Lines and textures will not show up together - c

Here is how I draw the line and I using mouse to draw the line
static struct
{
GLfloat p[MAX_POINTS][2];
GLuint point_cnt;
} contours [ MAX_CONTOURS ] ;
GLuint point_cnt_mouse;
point_cnt_mouse = contours[contour_cnt].point_cnt;
glColor3f( 0.0, 0.0, 0.0 );
glBegin(GL_LINES);
glLineWidth(5.0);
int i;
int j;
for(i = 0; i <= contour_cnt; i++)
{
GLuint point_cnt;
point_cnt = contours[i].point_cnt;
if (contours[i].point_cnt == 0)
{
glVertex2fv ( P );
glVertex2fv ( P );
}//if
else
{
for(j = 2; j <= point_cnt; j++)
{
glVertex2fv (contours[i].p[j-2]);
glVertex2fv (contours[i].p[j-1]);
}//for
}//else
}//for
if(point_cnt_mouse > 0)
{
glVertex2fv(contours[contour_cnt].p[point_cnt_mouse-1]);
glVertex2fv(P);
}//if
glEnd();
then I use glTexImage2D() to make GL_TEXTURE_2D then
my display is
void display()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glPushMatrix ();
glTranslatef(-4.0, 5.0, -6.0);
//this is box and load texture on it
drawPlane();
glPopMatrix();
glutSwapBuffers();
glFlush();
}
void myinit()
{
glClearColor(1.0, 1.0, 1.0, 1.0);
glEnable(GL_DEPTH_TEST);
//load png image
drawLogo();
glDisable(GL_DEPTH_TEST);
}
Logo won't show up with lines, why? Can any one tell what is wrong with my code?

Make sure to disable texturing (glDisable(GL_TEXTURE_2D)) before drawing your line(s). And re-enable (glEnable(GL_TEXTURE_2D)) before drawing your texture.
If you're using the default GL_MODULATE texture environment make sure to set the current color to white (glColor3ub(255,255,255)) before drawing with the texture. If you draw the texture after the glColor3f( 0.0, 0.0, 0.0 ) in your line routine then GL_MODULATE will multiply all your texel RGB values by zero, giving you black everywhere.

It looks kind of suspicious to me that your display() function never calls drawLogo().
void display()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glPushMatrix ();
glTranslatef(-4.0, 5.0, -6.0);
//this is box and load texture on it
drawPlane();
glPopMatrix();
glutSwapBuffers();
glFlush();
}

Related

Enlarging a cube and putting it into a 3D space

I've created a program to display a lined cube on a white canvas but I am unsure how to multiple that cube into lets say? 10 x 10.
Another question is how would I go about creating the same cube in a 3D space?
Here's my code:
void drawScene(void)
{
int i, j;
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0, 0.0, 0.0);
glLoadIdentity();
glTranslatef(0.0, 0.0, -25.0);
glutWireCube(5.0); // Box.
glColor3f(1.0, 0.0, 0.0);
for(i=5; i<5; i++)
{
for (j = -5; j < 5; j++)
{
glPushMatrix();
glTranslatef(i*5, j*5, -35.0);
glColor3f(1.0, 1.0, 0);
glutSolidCube(5.0);
glColor3f(0.0, 0.0, 1.0);
glutWireCube(5.0);
glPopMatrix();
}
}
glFlush();
}
void setup(void)
{
glClearColor(1.0, 1.0, 1.0, 0.0);
}
void resize (int w, int h)
{
glViewport(0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-10.0, 10.0, -10.0, 10.0, 10.0, 100.0);
glMatrixMode(GL_MODELVIEW);
}
void KeyInput(unsigned char key, int x, int y)
{
switch(key)
{
case 27:
exit(0);
break;
default:
break;
}
}
int main(int argc, char **argv)
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500,500); /* Size of the Program Window */
glutInitWindowPosition(100,100);
glutCreateWindow("Box.cpp");
setup();
glutDisplayFunc(drawScene);
glutReshapeFunc(resize);
glutKeyboardFunc(KeyInput);
glutMainLoop();
return 0;
}
The glu library has a lot of useful tidbits like gluLookAt( xfrom, yfrom, zfrom, xto, yto, zto, xup, yup, zup );
You can scale things using glScaled( factor ) / glScalef( factor )
You should poke around Google for some GL 1.X documentation.

