# CSE 167: Lecture #8: GLSL. Jürgen P. Schulze, Ph.D. University of California, San Diego Fall Quarter 2012

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1 CSE 167: Introduction to Computer Graphics Lecture #8: GLSL Jürgen P. Schulze, Ph.D. University of California, San Diego Fall Quarter 2012

2 Announcements Homework project #4 due Friday, November 2 nd Introduction: Oct 29 at 2:30pm This Friday: Late grading for project #3 Early grading for project #4 Midterm exam: Thursday, Oct 25, 2-3:20pm in classroom Midterm compatible index cards provided today and in instructor s office Reminder: Ted forums exist to discuss homework assignments and midterms 2

3 Review OpenGL s Shading Model 3

4 Complete Blinn-Phong Shading Model Blinn-Phong model with several light sources I All colors and reflection coefficients are vectors with 3 components for red, green, blue diffuse specular ambient 4

5 BRDFs Diffuse, Phong, Blinn models are instances of bidirectional reflectance distribution functions (BRDFs) For each pair of light directions L and viewing direction e, the BRDF returns the fraction of reflected light Shading with general BRDF f Many other forms of BRDFs exist in graphics, often named after inventors: Cook-Torrance, Ward, etc. 5

6 Lecture Overview OpenGL Light Sources Types of Geometry Shading Shading in OpenGL Fixed-Function Shading Programmable Shaders Vertex Programs Fragment Programs GLSL 6

7 Light Sources Real light sources can have complex properties Geometric area over which light is produced Anisotropy (directionally dependent) Reflective surfaces act as light sources (indirect light) OpenGL uses a drastically simplified model to allow real-time rendering 7

8 OpenGL Light Sources At each point on surfaces we need to know Direction of incoming light (the L vector) Intensity of incoming light (the c l values) Standard light sources in OpenGL Directional: from a specific direction Point light source: from a specific point Spotlight: from a specific point with intensity that depends on direction 8

9 Directional Light Light from a distant source Light rays are parallel Direction and intensity are the same everywhere As if the source were infinitely far away Good approximation of sunlight Specified by a unit length direction vector, and a color 9 Light source Receiving surface

10 Point Lights Similar to light bulbs Infinitely small point radiates light equally in all directions Light vector varies across receiving surface What is light intensity over distance proportional to? Intensity drops off proportionally to the inverse square of the distance from the light Reason for inverse square falloff: Surface area A of sphere: A = 4 π r 2 10

11 Point Lights in Theory p c src Light source At any point v on the surface: c l v c l Receiving surface v 11

12 Point Lights in OpenGL OpenGL model for distance attenuation: c l = Attenuation parameters: c src k c + k l p v + k q p v 2 k c = constant attenuation, default: 1 k l = linear attenuation, default: 0 k q = quadratic attenuation, default: 0 Default: no attenuation: c l =c src Change attenuation parameters with: GL_CONSTANT_ATTENUATION GL_LINEAR_ATTENUATION GL_QUADRATIC_ATTENUATION 12

13 Spotlights Like point source, but intensity depends on direction Parameters Position: location of the light source Spot direction: center axis of the light source Falloff parameters: Beam width (cone angle) The way the light tapers off at the edges of the beam (cosine exponent) 13

14 Spotlights Light source Receiving surface 14

15 Spotlights Photograph of real spotlight Spotlights in OpenGL 15

16 Lecture Overview OpenGL Light Sources Types of Geometry Shading Shading in OpenGL Fixed-Function Shading Programmable Shaders Vertex Programs Fragment Programs GLSL 16

17 Types of Geometry Shading Per-triangle Per-vertex Per-pixel 17

19 Per-Vertex Shading Known as Gouraud shading (Henri Gouraud, 1971) Interpolates vertex colors across triangles with Barycentric Interpolation Advantages Fast Smoother surface appearance than with flat shading Disadvantage Problems with small highlights 19

20 Per-Pixel Shading Also known as Phong Interpolation (not to be confused with Phong Illumination Model) Rasterizer interpolates normals (instead of colors) across triangles Illumination model is evaluated at each pixel Simulates shading with normals of a curved surface Advantage Higher quality than Gouraud shading Disadvantage Slow 20 Source: Penny Rheingans, UMBC

21 Gouraud vs. Per-Pixel Shading Gouraud has problems with highlights More triangles would improve result, but reduce frame rate Gouraud Per-Pixel 21

22 Lecture Overview OpenGL Light Sources Types of Geometry Shading Shading in OpenGL Fixed-Function Shading Programmable Shaders Vertex Programs Fragment Programs GLSL 22

23 Shading with Fixed-Function Pipeline Fixed-function pipeline only allows Gouraud (pervertex) shading We need to provide a normal vector for each vertex Shading is performed in camera space Position and direction of light sources are transformed by GL_MODELVIEW matrix If light sources should be in object space: Set GL_MODELVIEW to desired object-to-camera transformation Use object space coordinates for light positions More information:

24 Tips for Transforming Normals If you need to (manually) transform geometry by a transformation matrix M, which includes shearing or scaling: Transforming the normals with M will not work: transformed normals are no longer perpendicular to surfaces Solution: transform the normals differently: Either transform the end points of the normal vectors separately Or transform normals with Find derivation on-line at: OpenGL does this automatically if the following command is used: glenable(gl_normalize) 24

25 Lecture Overview OpenGL Light Sources Types of Geometry Shading Shading in OpenGL Fixed-Function Shading Programmable Shaders Vertex Programs Fragment Programs GLSL 25

