34. `OpenGL` Programming 2

Dated: 04-07-2025

#include "GL/glut.h"
#include <stdlib.h>

static void reshape(int w, int h) {
    glViewport(0, 0, w, h);
}

static void keyboard(unsigned char key, int x, int y) {
    switch(key) {
        case 27: // Escape
            exit(0);
            break;
    }
}

static void display() {
}

static void idle() {
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // glRotatef(0.5f, 0.0f, 1.0f, 0.0f);
    glBegin(GL_TRIANGLES);
    glVertex3f(0.5f, 0.2f, 0.0f);
    glVertex3f(0.5f, 0.0f, 0.0f);
    glVertex3f(0.0f, 0.0f, 0.0f);
    glEnd();

    glutSwapBuffers();
}

static void initGL() {
    float ratio = (float)640 / 480;
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45.0, ratio, 1.0, 100.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslated(0.0, 0.0, -5.0);
}

int main(int argc, char* argv[]) {
    // Initialize the GLUT library.
    glutInit(&argc, argv);

    // Set up the OpenGL rendering context.
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);

    // Create a window and set its width and height.
    glutCreateWindow("Deform");
    glutReshapeWindow(640, 480);

    // Initialize our data structures.
    initGL();

    // The keyboard function gets called whenever we hit a key.
    glutKeyboardFunc(keyboard);

    // The display function gets called whenever the window
    // requires an update or when we explicitly call
    // glutPostRedisplay()
    glutDisplayFunc(display);

    // The reshape function gets called whenever the window changes
    // shape.
    glutReshapeFunc(reshape);

    // The idle function gets called when there are no window
    // events to process.
    glutIdleFunc(idle);

    // Get the ball rolling!
    glutMainLoop();

    return 0;
}

`void glMatrixMode(GLenum mode);`

Specified which matrix¹ is the current one.

Value	Meaning
`GL_MODELVIEW`	Applies subsequent `matrix`¹ operations to the modelview matrix stack.
`GL_PROJECTION`	Applies subsequent `matrix`¹ operations to the projection matrix stack.
`GL_TEXTURE`	Applies subsequent `matrix`¹ operations to the texture `matrix`¹ stack.

Error code	Condition
`GL_INVALID_ENUM`	`mode` was not an accepted value.
`GL_INVALID_OPERATION`	`glMatrixMode` was called between a call to `glBegin` and the corresponding call to `glEnd`.

`void glLoadIdentity(void);`

Replaces current matrix¹ with identity matrix¹

\[ \begin{bmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \\ \end{bmatrix} \]

Error code	Condition
`GL_INVALID_OPERATION`	`glLoadIdentity` was called between a call to `glBegin` and the corresponding call to `glEnd`.

`void glTranslated(GLdouble x, GLdouble y, GLdouble z);`

Multiplies the current matrix¹ with a translation matrix.¹

`void glTranslatef(GLfloat x, GLfloat y, GLfloat z);`

Multiplies the current matrix¹ with a translation matrix.¹

\[ \begin{bmatrix} 1 & 0 & 0 & x \\ 0 & 1 & 0 & y \\ 0 & 0 & 1 & z \\ 0 & 0 & 0 & 1 \\ \end{bmatrix} \]

Use glPushMatrix and glPopMatrix to save and restore the untranslated coordinate system.²

Error code	Condition
`GL_INVALID_OPERATION`	`glTranslate` was called between a call to `glBegin` and `glEnd`.

`void gluPerspective(GLdouble fovy, GLdouble aspect, GLdouble zNear, GLdouble zFar);`

Term	Description
`fovy`	The field of view angle, in degrees, in the y-direction.
`aspect`	The aspect ratio that determines the field of view in the x-direction. The aspect ratio is the ratio of \(x\) (width) to \(y\) (height).
`zNear`	The distance from the viewer to the near clipping plane (always positive).
`zFar`	The distance from the viewer to the far clipping plane (always positive).

void glRotated(GLdouble angle, GLdouble x, GLdouble y, GLdouble z);
void glRotatef(GLfloat angle, GLfloat x, GLfloat y, GLfloat z);

Term	Description
`angle`	The angle of rotation, in degrees.
`x`, `y`, `z`	The x, y, and z coordinates of a `vector`,³ respectively.

These functions compute a matrix¹ that performs counter clock wise rotation of angle degrees about the vector³ from origin to \(\langle x, y ,z\rangle\).

Use glPushMatrix and glPopMatrix to save and restore the unrotated coordinate system.²

Error code	Condition
`GL_INVALID_OPERATION`	`glRotate` was called between a call to `glBegin` and `glEnd`.

`void glClearColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);`

Specifies the red, green, blue, and alpha values used by glClear to clear the color buffers, clamped to range \([0, 1]\).

Error code	Condition
`GL_INVALID_OPERATION`	`glClearColor` was called between a call to `glBegin` and the corresponding call to `glEnd`.

Following functions set the current color.

glColor3b
glColor3d
glColor3f
glColor3i
glColor3s
glColor3ub
glColor3ui
glColor3us
glColor4b
glColor4d
glColor4f
glColor4i
glColor4s
glColor4ub
glColor4ui
glColor4us
glColor3bv
glColor3dv
glColor3fv
glColor3iv
glColor3sv
glColor3ubv
glColor3uiv
glColor3usv
glColor4bv
glColor4dv
glColor4fv
glColor4iv
glColor4sv
glColor4ubv
glColor4uiv
glColor4usv

In particular, you can call glColor between a call to glBegin and the corresponding call to glEnd.