Rotating object around itself in OpenGL

i'm having trouble rotating a room around itself. its center is at (5,5,0) so i thought if i translated the room and the objects in the room with glTranslatef(5,5,5) then glRotatef(rotateroom,0,0,1) and then draw the items and use glTranslate(-5,-5,0) it would rotate the room and everything in it around 5,5,0 but it seems to still be rotating around (0,0,0) and i'm not really sure what I'm doing wrong. Thanks for the help in advance.
void drawside(){
int i,j;
/*for (j = 0; j <= 8; j++) {
glBegin(GL_LINE_STRIP);
for (i = 0; i <= 30; i++)
glEvalCoord2f((GLfloat)i/30.0, (GLfloat)j/8.0);
glEnd();
glBegin(GL_LINE_STRIP);
for (i = 0; i <= 30; i++)
glEvalCoord2f((GLfloat)j/8.0, (GLfloat)i/30.0);
glEnd();
}*/
glEvalMesh2(GL_FILL, 0, 20, 0, 20);
}
void drawRoom(){
//floor
glBegin(GL_POLYGON);
glColor3f(1,1,1);
glNormal3f(0,0,0);
glVertex3f(0,0,0);
glNormal3f(0,10,0);
glVertex3f(0,10,0);
glNormal3f(10,10,0);
glVertex3f(10,10,0);
glNormal3f(10,0,0);
glVertex3f(10,0,0);
glEnd(
);
//wall
glBegin(GL_POLYGON);
glColor3f(0,0,1);
glNormal3f(0,10,0);
glVertex3f(0,10,0);
glNormal3f(0,10,10);
glVertex3f(0,10,10);
glNormal3f(10,10,10);
glVertex3f(10,10,10);
glNormal3f(10,10,0);
glVertex3f(10,10,0);
glEnd();
//wall2
glBegin(GL_POLYGON);
glColor3f(0,1,0);
glNormal3f(10,10,0);
glVertex3f(10,10,0);
glNormal3f(10,10,0);
glVertex3f(10,10,10);
glNormal3f(10,0,10);
glVertex3f(10,0,10);
glNormal3f(10,0,0);
glVertex3f(10,0,0);
glEnd();
}
void drawObjects(){
glColor3f(1,0,1);
glTranslatef(2,2,0);
if(conefill == 1)
glShadeModel(GL_FLAT);
else
glShadeModel(GL_SMOOTH);
glutSolidCone(cone,5,10,2);
glTranslatef(5,5,0);
glColor3f(1,0,0);
if(spherefill == 1)
glShadeModel(GL_FLAT);
else
glShadeModel(GL_SMOOTH);
glutSolidSphere(sphere,20,20);
}
void display(){
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(viewx,viewy, viewz,viewx +lx, 5, viewz + ly,0.0f, 0.0f, 1.0f);
//gluLookAt(viewx,viewy,viewz,headup,headright,5,0,0,1);
glClear(GL_COLOR_BUFFER_BIT);
GLfloat light_position[] = {-1.0,5.0,5.0,0.0};
//GLfloat light_direction[] = {-5,-5,-5};
//GLfloat amb[] = {0,0,0,1};
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
//glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION,light_direction);
//glLightfv(GL_LIGHT0, GL_AMBIENT,amb);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glPushMatrix();
glTranslatef(5,5,0);
glRotatef(rotateroom,0,0,1);
glPushMatrix();
drawRoom();
glPopMatrix();
glColor3f(0.0, 0.0, 1.0);
glPushMatrix();
glTranslatef(6,3,1);
glPushMatrix ();
drawside();
glPopMatrix();
glPushMatrix();
//glTranslatef(5,0,0);
glRotatef(90,1,0,0);
//glTranslatef(-5,0,0);
glTranslatef(0,.5,.5);
drawside();
glPopMatrix();
glPushMatrix();
//glTranslatef(5,0,0);
glRotatef(180,1,0,0);
//glTranslatef(-5,0,0);
glTranslatef(0,1,0);
drawside();
glPopMatrix();
glPushMatrix();
glRotatef(90,0,0,1);
glTranslatef(-.5,.5,0);
drawside();
glPopMatrix();
glPushMatrix();
glRotatef(270,0,0,1);
glTranslatef(.5,.5,0);
drawside();
glPopMatrix();
glPopMatrix();
drawObjects();
glTranslatef(-5,-5,0);
glPopMatrix();
glutSwapBuffers();
glFlush();
}
You only need to invert the order of the istruction:
glRotatef(rotateroom,0,0,1);
glTranslatef(5,5,0);
Whenever you trasform a point you must invert the sequence of trasformation you want to apply. This dipend on the way the transformation matrix are multiplicated.

