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Preview of 3-D Graphics

This preview provides an overview of 3-D graphics concepts, including changing the display based on keyboard input, automatically changing the display, and quick solutions for viewing, hidden-surface removal, lighting, and smooth animation.

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Preview of 3-D Graphics

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  1. Preview of 3-D Graphics Glenn G. ChappellCHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 381 Lecture Notes Wednesday, September 17, 2003

  2. Review:intro2d.cpp • GLUT calls the keyboard function whenever an ASCII keypress happens. • Use the GLUT “special” function for non-ASCII (like arrow keys). • The ASCII value of the keypress is in key. Mouse position is in x, y. • Your keyboard (or special) function will usually be one big switch. • GLUT calls the idle function whenever nothing else needs doing. • It is useful for making time-dependent changes to the scene. • Think “animation”. CS 381

  3. Review:Making a Changing Display [1/3] • We looked at how to make the display change: • Based on keyboard input. • Using the keyboard (or special) function. • Automatically. • Using the idle function. • These ideas will be covered in greater detail in chapter 3. • Coming up on Friday. CS 381

  4. Review:Making a Changing Display [2/3] • To add keypress-based display changes to a GLUT program: • A global variable is needed to hold the current state of whatever part of the display is to change. • Declare the variable. • Initialize this variable to an appropriate value somewhere. • In its declaration? • In the init function? • In the display function: • Use the value of this variable. • Draw whatever should be drawn, according to the current value of the variable. • Do not change the variable in the display function. • In the keyboard function, when the appropriate key is pressed: • Change the value of the variable. • Call “glutPostRedisplay();”. • Do not call the display function. CS 381

  5. Review:Making a Changing Display [3/3] • To add automatic display changes to a GLUT program: • So it just like keyboard input, except: • Modify the idle function instead of the keyboard function. • Do not worry about the ASCII value of a key. • Doing this kind of programming may require a new mindset: • Remember to handle events in the proper callbacks. • Display does display, keyboard handles ASCII keypresses, etc. • Remember that your functions can be called in just about any order! • Again, we’ll look at this in more detail in chapter 3. CS 381

  6. 3-D Preview:Introduction • From a certain point of view, 3-D graphics is easy. • Just use 3 coordinates instead of 2 in your glVertex* commands. • In practice, it is much trickier. • We will be studying 3-D CG in detail starting with chapter 4. For now, a preview. CS 381

  7. 3-D Preview:Three Issues in 3-D CG • Viewing and Transformations • How do we look at a scene from any point of view within the scene, looking in any direction? • How do we do perspective projection? • How do we rotate things about arbitrary axes in 3-D? • Hidden-Surface Removal • When one object is behind another, we don’t want to see the hidden one. • Then there is transparency … • Lighting • 3-D CG is lousy without lighting. How do we do it? • In addition, we want to do smooth animation. CS 381

  8. 3-D Preview:Quick & Dirty Solutions [1/4] • Viewing & Transformations • By default, you are looking in the –z direction. • So the +z axis points toward you. • More negative values are farther away. • The rest will have to wait a little. • See sample3d.cpp for a few hints. CS 381

  9. 3-D Preview:Quick & Dirty Solutions [2/4] • Hidden-Surface Removal • Add GLUT_DEPTH to your glutInitDisplayMode call. • This allocates a depth buffer. • Remember, the various constants are bitwise-or’ed together. • Add GL_DEPTH_BUFFER_BIT to your glClear call. • This clears the depth buffer. • Again, bitwise-or. • Put glEnable(GL_DEPTH_TEST) somewhere in your initialization. • This enables hidden-surface removal. CS 381

  10. 3-D Preview:Quick & Dirty Solutions [3/4] • Lighting • This is tough to explain quickly. • For now, either • Ignore lighting, and make polygons lots of different colors, so you can tell them apart. • Or, base your programs on sample3d.cpp (or something similar), and use the GLUT built-in shape functions. • Like glutSolidTorus. CS 381

  11. 3-D Preview:Quick & Dirty Solutions [4/4] • Smooth Animation • Use double buffering. • Change GLUT_SINGLE to GLUT_DOUBLE in your glutInitDisplayMode call. • Change glFlush to glutSwapBuffers in your display callback. • Now the user only sees completed frames. • No flicker in animation. • Faster sometimes, slower others. • User cannot see the frame being built. • Sometimes this is bad, as with some complex fractals. CS 381

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