CODEWITHSUNDEEP
c++opengl
alhelal
alhelal

Reputation: 936

glBegin(GL_LINES) works but glBegin(GL_POINTS) doesn't work correctly?

I have following code to read data from Lion.bmp and show the image. 

#include<GL/gl.h>
#include<GL/glut.h>
#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<unistd.h>

unsigned char data[1000000]; // allocate 3 bytes per pixel
int size;
int width;
int height;

void
drawRGB()
{
    glClearColor(0, 0, 0, 1);
    glClear(GL_COLOR_BUFFER_BIT);
    int i,j;
    for(i=0;i<height;i++)
    {
        for(j=0;j<width;j++)
        {
            /* double R=data[3 * (i * width + j)]/255.0; */
            /* double G=data[3 * (i * width + j) + 1]/255.0; */
            /* double B=data[3 * (i * width + j) + 2]/255.0; */
            /* printf("R=%lf G=%lf B=%lf\n",R,G,B); */
            printf("i=%d j=%d\n",i,j);
            glColor3f(1,0,0);
            glBegin(GL_POINTS);
            glVertex2i(i,j);
            glEnd();
            glFlush();
        }
    }
}

void readBMP(char* filename)
{
    printf("inside readBMP\n");
    int i;
    FILE* f = fopen(filename, "rb");
    unsigned char info[54];
    fread(info, sizeof(unsigned char), 54, f); // read the 54-byte header

    // extract image height and width from header
    width = *(int*)&info[18];
    height = *(int*)&info[22];
    printf("widht = %d height= %d\n",width, height);

    int size = 3 * width * height;
    fread(data, sizeof(unsigned char), size, f); // read the rest of the data at once
    fclose(f);

    for(i = 0; i < size; i += 3)
    {
            unsigned char tmp = data[i];
            data[i] = data[i+2];
            data[i+2] = tmp;
    }
    printf("last in readBMP\n");
}


void
init(void)
{
    /*initialize the x-y co-ordinate*/

    /* glClearColor(0,0,0,0); */
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, width,0, height);
    /* glClear(GL_COLOR_BUFFER_BIT); */
    glFlush();
}



int
main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    /* glutInitWindowSize(1000, 1000); */
//  glutInitWindowPosition(400, 400);
    readBMP("Lion.bmp"); 
    init();
    printf("after\n");
    glutCreateWindow("RGB");
    glutDisplayFunc(drawRGB);
    /* drawRGB(); */
    glutMainLoop();
}

For debugging when I put glBegin(GL_LINES) it works but if put glBegin(GL_POINTS) it doesn't works. Only two points are draw.

glBegin(GL_LINES);
glVertex2i(0,0);
glVertex2i(i,j);
glEnd();

Above replacement works.

Lion.bmp

For line

For point

Compile
gcc bmpRead.c -lglut -lGLU -lGL

Upvotes: 1

Views: 1567

Answers (1)

Rabbid76
Rabbid76

Reputation: 210908

The setting of the orthographic projection (gluOrtho2D) does not work, because a valide and current OpenGL Context is need for any OpenGL introduction.

The OpenGL window and context is created by glutCreateWindow, but this is done after the call of init() (which does the scene initializations like gluOrtho2D).

Call init() after glutCreateWindow, to solve your issue:

readBMP("Lion.bmp"); 

glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutCreateWindow("RGB");
init(); // <------------------- after "glutCreateWindow"    

glutDisplayFunc(drawRGB);
glutMainLoop();

Upvotes: 1

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