Polygon Won't Show Up In The Screen

I am trying to draw a 6 sided obelisk using opengl.
Below is the code I use to model the bottom surface of the obelisk:
void drawObelisk() {
glColor3f(1.0,1.0,0.0);
glBegin(GL_POLYGON);
for ( x=0.0; x<2.0*pi/3; x=x+inc )
{
glVertex3f(0.3*cos(x),0.3*sin(x),0.0);
}
glEnd();
}
This code is used to render the model:
void myDisplay(void)
{
glEnable(GL_CULL_FACE);
glEnable(GL_DEPTH_TEST);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glPushMatrix();
drawObelisk();
glPopMatrix();
glFlush();
}
void SetupRC(void)
{
glClearColor(0.0, 0.0, 1.0, 1.0);
glOrtho(-6.0,6.0,-6.0,6.0,-6.0,6.0);
}
void main(void)
{
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(640,480);
glutInitWindowPosition(10,15);
glutCreateWindow("Test");
glutDisplayFunc(myDisplay);
SetupRC();
glutIdleFunc(idle);
glutMainLoop();
}
For certain reasons the polygon surface that I drew didn't show up on the screen once the model is rendered.
Give this a shot:
#include <GL/glut.h>
#include <math.h>
void drawObelisk()
{
double pi = 3.14159;
double inc = (2*pi) / 6;
glColor3f(1.0,1.0,0.0);
glBegin(GL_POLYGON);
for( double theta = 0.0; theta < 2*pi; theta += inc )
{
double scale = 2.0;
double x = scale * cos( theta );
double y = scale * sin( theta );
glVertex3d(x,y,0.0);
}
glEnd();
}
void myDisplay(void)
{
glEnable(GL_CULL_FACE);
glEnable(GL_DEPTH_TEST);
glClearColor(0.0, 0.0, 1.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-6.0,6.0,-6.0,6.0,-6.0,6.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glPushMatrix();
drawObelisk();
glPopMatrix();
glFlush();
}
void main( int argc, char** argv )
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(640,480);
glutInitWindowPosition(10,15);
glutCreateWindow("Test");
glutDisplayFunc(myDisplay);
glutMainLoop();
}
Things that were wonky:
Missing glutInit() call.
glClear()ing the depth buffer without requesting one via GLUT_DEPTH.
Not switching to GL_MODELVIEW after setting your projection matrix.
Original drawObelisk() for-loop only generated two vertices. GL_POLYGON needs a minimum of three.