27 Demo NVIDIA Froggy Features Bump mapping shader for Froggy s skin Physically-based lighting model simulating sub-surface scattering Supersampling for scene anti-aliasing Raytracing shader for irises to simulate refraction for wet and shiny eyes Dynamically-generated lights and shadows 27

29 Programmable Pipeline Scene Modeling and viewing transformation Shading Executed once per vertex: Vertex Shader Tessellation Shader Geometry Shader Projection Rasterization Fragment processing Frame-buffer access (z-buffering) Executed once per fragment: Fragment Shader 29 Image

30 Vertex Shader Executed once per vertex Cannot create or remove vertices Does not know the primitive it belongs to Replaces functionality for Model-view, projection transformation Per-vertex shading If you use a vertex program, you need to implement behavior for the above functionality in the program! Typically used for: Character animation Particle systems 30

31 Tessellation Shader Executed once per primitive Generates new primitives by subdividing each line, triangle or quad primitive Typically used for: Adapting visual quality to the required level of detail For instance, for automatic tessellation of Bezier curves and surfaces Geometry compression: 3D models stored at coarser level of resolution, expanded at runtime Allows detailed displacement maps for less detailed geometry 31

32 Geometry Shader Executed once per primitive (triangle, quad, etc.) Can create new graphics primitives from output of tessellation shader (e.g., points, lines, triangles) Or can remove the primitive Typically used for: Per-face normal computation Easy wireframe rendering Point sprite generation Shadow volume extrusion Single pass rendering to a cube map Automatic mesh complexity modification (depending on resolution requirements) 32

33 Fragment Shader A.k.a. Pixel Shader Executed once per fragment Cannot access other pixels or vertices Makes execution highly parallelizable Computes color, opacity, z-value, texture coordinates Typically used for: Per-pixel shading (e.g., Phong shading) Advanced texturing Bump mapping Shadows 33

34 Lecture Overview OpenGL Light Sources Types of Geometry Shading Shading in OpenGL Fixed-Function Shading Programmable Shaders Vertex Programs Fragment Programs GLSL 34

35 Vertex Programs Vertex Attributes From Application Uniform Parameters Vertex program To Rasterizer Output Variables 35

36 Vertex Attributes Declared using the attribute storage classifier Different for each execution of the vertex program Can be modified by the vertex program Two types: Pre-defined OpenGL attributes. Examples: attribute vec4 gl_vertex; attribute vec3 gl_normal; attribute vec4 gl_color; User-defined attributes. Example: attribute float myattrib; 36

37 Uniform Parameters Declared by uniform storage classifier Normally the same for all vertices Read-only Two types: Pre-defined OpenGL state variables User-defined parameters 37

38 Uniform Parameters: Pre-Defined Provide access to the OpenGL state Examples for pre-defined variables: uniform mat4 gl_modelviewmatrix; uniform mat4 gl_modelviewprojectionmatrix; uniform mat4 gl_projectionmatrix; uniform gl_lightsourceparameters gl_lightsource[gl_maxlights]; 38

39 Uniform Parameters: User-Defined Parameters that are set by the application Should not be changed frequently Especially not on a per-vertex basis! To access, use glgetuniformlocation, gluniform* in application Example: In shader declare uniform float a; Set value of a in application: GLuint p; int I = glgetuniformlocation(p, a ); gluniform1f(i, 1.0f); 39

40 Vertex Programs: Output Variables Required output: homogeneous vertex coordinates vec4 gl_position varying output variables Mechanism to send data to the fragment shader Will be interpolated during rasterization Fragment shader gets interpolated data Pre-defined varying output variables, for example: varying vec4 gl_frontcolor; varying vec4 gl_texcoord[]; Any pre-defined output variable that you do not overwrite will have the value of the OpenGL state. User-defined varying output variables, e.g.: varying vec4 vertex_color; 40

41 Lecture Overview OpenGL Light Sources Types of Geometry Shading Shading in OpenGL Fixed-Function Shading Programmable Shaders Vertex Programs Fragment Programs GLSL 41

42 Fragment Programs Fragment Data From Rasterizer Uniform Parameters Fragment program To Frame Buffer Output Variables 42

43 Fragment Data Changes for each execution of the fragment program Fragment data includes: Interpolated standard OpenGL variables for fragment shader, as generated by vertex shader, for example: varying vec4 gl_color; varying vec4 gl_texcoord[]; Interpolated varying variables from vertex shader Allows data to be passed from vertex to fragment shader 43

44 Uniform Parameters Same as in vertex programs 44

45 Output Variables Pre-defined output variables: gl_fragcolor gl_fragdepth OpenGL writes these to the frame buffer Result is undefined if you do not set these variables! 45

46 Lecture Overview OpenGL Light Sources Types of Geometry Shading Shading in OpenGL Fixed-Function Shading Programmable Shaders Vertex Programs Fragment Programs GLSL 46

47 GLSL Main Features Similar to C language attribute, uniform, varying storage classifiers Set of predefined variables Access to per-vertex, per-fragment data Access OpenGL state Built-in vector data types, vector operations No pointers No direct access to data or variables in your C++ code 47

48 Example: Treat normals as colors // Vertex Shader varying vec4 color; void main() { // Treat the normal (x, y, z) values as (r, g, b) color components. color = vec4(clamp(abs((gl_normal + 1.0) * 0.5), 0.0, 1.0), 1.0); } gl_position = ftransform(); // Fragment Shader varying vec4 color; void main() { gl_fragcolor = color; } 48

49 Creating Shaders in OpenGL 49 Source: Gabriel Zachmann, Clausthal University

50 Tutorials and Documentation OpenGL and GLSL specifications GLSL tutorials OpenGL Programming Guide (Red Book) OpenGL Shading Language (Orange Book) OpenGL API Reference Card 50

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