OPENGL INTRODUCTION

林宏祥 2014/10/06

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TODAY WE WILL TALK ABOUT…

1. Rasterization and transformation implementation

2. OpenGL progress

RASTERIZATION

3D model

2D image

RASTERIZATION

transformation

clipping

scan conversion

suited for hardware acceleration

TRANSFORMATION

=> matrix vector multiplication

=> from object space to clipping space

Space transition: a vector is multiplied by a corresponding transformation matrix.

The series of space: a definition describe the rendering process

LOADING MODEL FILETriangle

126.000000 202.500000 0.000000 -0.902861 -0.429933 -0.00000012

89.459999 202.500000 89.459999 -0.637936 -0.431364 -0.63793612

88.211967 209.144516 88.211967 -0.703896 0.095197 -0.70389512

Triangle

88.211967 209.144516 88.211967 -0.703896 0.095197 -0.70389512

124.242210 209.144516 0.000000 -0.995496 0.094807 -0.00000012

126.000000 202.500000 0.000000 -0.902861 -0.429933 -0.00000012

Triangle

89.459999 202.500000 89.459999 -0.637936 -0.431364 -0.63793612

0.000000 202.500000 126.000000 -0.000000 -0.429933 -0.90286112

0.000000 209.144516 124.242210 -0.000000 0.094807 -0.99549612

Triangle

0.000000 209.144516 124.242210 -0.000000 0.094807 -0.99549612

88.211967 209.144516 88.211967 -0.703895 0.095197 -0.70389612

89.459999 202.500000 89.459999 -0.637936 -0.431364 -0.63793612

object coordinate

Rotation

Translation

MOVE AND ROTATE OBJECT

world coordinate

camera coordinate

camera parameters in : eye point, reference point, up vector

ADD CAMERA

NOW WE ENTER CLIPPING SPACE

Clipping space: a viewing volume in 3D space.

The viewing volume is related to projection model.

Orthogonal Projection

-Z

X

xcamerax

cameraxx

Perspective Projection

-Z

X

xcamera

x

d

z

dz

xx camera

Viewing volume of orthogonal projection

Near Clipping Plane

Far Clipping PlaneLeft Clipping Plane

Right Clipping Plane

Bottom Clipping Plane

Top Clipping Plane

x

y

-z

(camera coordinates)

Viewing volume of perspective projection

Far Clipping Plane

Near Clipping Plane

Left Clipping Plane

Right Clipping Plane

Bottom Clipping Plane

Top Clipping Plane

x

y

-z

(camera coordinates)

What is the transformation from camera space to clipping space?

Let’s see normalized device coordinates space.

NDC SPACE

NDC space: a viewing volume in 3D space (cuboid).

x

y

z

Note: NDC is a left-hand coordinate system (because of z-buffer)

enclosed by (in OpenGL system): xNDC = 1, xNDC=-1, yNDC=1, yNDC=-1, zNDC =1, zNDC=-1

FROM CLIPPING SPACETO NDC SPACE

viewing volume in clipping space == viewing volume in NDC space after the division operation.

(Clipping coordinates is also a left-hand coordinate system)

Viewing volume of orthogonal projection

nearVal

farValleft

right

bottom

top

x

y

-z

(camera coordinates)

1

1000

200

02

0

002

1camera

camera

camera

clipping

clipping

clipping

z

y

x

nearValfarVal

nearValfarVal

nearValfarVal

bottomtop

bottomtop

bottomtop

leftright

leftright

leftright

z

y

x

1NDC

NDC

NDC

z

y

x

NDC space

A CANONICAL VIEW

xNDC0

1-1

xcamera

rightleft

Viewing volume on NDC space

Viewing Volume on camera space

2

rightleft

1)(1

0x

leftright2

leftrightx

clippingcamera

same mapping in y direction and z direction

The z component will multiply (-1) for flipping.

zFar

zNear

Viewing volume of perspective projection

(camera coordinates)

x

y

-z

)2/tan(

1

fovyf fovy: the viewing angle in y direction

Perspective Projection

-Z

X

xcamera

x

d = 1

some scaling

-Z

X

d

z

dz

xx camera

NDC

A CANONICAL VIEW..

-Z

-zNear

-zFar

y0

zcamerafovy

Y

f

1

z-

y)

2

fovytan(

camera

0

-Z

-zNear

-zFar

y0

zcamerafovy

Y

After projection, the boundaries should map to 1, -1 )

fz-

(

yy

camera

cameraNDC

-Z

-zNear

-zFar

y0

zcamerafovy

Y

ycamera

Similarly in x, but consider the aspect.