Problems when dealing with lighting and shadowing in opengl

I am trying to put lights, materials and shadows to my robot arm but unfortunately something weird happens (please compile or see the below picture), now I am still annoying by
1) Not showing correct lighting and reflection properties as well as material properties
2) No shadow painted, although I have done the shadow casting in function "void showobj(void)"
I appreciate if anyone can help, I have already working for it 2 days with no progress :(
The following is my code
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <time.h>
#include <math.h>
#include <GL/glut.h>
#include "gsrc.h"
#include <Windows.h>
const double PI = 3.14159265;
// angles to rotate the base, lower and upper arms of the robot arm
static GLfloat theta, phi, psi = 0.0;
//Starting time
double startT;
//Time Diff variable
double dif,startTime,endTime,deltaT;
//define n
double n = 3;
//Set the parameters of the light number one
GLfloat Xs = 35.0;
GLfloat Ys = 35.0;
GLfloat Zs = 35.0;
//Shadow color
GLfloat shadowcolor[] = {0.0,0.0,0.0};
//initialize the window and everything to prepare for display
void init_gl() {
//set display color to white
glClearColor(1,1,1,0);
//clear and enable z-buffer
glClear (GL_DEPTH_BUFFER_BIT);
glEnable (GL_DEPTH_TEST);
//clear display window
glClear(GL_COLOR_BUFFER_BIT);
}
//Draw the base of the robot arm
void draw_base(){
glPushMatrix();
//to create the quadric objects
GLUquadric *qobj,*qobjl,*qobju;
qobj = gluNewQuadric();
qobjl = gluNewQuadric();
qobju = gluNewQuadric();
//set the color of the cylinder
glColor3f(1.0,0.0,0.0);
//Re-position the cylinder (x-z plane is the base)
glRotatef(-90,1.0,0.0,0.0);
//Draw the cylinder
gluCylinder(qobj, 30.0, 30.0, 40.0, 40.0, 40.0);
//Draw the upper disk of the base
gluDisk(qobju,0,30,40,40);
glPushMatrix();
//Change the M(lowdisk<updisk)
glTranslatef(0,0,40);
glColor3f(0,0,0);
//Draw the lower disk of the base
gluDisk(qobjl,0,30,40,40);
glPopMatrix();
glPopMatrix();
}
/***********************Texture Work Starts************************************/
//Load the raw file for texture
/* Global Declarations */
#define IW 256 // Image Width
#define IH 256 // Image Height
//3D array to store image data
unsigned char InputImage [IW][IH][4];
// Read an input image from a .raw file with double
void ReadRawImage ( unsigned char Image[][IH][4] )
{
FILE *fp;
int i, j, k;
char* filename;
unsigned char temp;
filename = "floor.raw";
if ((fp = fopen (filename, "rb")) == NULL)
{
printf("Error (ReadImage) : Cannot read the file!!\n");
exit(1);
}
for ( i=0; i<IW; i++)
{
for ( j=0; j<IH; j++)
{
for (k = 0; k < 3; k++) // k = 0 is Red k = 1 is Green K = 2 is Blue
{
fscanf(fp, "%c", &temp);
Image[i][j][k] = (unsigned char) temp;
}
Image[i][j][3] = (unsigned char) 0; // alpha = 0.0
}
}
fclose(fp);
}
/****************************Texture Work Ends***************************************/
/****************************Light and Shadows***************************************/
void lightsrc(){
GLfloat light1PosType [] = {Xs, Ys, Zs, 1.0};
//GLfloat light2PosType [] = {0.0, 100.0, 0.0, 0.0};
glLightfv(GL_LIGHT1, GL_POSITION, light1PosType);
//glEnable(GL_LIGHT1);
//glLightfv(GL_LIGHT2, GL_POSITION, light2PosType);
//glEnable(GL_LIGHT2);
GLfloat whiteColor[] = {1.0, 1.0, 1.0, 1.0};
GLfloat blackColor[] = {0.0, 0.0, 0.0, 1.0};
