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LAB (1) : OpenGL Tutorial

2018. 03. 05

CS 380

Introduction to Computer Graphics

Reference : Foundations of 3D Computer Graphics, Steven J. Gortler

Goals

• Understand OpenGL pipeline

• Practice basic OpenGL programming

• Set up a project for OpenGL programs

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OpenGL with GLSL• OpenGL – Open Graphics Library

– Cross-language and multi-platform API for rendering vector graphics

• GLSL– OpenGL Shading Language

– Allow application programmers to express the processing that occurs at those programmable points of the OpenGL pipeline

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GLUT, GLEW

• GLUT : The OpenGL Utility Toolkit

– To open windows and to allow our program to

respond to mouse and keyboard events

• GLEW : The OpenGL Extension Wrangler Library

– To gain access to the latest features of OpenGL in

Windows

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OpenGL Installation (Windows)

• Download and install Visual Studio

• Download ‘asst0.zip’ and extract it

• After compilation of the project, the debug or release

folder would be created on the project folder

• Two files (glut32.dll, glew32.dll) in dll directory should

be copied into directory which the exe file is in (debug

or release folder)

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OpenGL Installation (Mac)• GLUT should be already installed in Mac• Install GLEW– Download GLEW, compile (by typing make), and

install (by typing make install)– http://glew.sourceforge.net/

• When linking your code, use following options-framework OpenGL -framework GLUT

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OpenGL Installation (Ubuntu)• Install glut & glew– sudo apt-get install freeglut3-dev – sudo apt-get install libglew-dev

• Compile with following options-lGL -lGLU -lglut –lGLEWex) g++ -o main main.cpp –lGL -lGLU -lglut -lGLEW

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First Screen

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OpenGL Pipeline

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Vertex Buffer Vertex Shader Assembler

RasterizerFragment ShaderFrame Buffer

On your display

Basic OpenGL• APIs

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Basic OpenGL• Main Program

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Basic OpenGL• Main Program

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Basic OpenGL• initGlutState()

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Callback Functions

• Whenever some events occur, callback functions

catch them and do some actions already defined

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Callback Functions• reshape();

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Vertex Buffer

Pass the program data to the OpenGL buffer

Determine the initial coordinates and colors of vertices

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Callback Functions

• Whenever some events occur, callback functions

catch them and do some actions already defined

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Display

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GLSL• Shader language for OpenGL• Different grammar along versions– asst1-gl2 for version 1.2, asst1-gl3 for version 1.3

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version 1.2

attribute

varying

vertex shader fragment shader

varyingprogram

version 1.3

in

out

vertex shader fragment shader

in

out

program

• version 1.2 uses built-in variables such as gl_FragColor for fragment output

change g_G12Compatible to false for activating 1.3

Vertex Shader

• Usually final positions of vertices on the screen

are determined in the vertex shader

• When draw function is called, each vertex data

in the vertex buffer passes through the vertex

shader

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Vertex Buffer Vertex Shader AssemblerAttributes

(in)

Uniform variables

gl_Position,Varying

variables(out)

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Vertex Shader

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Vertex Buffer Vertex Shader AssemblerAttributes

(in)

Uniform variables

gl_Position,Varying

variables(out)

Fragment Shader

• Final pixel colors on the screen are determined

in the fragment shader

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Fragment Shader Frame BufferVarying variables

(in)

Uniform variables

gl_FragColor, Screen color

(out)

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Fragment Shader

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Fragment Shader Frame BufferVarying variables

(in)

Uniform variables

gl_FragColor, Screen color

(out)

Exercise: Draw the First Polygon

• Vertices are stored in a vertex buffer

• Positions, color and other attributes

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struct SquareGeometry {static GLfloat sqPos[6 * 2] = {

-0.5, -0.5,0.5, 0.5,0.5, -0.5

};

static GLfloat sqCol[6 * 3] = {1, 1, 0,1, 1, 0,1, 1, 0

};..

}

Vertex positions (x, y) of the triangle

Colors (R,G,B) of the triangle

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Exercise: Draw Your Own Polygon

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e.g.)

Exercise: Keyboard Input

void keyboard(unsigned char key, int x, int y) {switch (key) {case 'h':cout << "Welcome to OpenGL World!\n " << " Please enjoy your

self!\n\n ";break;}glutPostRedisplay();

}

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• Callback functions are provided

• Add the following code into the ‘keyboard’ function

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Keyboard Input

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Mouse Input

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• Callback function are provided

• Add the code into the ‘mouse’ function

static void mouse(int button, int state, int x, int y) {

if (button == GLUT_LEFT_BUTTON) {

if (state == GLUT_DOWN) {

cout << "Mouse Left Button has been Clicked\n";

}

else {

cout << "Mouse Left Button has been Released\n";

}

}

glutPostRedisplay();

}

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Mouse Input

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Exercise 1• If you press ‘r’, ‘g’ or ‘b’, the color of the polygon changed to red,

green or blue.

• Then, if you click ‘p’, the color returned to the original color.

• Hint: modify shader

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r g boriginal

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Exercise 2• Change color over time

• Use “uTime” in shader code

• Use periodic function such as sin and cos.

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Exercise 3• If you click the ‘mouse left or right button’, then a

polygon should move to left or right.

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original right

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Exercise 3 (Hint)• In vertex shader, we can determine the final positions of vertices

• In fragment shader, we can determine the final colors of vertices

• You should modify the uniform variables provided in the shader file

static float g_vertexTemp = 0.0; // for vertex movingstatic float g_colorTemp = 0.0; // for changing color

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