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Graphics Systems and OpenGLPowerPoint Presentation

Graphics Systems and OpenGL

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## PowerPoint Slideshow about ' Graphics Systems and OpenGL' - beck-patel

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Presentation Transcript

Business of Generating Images

- Images are made up of pixels

RGB

RGB Color cube (what we use in computer graphics)

Other color spaces include HSV, YUV, YCrCb, and YIQ

The “goal” of computer graphics

- Solve the function
- Red @ a pixel is f(i,j)=…
- Green @ a pixel is f(i,j)=…
- Blue @ a pixel is f(i,j)=…

Two Dimensional Images

+Y

- Images (at least the ones in this class) are two dimensional shapes.
- The two axes we will label as X (horizontal), and Y (vertical).

Y

Axis

(0,0)

X Axis

+X

Hardware Pipeline

Input

Computation

Output

We want to draw a rectangle, how do we describe it to a computer?

Model (n) - object description that a computer understands.

Partition the space

1. Define a set of points (vertices) in 2D space.

2. Given a set of vertices, draw lines between consecutive vertices.

(7,9)

(14,9)

(7,3)

(14,3)

Vertex (pl. Vertices) - a point in 2 or 3 dimensional space.

Record every position

Bitmap - a rectangular array of bits mapped one-to-one with pixels.

Representing Objects

- Most common method is the VERTEX method. Define the object as a set of points with connectivity information.
- Why is connectivity important?

Connectivity - information that defines which vertices are connected to which other vertices via edges.

Edge - connects two vertices

A Simple Program

Generate a square on a solid background

simple.c

#include <GL/glut.h>

void mydisplay(){

glClear(GL_COLOR_BUFFER_BIT);

glBegin(GL_QUADS);

glVertex2f(-0.5, -0.5);

glVertex2f(-0.5, 0.5);

glVertex2f(0.5, 0.5);

glVertex2f(0.5, -0.5);

glEnd();

glFlush();

}

int main(int argc, char** argv){

glutCreateWindow("simple");

glutDisplayFunc(mydisplay);

glutMainLoop();

}

Practical Approach

- Process objects one at a time in the order they are generated by the application
- Pipeline architecture:
- All steps can be implemented in hardware on the graphics card

Input

Computation

Output

application

program

display

simple.c

#include <GL/glut.h>

void mydisplay(){

glClear(GL_COLOR_BUFFER_BIT);

glBegin(GL_QUADS);

glVertex2f(-0.5, -0.5);

glVertex2f(-0.5, 0.5);

glVertex2f(0.5, 0.5);

glVertex2f(0.5, -0.5);

glEnd();

glFlush();

}

int main(int argc, char** argv){

glutCreateWindow("simple");

glutDisplayFunc(mydisplay);

glutMainLoop();

}

Vertex Processing

- Much of the work in the pipeline is in converting object representations from one coordinate system to another
- Object coordinates
- Camera (eye) coordinates
- Screen coordinates

- Every change of coordinates is equivalent to a matrix transformation
- Vertex processor also computes vertex colors

Projection

- Projection is the process that combines the 3D viewer with the 3D objects to produce the 2D image
- Perspective projections: all projectors meet at the center of projection
- Parallel projection: projectors are parallel, center of projection is replaced by a direction of projection

Primitive Assembly

Vertices must be collected into geometric objects before clipping and rasterization can take place

- Line segments
- Polygons
- Curves and surfaces

Clipping

Just as a real camera cannot “see” the whole world, the virtual camera can only see part of the world or object space

- Objects that are not within this volume are said to be clipped out of the scene

Rasterization

- If an object is not clipped out, the appropriate pixels in the frame buffer must be assigned colors
- Rasterizer produces a set of fragments for each object
- Fragments are “potential pixels”
- Have a location in frame bufffer
- Color and depth attributes

- Vertex attributes are interpolated over objects by the rasterizer

Fragment Processing

- Fragments are processed to determine the color of the corresponding pixel in the frame buffer
- Colors can be determined by texture mapping or interpolation of vertex colors
- Fragments may be blocked by other fragments closer to the camera
- Hidden-surface removal

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