glLightfv(GL_LIGHT1, GL_AMBIENT, blackColor);
glLightfv(GL_LIGHT1, GL_DIFFUSE, whiteColor);
glLightfv(GL_LIGHT1, GL_SPECULAR, whiteColor);
glEnable(GL_LIGHT1);
glEnable( GL_LIGHTING );
}
/****************************Light and Shadows work ends***************************************/
//Draw the 2x2x2 cube with center (0,1,0)
void cube(){
glPushMatrix();
glTranslatef(0,1,0);
glutSolidCube(2);
glPopMatrix();
}
//Draw the lower arm
void draw_lower_arm(){
glPushMatrix();
glScalef(15.0/2.0,70.0/2.0,15.0/2.0);//scale half is enough (some part is in the negative side)
cube();
glPopMatrix();
}
//Draw the upper arm
void draw_upper_arm(){
glPushMatrix();
glScalef(15.0/2.0,40.0/2.0,15.0/2.0);//scale half is enough (some part is in the negative side)
cube();
glPopMatrix();
}
void drawCoordinates(){
glBegin (GL_LINES);
glColor3f (1,0,0);
glVertex3f (0,0,0);
glVertex3f (600,0,0);
glColor3f (0,1,0);
glVertex3f (0,0,0);
glVertex3f (0,600,0);
glColor3f (0,0,1);
glVertex3f (0,0,0);
glVertex3f (0,0,600);
glEnd();
}
//To draw the whole robot arm
void drawRobot(){
//Robot Drawing Starts
//Rotate the base by theta degrees
glRotatef(theta,0.0,1.0,0.0);
//Draw the base
draw_base();
//M(B<La)
glTranslatef(0.0,40.0,0.0);
//Rotate the lower arm by phi degree
glRotatef(phi,0.0,0.0,1.0);
//change the color of the lower arm
glColor3f(0.0,0.0,1.0);
//Draw the lower arm
draw_lower_arm();
//M(La<Ua)
glTranslatef(0.0,70.0,0.0);
//Rotate the upper arm by psi degree
glRotatef(psi,0.0,0.0,1.0);
//change the color of the upper arm
glColor3f(0.0,1.0,0.0);
//Draw the upper arm
draw_upper_arm();
//Drawing Finish
glutSwapBuffers();
}
void showobj(void) {
//set the projection and perspective parameters/arguments
GLint viewport[4];
glGetIntegerv( GL_VIEWPORT, viewport );
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective( 45, double(viewport[2])/viewport[3], 0.1, 1000 );
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(-200, 300, 200, 0, 0, 0, 0,1,0 );
// get the rotation matrix from the rotation user-interface
glMultMatrixf(gsrc_getmo() );
//Clear the display and ready to show the robot arm
init_gl();
//put the light source
lightsrc();
//Draw coordinates
drawCoordinates();
//give material properties
GLfloat diffuseCoeff[] = {0.2, 0.4, 0.9, 1.0}; // kdR= 0.2, kdG= 0.4, kdB= 0.9
GLfloat specularCoeff[] = {1.0, 1.0, 1.0, 1.0}; //
glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE, diffuseCoeff);
glMaterialfv(GL_FRONT_AND_BACK, GL_SPECULAR, specularCoeff);
glMaterialf(GL_FRONT_AND_BACK, GL_SHININESS, 100.0 ); // ns= 25
//Draw the ground floor
glColor3f(0.4,0.4,0.4);
glPushMatrix();
glRotatef(90,1,0,0);
glRectf(-500,-500,500,500);
glPopMatrix();
int i,j;
GLfloat M[4][4];
for (i=0; i<4; i++){
for (j=0; j<4; j++){
M[i][j] = 0;
}
M[0][0]=M[1][1]=M[2][2]=1;
M[2][3]=-1.0/Zs;
}
//Start drawing shadow
drawRobot(); // draw the objects
glPushMatrix( ); // save state
glMatrixMode(GL_MODELVIEW);
glTranslatef(Xs, Ys, Zs);// Mwc←s
glMultMatrixf(M[4]);// perspective project
glTranslatef(-Xs, -Ys, -Zs);// Ms←wc
glColor3fv (shadowcolor);
//Draw the robot arm
drawRobot();
glPopMatrix(); // restore state
//Shadow drawing ends
glFlush ();
}
//To animate the robot arm
void animate(void)
{
//get the end time
endTime = timeGetTime();
//float angle;
//calculate deltaT
deltaT = (endTime - startTime); //in msecs
//float test;
float deltaTSecs = deltaT/1000.0f; //in secs
