY_LIGHTMODEL_AMBIENT;
// Compute the diffuse term.
float3 N = normalize(normal);
float3 L = normalize(_WorldSpaceLightPos0.xyz - position.xyz);
float3 diffuse = _Kd * _LightColor0 * max(dot(N, L), 0);
// Compute the specular term.
float facing = dot(N, L) > 0 ? 1 : 0;
float3 V = normalize(WorldSpaceViewDir(position));
float3 H = normalize(L + V);
float3 specular = _Ks * _LightColor0 * facing * pow(max(dot(N, H), 0), _Shininess);
// Compute final color.
OUT.color.xyz = emissive + ambient + diffuse + specular;
OUT.color.w = 1;
return OUT;
}

Toon Shading
• Common and useful non-photorealistic rendering
technique
• Shades objects with constant, sharply delineated
colors
• Renderings look like cartoons without having to
change how we represent our characters and
objects (e.g. draw everything as 2D images)
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Toon Shading
1. The diffuse shading needs to be represented by
just two values: one for bright regions, and
another for dark regions.
2. Specular highlights need to be identified and
represented as a single color where their
intensity is sufficiently high.
3. Objects need to be outlined to complete the
cartoon look.
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Toon Shading
Cg
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v_Output vert(float4 position : POSITION,
float3 normal : NORMAL,
float2 texCoord : TEXCOORD0,
const uniform float _Shininess,
const uniform float4 _LightColor0)
{
v_Output OUT;
// Compute the diffuse term.
float3 N = normalize(normal);
float3 L = normalize(_WorldSpaceLightPos0.xyz - position.xyz);
OUT.diffuse = max(dot(N, L), 0);
// Compute the specular term.
float facing = dot(N, L) > 0 ? 1 : 0;
float3 V = normalize(WorldSpaceViewDir(position));
float3 H = normalize(L + V);
OUT.specular = facing * pow(max(dot(N, H), 0), _Shininess);
// Perform edge detection
OUT.edge = max(dot(N, V), 0);
OUT.texCoord = texCoord;
return OUT;
}

Toon Shading
Cg
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f_Output frag(v_Output v,
sampler2D _Texture,
const uniform float4 _Kd,
const uniform float4 _Ks)
{
f_Output OUT;
// Apply step functions.
float diffuse = v.diffuse <= 0.5 ? 0.5 : 1;
float specular = v.specular <= 0.5 ? 0.5 : 1;
float edge = v.edge <= 0.1 ? 0 : 1;
// Compute the final color
OUT.color = tex2D(_Texture, v.texCoord) * edge * (_Kd * diffuse + _Ks * specular);
return OUT;
}

Toon Shading
76 / 77Unity Construction Worker model with default diffuse shader

Toon Shading
77 / 77Unity Construction Worker model with custom Toon shader

References
• Fernando, Kilgard. The Cg Tutorial. Addison-Wesley, March
8, 2003.
• Blythe. Per-Fragment Operations.
https://www.opengl.org/documentation/specs/version1.1/g
lspec1.1/node93.html, 1997.
• Unity Technologies. Vertex and Fragment Shader Examples.
http://docs.unity3d.com/Manual/SL-
VertexFragmentShaderExamples.html, 2016.
• Unity Technologies. Accessing shader properties in Cg.
http://docs.unity3d.com/Manual/SL-
PropertiesInPrograms.html, 2016.
• Unity Technologies. Built-in shader include files.
http://docs.unity3d.com/Manual/SL-BuiltinIncludes.html,
2016.
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Thank you!
http://www.npruehs.de
https://github.com/npruehs
@npruehs
dev@npruehs.de

5 Minute Review Session
• What are the major stages of the graphics pipeline? In what
order are the stages arranged?
• Where do vertex and fragment programs fit into the
pipeline?
• What is a vertex? What is a fragment? Distinguish a
fragment from a pixel.
• What are shader semantics?
• What are shader parameters?
• Which transformations happen in the graphics pipeline from
geometric data in three dimensions and the final two-
dimensional image?
• The Phong shading model computes final color as the sum
of four lighting contributions – which ones?

Game Programming 12 - Shaders

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