)f

z-(aspect

xx

camera

cameraNDC

)f

z-(

yy

camera

cameraNDC

cz

m

camera

NDCz

cameraz

1NDCz

Mapping on z: inverse proportional to camera coordinates.

cz

m

camera

NDCz

-zFar is mapping to 1, -zNear is mapping to -1, then

czFar-

m1

czNear-

m1

zFar-zNear

zNearzFar c ,

zFar)-(zNear

zNearzFar2m

zFar-zNear

zNearzFar

)zFar)(-z-(zNear

zNearzFar2z

cameraNDC

)f

z-(

yy

camera

cameraNDC

)f

z-(aspect

xx

camera

cameraNDC

1

0100

200

100

100

camera

camera

camera

camera

clipping

clipping

clipping

z

y

x

zFarzNear

zNearzFar

zFarzNear

zNearzFarf

aspect

f

z

z

y

x

1

)/(

)/(

)/(

cameraclipping

cameraclipping

cameraclipping

zz

zy

zx

NDC space

FROM NDC SPACE TO WINDOW SPACE

02)1( xw

xx NDCwindow

02)1( yh

yy NDCwindow

(x0, y0): window position

(w, h): size of the window

SCAN CONVERSION

Vector to fragments

OPENGL PROGRESS

Fixed openGL pipeline (openGL 1.x)

Programmable openGL pipeline (above openGL 2.x)

Fixed openGL pipeline Programmable openGL pipeline

The programming becomes very different since modern hardware (GPU) has changed.

ADVANTAGE ON FIXED PIPELINE

Easy to learn: regard the pipeline as a black box.

DIS-ADVANTAGE ON FIXED PIPELINE PROGRAMMING

Debugging is hard if you do not know openGL pipeline.

(In modern graphic programming, we “must” understand the programmable system or we cannot programming easily)

Only provide specific functions, thus limiting the creativity and problem solving.

The programming on modern programmable GPU is totally different from that of fixed pipeline. (You must learn from the start).

DEPRECATED聲明不贊成 & EVIL OPENGL

OpenGL 4.2 Reference card: Blue means deprecated www.khronos.org/files/opengl42-quick-reference-card.pdf

Current version: OpenGL & GLSL 4.3

OpenGL 4.3 Reference card: Deprecated functions are gone!

http://www.khronos.org/files/opengl43-quick-reference-card.pdf

and many many many more…

…since 2008!(GLSL since 2004)

GPUs have changed!

(from “progressive openGL” 2012 slides)

RENDER LOOP(CLIENT)

initialize window

load shader program

clear frame buffer

Update transformation

Update Objects

Draw Object

SwapBuffers

while (running):

a linear array on GPU memory

SHADER DATA Uniform= Shared Constant

Vertex Data= ANYTHING YOU WANT!

Example?Positions…Normals…Colors…

Texture Coordinates…

“Per-object constant”(from “progressive openGL” slides, 2012)

SHADER DATA AND GLSL

OpenGL 4.2 Reference card: Blue means deprecated www.khronos.org/files/opengl42-quick-reference-card.pdf

“Per-object constant”

(from “progressive openGL” slides, 2012)

IN VS. OUT

Vertex Shader

in = Data from Vertex Buffer

out = Rasterizer in

FragmentShader

in = Rasterizer out

out = ANYTHING YOU WANT!

http://en.wikibooks.org/wiki/GLSL_Programming/Rasterization

http://en.wikibooks.org/wiki/GLSL_Programming/Rasterization

Rasterizer

GPU Memory

Fragment

Pixel… but usually pixel color and depth

(from “progressive openGL” slides, 2012)

EXAMPLE: DRAW AN TRIANGLE

CLIENT//declare a linear array on CPU memory

static const GLfloat g_vertex_buffer_data[] = {

-1.0f, -1.0f, 0.0f, (use xyz vector to represent a vertex)

1.0f, -1.0f, 0.0f,

0.0f, 1.0f, 0.0f, };

GLuint vertexbuffer;

// generate the vertex buffer object(VBO)

glGenBuffers(1, &vertexbuffer);

// bind VBO to GL_ARRAY_BUFFER, a state in OpenGL context

glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);

// allocate GPU memory and copy VBO content to the memory

glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);

CLIENTdo{

…

// Use shader program

glUseProgram(programID);

// Give an attribute id to the VBO ( a VBO can be assigned many attribute ids)

glEnableVertexAttribArray(0);

//bind VBO vertexbuffer again (in other application we may have many VBOs)

glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);

glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0 );//describe the content in VBO

// Draw the triangle

glDrawArrays(GL_TRIANGLES, 0, 3); // 3 indices starting at 0 -> 1 triangle

….

// Swap buffers

glfwSwapBuffers();

} while( ……);

(3 floats for a point) (offset) (padding)

(read 3 vertices once)

VERTEX SHADER

//specify GLSL version#version 330 core// Get vertex data from the VBO according to the vertex attribute id. The vertex data will stored in declared variable “vertex_position”layout(location = 0) in vec3 vertex_position;

void main(){// gl_Position is a built-in variable in GLSL, which is an output variable of the vertex shadergl_Position = vec4(vertex_position, 1.0);

}

note: vertex shader “must” output vertex position (in clipping coordinate space) to let OpenGL system perform scan conversion

FRAGMENT SHADER

#version 330//declare an output variable “color” to the imageout vec3 color;void main(){// output red color for each segmentcolor = vec3(1,0,0);}

Note:

In fragment shader, it receives a fragment in a “triangle” when vertex shader finish processing three vertices (remember GL_TRIANGLE in client code?)

The fragment is already in window coordinate here. We can derive the coordinate from the built-in variable for other application

REFERENCE

Dominik Seifert, “Progressive OpenGL” slides, 2012 ICG course.

Hong-Shiang Lin, “ICG clipping” slides, 2012 ICG course.

Jason L.McKesson, “Learning Modern 3D Graphics Programming” website. 2012