//apply moving equation
psi = (90.0) * 0.50 * (1-cos((deltaTSecs/(n+1)) * PI));
glutPostRedisplay ();
}
void main (int argc, char** argv)
{
glutInit(&argc, argv);
//DOUBLE mode better for animation
// Set display mode.
glutInitDisplayMode( GLUT_DOUBLE | GLUT_RGB);
glutInitWindowPosition( 50, 100 ); // Set top-left display-window position.
glutInitWindowSize( 400, 300 ); // Set display-window width and height.
glutCreateWindow( "Robot arm : my first self-learning opengl program" ); // Create display window.
// Register mouse-click and mouse-move glut callback functions
// for the rotation user-interface.
//Allow user to drag the mouse and view the object
glutMouseFunc( gsrc_mousebutton );
glutMotionFunc( gsrc_mousemove );
//record the starting time
startTime = timeGetTime();
// Display everything in showobj function
glutDisplayFunc(showobj);
//Perform background processing tasks or continuous animation
glutIdleFunc(animate);
glutMainLoop();
}
your screen flashes because you are calling glutSwapBuffers() in drawRobot(). That makes your screen repaint two times, once when you draw the robot, and once more when you draw the shadow. Also, you are missing glPushMatrix() at the beginning of drawRobot() and glPopMatrix() at the end. You need to put it there, otherwise it will affect rendering afterwards (the shadow will move with the upper link of the arm).
Then, you specify the shadow matrix wrong. Let's try this:
int i,j;
GLfloat M[4][4];
for (i=0; i<4; i++){
for (j=0; j<4; j++){
M[i][j] = 0;
}
}
M[0][0]=M[1][1]=M[2][2]=1;
M[2][3]=-1.0/Zs;
drawRobot(); // draw the objects
//Start drawing shadow
glEnable(GL_CULL_FACE);
glDisable(GL_LIGHTING); // want constant-color shadow
glPushMatrix( ); // save state
glMatrixMode(GL_MODELVIEW);
glTranslatef(Xs, Ys, Zs);// Mwc←s
glMultMatrixf(&M[0][0]);// perspective project
glTranslatef(-Xs, -Ys, -Zs);// Ms←wc
glColor3fv (shadowcolor);
//Draw the robot arm
drawRobot();
glPopMatrix(); // restore state
glDisable(GL_CULL_FACE);
glEnable(GL_LIGHTING); // enable again ...
//Shadow drawing ends
Also, you can see i've added GL_CULL_FACE arround the shadow, it is to avoid depth fighting. This more or less fixes it technically.
But still - the shadow position is calculated incorrectly. Let's try looking at projection shadows.
So first, we need to have position for the ground plane and for the light:
float g[] = {0, 1, 0, 0}; // ground plane
float l[] = {20, 300, 50, 1}; // light position and "1"
That is a plane equation and a homogenous light position (normal 3D position, padded with a "1"). Then you throw away your shadow matrix setup (glTranslatef(), glMultMatrixf() and glTranslatef()) and call myShadowMatrix(g, l) instead, so it becomes:
glPushMatrix( ); // save state
glMatrixMode(GL_MODELVIEW);
float g[] = {0, 1, 0, 0}; // ground plane
float l[] = {20, 300, 50, 1}; // light position and "1"
myShadowMatrix(g, l);
glColor3fv (shadowcolor);
//Draw the robot arm
drawRobot();
glPopMatrix(); // restore state
And that mostly does work. There is still a lot of z-fighting going on, and the shadow has four different colors. As for the colors, stop calling glColor3f() in drawRobot(), as for the z-fighting, use this:
glPolygonOffset(-1, -1);
glEnable(GL_POLYGON_OFFSET_FILL);
// before
// draw shadow
glDisable(GL_POLYGON_OFFSET_FILL);
// afterwards
And that makes one nice planar shadows demo :). Cheers ...
sw.

