OpenGL Introduction

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Introduction

OpenGL

OpenGL is an API for computer graphics.

• Hardware-independent
  • Windowing or getting input is not included in the API
• Low-level
  • Only knows about triangles (kind of, but also: points, lines, polygons)
  • Libraries are often built on top of OpenGL
• State machine
  • The various API calls change the OpenGL state, query some part of that state, or cause OpenGL to use its current state to render something

GLUT (OpenGL Utility Toolkit) or FreeGLUT

API for creating a display window and using keyboard/mouse interations.

• See RayWindow.cpp to see how these are used for Assignment3

GLU (OpenGL Utility Library)

Provides commonly used functions on top of OpenGL.

• Viewing/Camera functions
  • gluLookAt (Sets the view matrix that transforms model coordinates to world coordinates)
  • gluPerspective (Sets up perspective projection given a field of view and aspect ratio) -- (RayWindow::DisplayFunction)
• Higher level shape drawing functions
  • gluCylinder
  • gluSphere

Other

• GLEW - OpenGL Extension Wrangler
  • Helps loading extensions
• GLFW
  • Similar to GLUT and FreeGLUT
• Many more

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Tutorial 1 - Hello World! (First Window)

• glutInit
  • Primary initialization routine
• glutInitDisplayMode
  • Creates windows with certain types of windows
• glutCreateWindow
  • Creates the window with the provided name
• glutReshapeWindow
  • Changes the size of the window to the width and height provided
• glutDisplayFunc
  • Sets a callback function for the window. GLUT determines when the display callback should be triggered based on the window's redisplay state (can set explicitly by calling glutPostRedisplay).
• glutMainLoop
  • Starts the GLUT event processing loop, all registered callbacks will now get called.

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Tutorial 2 - First Rendering

• glClearColor
  • Establishes what color the window will be cleared to (background color)
• glClear
  • Actually clears the window (by clearing the framebuffers)
• glGetFloatv
  • Get values from the current OpenGL state
• GL_MODELVIEW_MATRIX, GL_PROJECTION_MATRIX
  • Model coordinates (vertices defined relative to the model/object)
    • Model Matrix
  • World coordinates (vertices defined relative to the center of the world)
    • View Matrix
  • Camera coordinates (vertices defined relative to the camera)
    • Projection Matrix
  • Homogeneous coordinates (vertices defined relative to the camera's perspective)
• glBegin & glEnd
  • Define what type of objects to draw
  • Possible types: GL_POINTS, GL_LINES, GL_LINE_STRIP, GL_LINE_LOOP, GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_QUADS, GL_QUAD_STRIP, and GL_POLYGON
• glColor3f
  • Set the color for a vertex (or any object)
  • All objects after this call will use this color, need to call it again to change
• glVertex3f
  • Vertices of a shape
• glutSwapBuffers
  • Tells the window to use the current buffers in the OpenGL state (calls glFlush)

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Tutorial 3 - Model View and Projection Matrices

• glMatrixMode
  • Specifies the matrix that will be operated on
• glLoadIdentity
  • Replaces the specified current matrix with the identity matrix
• gluPerspective
  • For setting the perspective transformation (projection matrix)
  • 4 arguments:
    • fovy - field of view (degrees) in y direction
    • aspect - aspect ratio (width/height)
    • zNear - near clipping plane
    • zFar - far clipping plane
• gluLookAt
  • For setting the viewing transformation (view matrix)
  • 3 sets of arguments:
    • Eye x,y,z - location of the camera (3D point)
    • Center/Reference x,y,z - where camera is looking (3D point)
    • Up x,y,z - rotation of the camera (3D vector)
  • You want to move forward --> What is the direction vector (forward vector) of the camera???
  • You want to move right --> What is the vector pointing to the right of the camera???
• glRotatef
  • Rotates the current object angle degrees around the vector x,y,z
• glTranslatef
  • Translates in the given x,y,z direction

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Tutorial 4 - Transformation Stack

• glPushMatrix
  • Pushes the current matrix stack down by one, duplicating the current matrix
• glPopMatrix
  • Pops the current matrix stack, replacing the current matrix with the one below it
• Explanation:
  • You are applying transformations to the modelview matrix (with glTranslate or glRotate) and then you are in some good reference position. You can push a duplicate matrix onto the stack, perform a transformation on that, and then pop it off and you are back at that reference position.
  • This avoids having to either make backwards transformations to get back to your reference position or making the whole original sequence of transformations again.

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Tutorial 5, 6 - Call lists

Not used in Assignment 3 anymore

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Tutorial 7 - Lighting

• glEnable
  • Enables a state variable
• glEnable(GL_LIGHTING)
  • If not enabled (or glDisable is called) then everything is colored according to the glColor calls made
• glEnable(GL_LIGHT0)
• glLightfv
  • Sets parameters for a light
  • Parameters:
    • light - which light it is
    • pname - which parameter name to change for the light
    • params - the value(s) of the parameters
  • pname options: (GL_AMBIENT, GL_DIFFUSE, GL_SPECULAR, GL_POSITION, GL_SPOT_CUTOFF, GL_SPOT_DIRECTION, GL_SPOT_EXPONENT, GL_CONSTANT_ATTENUATION, GL_LINEAR_ATTENUATION, and GL_QUADRATIC_ATTENUATION)
• glMaterialfv
  • Sets parameters for a material (with respect to a light)
  • Parameters:
    • face - face being updated (GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK)
    • pname - which material parameter to change
    • params - the value(s) of the parameters
  • pname options: (GL_AMBIENT, GL_DIFFUSE, GL_SPECULAR, GL_EMISSION, GL_SHININESS, GL_AMBIENT_AND_DIFFUSE, or GL_COLOR_INDEXES)

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Tutorial 8 - Shading

• glShadeModel
  • GL_SMOOTH: interpolate colors across vertices
  • GL_FLAT: solid color over the whole polygon
• glNormal
  • You have to specify the normals at each vertex
  • This means you have to perform the calculation (derivatives)

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Tutorial 9 - Texture

• glGenTextures
  • Create a texture object
• glBindTexture
  • Name a texture object
• glTexImage2D
  • Set the texture map
  • Parameters:
    • Target texture type
    • Level of detail (mipmap reduction)
    • Number of color components
    • Size (width, height)
    • Border
    • Format, data type of the pixels
    • Pointer to the image data in memory (pixels)
• glEnable(GL_TEXTURE_2D)
  • Enable 2D texture mapping
• glTexEnvi
  • How the final color of a fragment is chosen
  • Decal: texture painted on top of the surface (don't worry about original color)
  • Modulate: multiplies the original color with the texture (specular highlights will show through)
  • Others...
• glTexParameteri
  • How to wrap the texture if the image isn't the same size as the fragment
• glTexCoord2d
  • Sets the texture coordinates on the texture
  • Then, the next specified vertex coordinates will have those coordinates

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Tutorial 10 - Depth Test

• GL_DEPTH_BUFFER_BIT
  • Stores the distance from the view plane
• glEnable(GL_DEPTH_TEST)
  • When filling in pixels, comparison is done with the existing depth value at the pixel being drawn

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Tutorial 11 - Transparency

• glEnable(GL_BLEND)
  • Enable blending to blend a pixel with an alpha value with the pixel already drawn at that location
• glBlendFunc
  • Specifies how the source (current pixel to draw) should be computed and how the destination (existing pixel) should be computed