Display list -- a group of OpenGL commands that have been stored for later execution.
Properties:
Why Use Display Lists?
Drawing a torus
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Rotate about
Rotate about |
| GLuint glGenLists(GLsizei range); |
| Allocates range number of contiguous, previously unallocated display-list indices. The integer returned is the index that marks the beginning of a contiguous block of empty display-list indices. The returned indices are all marked as empty and used, so subsequent calls to glGenLists() don't return these indices until they're deleted. Zero is returned if the requested number of indices isn't available, or if range is zero. |
| void glNewList (GLuint list, GLenum mode); |
| Specifies the start of a display list. OpenGL routines that are called subsequently (until glEndList() is called to end the display list) are stored in a display list, except for a few restricted OpenGL routines that can't be stored. (Those restricted routines are executed immediately, during the creation of the display list.) list is a nonzero positive integer that uniquely identifies the display list. The possible values for mode are GL_COMPILE and GL_COMPILE_AND_EXECUTE. Use GL_COMPILE if you don't want the OpenGL commands executed as they're placed in the display list; to cause the commands to be executed immediately as well as placed in the display list for later use, specify GL_COMPILE_AND_EXECUTE. |
| void glEndList (void); |
| Marks the end of a display list. |
Drawing Triangles
//list.cpp
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <stdlib.h>
GLuint listName;
static void init (void)
{
listName = glGenLists (1);
glNewList (listName, GL_COMPILE);
glColor3f (1.0, 0.0, 0.0); /* current color red */
glBegin (GL_TRIANGLES);
glVertex2f (0.0, 0.0);
glVertex2f (1.0, 0.0);
glVertex2f (0.0, 1.0);
glEnd ();
glTranslatef (1.5, 0.0, 0.0); /* move position */
glEndList ();
glShadeModel (GL_FLAT);
}
static void drawLine (void)
{
glBegin (GL_LINES);
glVertex2f (0.0, 0.5);
glVertex2f (15.0, 0.5);
glEnd ();
}
void display(void)
{
GLuint i;
glClear (GL_COLOR_BUFFER_BIT);
glColor3f (0.0, 1.0, 0.0); /* current color green */
for (i = 0; i < 10; i++) /* draw 10 triangles */
glCallList (listName);
drawLine (); /* is this line green? NO! */
/* where is the line drawn? */
glFlush ();
}
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
if (w <= h)
gluOrtho2D (0.0, 2.0, -0.5 * (GLfloat) h/(GLfloat) w,
1.5 * (GLfloat) h/(GLfloat) w);
else
gluOrtho2D (0.0, 2.0*(GLfloat) w/(GLfloat) h, -0.5, 1.5);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void keyboard(unsigned char key, int x, int y)
{
switch (key) {
case 27:
exit(0);
}
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(650, 50);
glutCreateWindow(argv[0]);
init ();
glutReshapeFunc (reshape);
glutKeyboardFunc (keyboard);
glutDisplayFunc (display);
glutMainLoop();
return 0;
}
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bitmap -- a rectangular array of 0s and 1s that serves as a drawing mask for a corresponding rectangular portion of the window.
Suppose you're drawing a bitmap and that the current raster color is red. Wherever there's a 1 in the bitmap, the corresponding pixel is replaced by a red pixel
If there's a 0 in the bitmap, the contents of the pixel are unaffected. The most common use of bitmaps is for drawing characters on the screen.