Understanding opengl's (glut) resize

I'm reading the sample code for the bezier curve on the online version of opengl's tutorial.
I'm curious about how is resize being handled in the example cause I figure I might use it on my own version of this program , I placed my questions on its comments:
void reshape(int w, int h)
{
glViewport(0, 0, (GLsizei) w, (GLsizei) h); // what's GLsizei? Why is it called inside glViewPort?
glMatrixMode(GL_PROJECTION); // is it necessary to reset the projection matrix?
glLoadIdentity();
if (w <= h) // is this calculation universal, could I use it on another program?
glOrtho(-5.0, 5.0, -5.0*(GLfloat)h/(GLfloat)w,
5.0*(GLfloat)h/(GLfloat)w, -5.0, 5.0);
else
glOrtho(-5.0*(GLfloat)w/(GLfloat)h,
5.0*(GLfloat)w/(GLfloat)h, -5.0, 5.0, -5.0, 5.0);
glMatrixMode(GL_MODELVIEW); // why do I set to GL_MODELVIEW at the end?
glLoadIdentity(); // why does it get a reset?
}
Full code:
#include <stdlib.h>
#include <GL/glut.h>
GLfloat ctrlpoints[4][3] = {
{ -4.0, -4.0, 0.0}, { -2.0, 4.0, 0.0},
{2.0, -4.0, 0.0}, {4.0, 4.0, 0.0}};
void init(void)
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glShadeModel(GL_FLAT);
glMap1f(GL_MAP1_VERTEX_3, 0.0, 1.0, 3, 4, &ctrlpoints[0][0]);
glEnable(GL_MAP1_VERTEX_3);
}
void display(void)
{
int i;
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 1.0, 1.0);
glBegin(GL_LINE_STRIP);
for (i = 0; i <= 30; i++)
glEvalCoord1f((GLfloat) i/30.0);
glEnd();
/* The following code displays the control points as dots. */
glPointSize(5.0);
glColor3f(1.0, 1.0, 0.0);
glBegin(GL_POINTS);
for (i = 0; i < 4; i++)
glVertex3fv(&ctrlpoints[i][0]);
glEnd();
glFlush();
}
void reshape(int w, int h)
{
glViewport(0, 0, (GLsizei) w, (GLsizei) h); // what's GLsizei? Why is it called inside glViewPort?
glMatrixMode(GL_PROJECTION); // is it necessary to reset the projection matrix?
glLoadIdentity();
if (w <= h) // is this calculation universal, could I use it on another program?
glOrtho(-5.0, 5.0, -5.0*(GLfloat)h/(GLfloat)w,
5.0*(GLfloat)h/(GLfloat)w, -5.0, 5.0);
else
glOrtho(-5.0*(GLfloat)w/(GLfloat)h,
5.0*(GLfloat)w/(GLfloat)h, -5.0, 5.0, -5.0, 5.0);
glMatrixMode(GL_MODELVIEW); // why do I set to GL_MODELVIEW at the end?
glLoadIdentity(); // why does it get a reset?
}
void keyboard(unsigned char key, int x, int y)
{
switch (key) {
case 27:
exit(0);
break;
}
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow (argv[0]);
init ();
glutDisplayFunc(display);
glutReshapeFunc(reshape); // el reshape funciona correctamente
glutKeyboardFunc (keyboard);
glutMainLoop();
return 0;
}
GLsizei is a type, not a function. Therefore, it's not being called; its being casted; it's converting an int into a GLsizei. It's basically an int, but it makes it clear that you're using it for a size (I think). (I'm a Python programmer, so this is a guess)
glMatrixMode(...);
This does not reset the matrix, unless it is followed by a glLoadIdentity() call. Instead, it sets the current target of OpenGL's matrix operations to the matrix specified. This way, OpenGL knows what matrix you're trying to affect.
I've seen that calcuation before in the OpenGL superbible (or at least something like it); why don't you talk yourself through it and see what it's deciding and why? (Hint (I think): it's trying to keep the viewport square to avoid stretching)
At the end, it sets it back to GL_MODELVIEW since that matrix handles transformations, like translation and rotation, which are used to position vertices onscreen. It should be reset every frame so that your code can assume that the coordinate plane is currently at (0,0); that simplifies calculations about where to translate to.
Also note that reshape is not called by OpenGL, but by GLUT. OpenGL is platform-independent and does not handle windowing; this is why you need GLUT.
If you're new to OpenGL, you should work through it from the beginning - later tutorials will assume this sort of knowledge.

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