`void glClear(GLbitfield mask);`

Clears buffers to preset values.

Mask	Buffer to be Cleared
`GL_COLOR_BUFFER_BIT`	The buffers currently enabled for color writing.
`GL_DEPTH_BUFFER_BIT`	The depth buffer.
`GL_ACCUM_BUFFER_BIT`	The accumulation buffer.
`GL_STENCIL_BUFFER_BIT`	The stencil buffer.

The glClear function sets the bitplane area of the window to values previously selected by

glClearColor
glClearIndex
glClearDepth
glClearStencil
glClearAccum

You can clear multiple color buffers simultaneously by selecting more than one buffer at a time using glDrawBuffer.

Error code	Condition
`GL_INVALID_VALUE`	Any bit other than the four defined bits was set in `mask`.
`GL_INVALID_OPERATION`	`glClear` was called between a call to `glBegin` and `glEnd`.

The glBegin and glEnd functions delimit the vertices of a primitive or a group of like primitives.

void glBegin(GLenum mode);
void glEnd(void);

Primitive Type	Description
`GL_POINTS`	Treats each vertex as a single point. Vertex \(n\) defines point \(n\). \(N\) points are drawn.
`GL_LINES`	Each pair of vertices defines a line. Vertices \(2n+1\) and \(2n\) define line \(n\). \(N/2\) lines are drawn.
`GL_LINE_STRIP`	Connects vertices into a line strip. Vertices \(n\) and \(n+1\) define line \(n\). \(N-1\) lines are drawn.
`GL_LINE_LOOP`	Like `GL_LINE_STRIP`, but connects last vertex back to the first. Vertices \(n\) and \(n+1\), and then \(N\) and \(1\). \(N\) lines are drawn.
`GL_TRIANGLES`	Each group of 3 vertices forms a triangle. Vertices \(3n-2\), \(3n-1\), and \(3n\) define triangle \(n\). \(N/3\) triangles are drawn.
`GL_TRIANGLE_STRIP`	Connected group of triangles. For odd \(n\), vertices \(n\), \(n+1\), \(n+2\); for even \(n\), \(n+1\), \(n\), \(n+2\) define triangle \(n\). \(N-2\) triangles are drawn.
`GL_TRIANGLE_FAN`	Connected group of triangles sharing the first vertex. Vertices \(1\), \(n+1\), and \(n+2\) define triangle \(n\). \(N-2\) triangles are drawn.
`GL_QUADS`	Each group of 4 vertices defines a quad. Vertices \(4n-3\), \(4n-2\), \(4n-1\), and \(4n\) define quadrilateral \(n\). \(N/4\) quads are drawn.
`GL_QUAD_STRIP`	Connected group of quads. For each pair after the first, vertices \(2n-1\), \(2n\), \(2n+2\), and \(2n+1\) define quad \(n\). \(N\) quads are drawn.
`GL_POLYGON`	Draws a single convex polygon using all vertices \(1\) through \(N\).

You can use only a subset of OpenGL functions between glBegin and glEnd. The functions you can use are:

glVertex
glColor
glIndex
gINormal
glMaterial

Minimum number of vertices	Type of primitive
1	Point
2	Line
3	Triangle
4	Quadrilateral
3	Polygon

Modes that require a certain multiple of vertices are

GL_LINES
GL_TRIANGLES
GL_QUADS
GL_QUAD_STRIP

Error code	Condition
`GL_INVALID_ENUM`	`mode` was set to an unaccepted value.
`GL_INVALID_OPERATION`	A function other than `glVertex`, `glColor`, `glIndex`, `glNormal`, `glTexCoord`, `glEvalCoord`, `glEvalPoint`, `glMaterial`, `glEdgeFlag`, `glCallList`, or `glCallLists` was called between `glBegin` and the corresponding `glEnd`. The function `glEnd` was called before the corresponding `glBegin` was called, or `glBegin` was called within a `glBegin`/`glEnd` sequence.

These functions specify a vertex.

glVertex2d
glVertex2f
glVertex2i
glVertex2s
glVertex3d
glVertex3f
glVertex3i
glVertex3s
glVertex4d
glVertex4f
glVertex4i
glVertex4s
glVertex2dv
glVertex2fv
glVertex2iv
glVertex2sv
glVertex3dv
glVertex3fv
glVertex3iv
glVertex3sv
glVertex4dv
glVertex4fv
glVertex4iv
glVertex4sv

References

Read more about matrices. ↩↩↩↩↩↩↩↩↩↩↩↩
Read more about coordinate systems. ↩↩
Read more about vectors. ↩↩

34. OpenGL Programming 2

void glMatrixMode(GLenum mode);

void glLoadIdentity(void);

void glTranslated(GLdouble x, GLdouble y, GLdouble z);

void glTranslatef(GLfloat x, GLfloat y, GLfloat z);

void gluPerspective(GLdouble fovy, GLdouble aspect, GLdouble zNear, GLdouble zFar);

void glClearColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);

void glClear(GLbitfield mask);

References

34. `OpenGL` Programming 2

`void glMatrixMode(GLenum mode);`

`void glLoadIdentity(void);`

`void glTranslated(GLdouble x, GLdouble y, GLdouble z);`

`void glTranslatef(GLfloat x, GLfloat y, GLfloat z);`

`void gluPerspective(GLdouble fovy, GLdouble aspect, GLdouble zNear, GLdouble zFar);`

`void glClearColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);`

`void glClear(GLbitfield mask);`