| void glBitmap(GLsizei width, GLsizei height, GLfloat xbo, GLfloat ybo, GLfloat xbi, GLfloat ybi, const GLubyte *bitmap); |
| Draws the bitmap specified by bitmap, which is a pointer to the bitmap image. The origin of the bitmap is placed at the current raster position. If the current raster position is invalid, nothing is drawn, and the raster position remains invalid. The width and height arguments indicate the width and height, in pixels, of the bitmap. The width need not be a multiple of 8, although the data is stored in unsigned characters of 8 bits each. Use xbo and ybo to define the origin of the bitmap (positive values move the origin up and to the right of the raster position; negative values move it down and to the left); xbi and ybi indicate the x and y increments that are added to the raster position after the bitmap is rasterized. |
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Example: Defining and and Using a Complete Font
//font.cpp
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <stdlib.h>
#include <string.h>
GLubyte space[] =
{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
GLubyte letters[][13] = {
{0x00, 0x00, 0xc3, 0xc3, 0xc3, 0xc3, 0xff, 0xc3, 0xc3, 0xc3, 0x66, 0x3c, 0x18},
{0x00, 0x00, 0xfe, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe},
{0x00, 0x00, 0x7e, 0xe7, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xe7, 0x7e},
{0x00, 0x00, 0xfc, 0xce, 0xc7, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc7, 0xce, 0xfc},
{0x00, 0x00, 0xff, 0xc0, 0xc0, 0xc0, 0xc0, 0xfc, 0xc0, 0xc0, 0xc0, 0xc0, 0xff},
{0x00, 0x00, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xfc, 0xc0, 0xc0, 0xc0, 0xff},
{0x00, 0x00, 0x7e, 0xe7, 0xc3, 0xc3, 0xcf, 0xc0, 0xc0, 0xc0, 0xc0, 0xe7, 0x7e},
{0x00, 0x00, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xff, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3},
{0x00, 0x00, 0x7e, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x7e},
{0x00, 0x00, 0x7c, 0xee, 0xc6, 0x06, 0x06, 0x06, 0x06, 0x06, 0x06, 0x06, 0x06},
{0x00, 0x00, 0xc3, 0xc6, 0xcc, 0xd8, 0xf0, 0xe0, 0xf0, 0xd8, 0xcc, 0xc6, 0xc3},
{0x00, 0x00, 0xff, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0},
{0x00, 0x00, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xdb, 0xff, 0xff, 0xe7, 0xc3},
{0x00, 0x00, 0xc7, 0xc7, 0xcf, 0xcf, 0xdf, 0xdb, 0xfb, 0xf3, 0xf3, 0xe3, 0xe3},
{0x00, 0x00, 0x7e, 0xe7, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xe7, 0x7e},
{0x00, 0x00, 0xc0, 0xc0, 0xc0, 0xc0, 0xc0, 0xfe, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe},
{0x00, 0x00, 0x3f, 0x6e, 0xdf, 0xdb, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0x66, 0x3c},
{0x00, 0x00, 0xc3, 0xc6, 0xcc, 0xd8, 0xf0, 0xfe, 0xc7, 0xc3, 0xc3, 0xc7, 0xfe},
{0x00, 0x00, 0x7e, 0xe7, 0x03, 0x03, 0x07, 0x7e, 0xe0, 0xc0, 0xc0, 0xe7, 0x7e},
{0x00, 0x00, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0xff},
{0x00, 0x00, 0x7e, 0xe7, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3},
{0x00, 0x00, 0x18, 0x3c, 0x3c, 0x66, 0x66, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3},
{0x00, 0x00, 0xc3, 0xe7, 0xff, 0xff, 0xdb, 0xdb, 0xc3, 0xc3, 0xc3, 0xc3, 0xc3},
{0x00, 0x00, 0xc3, 0x66, 0x66, 0x3c, 0x3c, 0x18, 0x3c, 0x3c, 0x66, 0x66, 0xc3},
{0x00, 0x00, 0x18, 0x18, 0x18, 0x18, 0x18, 0x18, 0x3c, 0x3c, 0x66, 0x66, 0xc3},
{0x00, 0x00, 0xff, 0xc0, 0xc0, 0x60, 0x30, 0x7e, 0x0c, 0x06, 0x03, 0x03, 0xff}
};
GLuint fontOffset;
void makeRasterFont(void)
{
GLuint i, j;
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
fontOffset = glGenLists (128);
for (i = 0,j = `A'; i < 26; i++,j++) {
glNewList(fontOffset + j, GL_COMPILE);
glBitmap(8, 13, 0.0, 2.0, 10.0, 0.0, letters[i]);
glEndList();
}
glNewList(fontOffset + ` `, GL_COMPILE);
glBitmap(8, 13, 0.0, 2.0, 10.0, 0.0, space);
glEndList();
}
void init(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glClearColor( 1.0, 1.0, 1.0, 1.0 );
glShadeModel (GL_FLAT);
makeRasterFont();
}
void printString(char *s)
{
glPushAttrib (GL_LIST_BIT);
glListBase(fontOffset);
glCallLists(strlen(s), GL_UNSIGNED_BYTE, (GLubyte *) s);
glPopAttrib ();
}
/* Everything above this line could be in a library
* that defines a font. To make it work, you've got
* to call makeRasterFont() before you start making
* calls to printString().
*/
void display(void)
{
GLfloat black[3] = { 0.0, 0.0, 0.0 };
glClear(GL_COLOR_BUFFER_BIT);
glColor3fv( black );
glRasterPos2i(20, 60);
printString("THE QUICK BROWN FOX JUMPS");
glRasterPos2i(20, 40);
printString( "OVER A LAZY DOG" );
glFlush ();
}
void reshape(int w, int h)
{
glViewport(0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho (0.0, w, 0.0, h, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
}
void keyboard(unsigned char key, int x, int y)
{
switch (key) {
case 27:
exit(0);
}
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(300, 100);
glutInitWindowPosition (100, 100);
glutCreateWindow(argv[0]);
init();
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutDisplayFunc(display);
glutMainLoop();
return 0;
}
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An image is similar to a bitmap, but instead of containing only a single bit for each pixel in a rectangular region of the screen, an image can contain much more information such as a complete ( R, G, B, A ) color of each pixel.
Image sources:
Manipulating Image data:
Pixel Data Flow:
| void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels); |
| Reads pixel data from the framebuffer rectangle whose lower-left corner is at (x, y) and whose dimensions are width and height and stores it in the array pointed to by pixels. format indicates the kind of pixel data elements that are read (an index value or an R, G, B, or A component value, as listed in Table below), and type indicates the data type of each element shown below. |
Table: Pixel Formats for glReadPixels() or glDrawPixels()
format Constant | Pixel Format |
|---|---|
GL_COLOR_INDEX | A single color index |
GL_RGB | A red color component, followed by a green color component, followed by a blue color component |
GL_RGBA | A red color component, followed by a green color component, followed by a blue color component, followed by an alpha color component |
GL_RED | A single red color component |
GL_GREEN | A single green color component |
GL_BLUE | A single blue color component |
GL_ALPHA | A single alpha color component |
GL_LUMINANCE | |
GL_LUMINANCE_ALPHA | A luminance component followed by an alpha color component |
GL_STENCIL_INDEX | A single stencil index |
GL_DEPTH_COMPONENT | A single depth component |
Table: Data Types for glReadPixels() or glDrawPixels()
type Constant | Data Type |
|---|---|
GL_UNSIGNED_BYTE | unsigned 8-bit integer |
GL_BYTE | signed 8-bit integer |
GL_BITMAP | single bits in unsigned 8-bit integers using the same format as glBitmap() |
GL_UNSIGNED_SHORT | unsigned 16-bit integer |
GL_SHORT | signed 16-bit integer |
GL_UNSIGNED_INT | unsigned 32-bit integer |
GL_INT | signed 32-bit integer |
GL_FLOAT | single-precision floating point |
| void glDrawPixels(GLsizei width, GLsizei height, GLenum format, GLenum type, const GLvoid *pixels); | Draws a rectangle of pixel data with dimensions width and height. The pixel rectangle is drawn with its lower-left corner at the current raster position. format and type have the same meaning as with glReadPixels(). The array pointed to by pixels contains the pixel data to be drawn. |
| void glCopyPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum buffer); |
Copies pixel data from the framebuffer rectangle whose lower-left corner is
at (x, y) and whose dimensions are width and height. The data is copied to a
new position whose lower-left corner is given by the current raster position.
buffer is either GL_COLOR, GL_STENCIL, or GL_DEPTH, specifying the
framebuffer that is used. glCopyPixels() behaves similarly to a
glReadPixels() followed by a glDrawPixels(), with the following translation
for the buffer to format parameter:
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Example: image.cpp -- displaying a checker board
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You may zoom and drag the board:
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