[opengl]glDrawElements();

hylke · 06-22-2004, 04:21 AM

Hello
Could anyone tell me why the following piece of code doesn't work?

Code:

glEnableClientState(GL_VERTEX_ARRAY);
float test[9] = { 0.0, 0.5, 0.0,
         -0.5, 0.0, 0.0,
         0.5, 0.0, 0.0 };
glVertexPointer(3, GL_FLOAT, 0, test);
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_BYTE, test);
glDisableClientState(GL_VERTEX_ARRAY);

Thanx Hylke

hylke · 06-23-2004, 04:33 AM

So does anyone have an idea?

deiussum · 06-23-2004, 11:21 AM

Try changing GL_UNSIGNED_BYTE to GL_FLOAT, since test is a float array, not a unsigned char array.

Other than that, there are many other things that could cause it to not work, but seeing only that code segment won't help. Are you sure you have a GL context, etc?

hylke · 06-24-2004, 03:34 AM

Maybe it's because i have an old version of opengl because glMultiDrawElements(); doesn't work at all(gives compilation errors).
So i'il check if nvidia has released a new opengl version this month.
Hylke

deiussum · 06-24-2004, 12:44 PM

Did you try changing that constant? Because that is one definite problem in the small code segment you posted.

Just off the top of my head I can think of a number of possible reasons it's not displaying anything.

OpenGL isn't initialized, thus no context to render to
You have your MODELVIEW and PROJECTION matrices setup wrong
It could be getting clipped by the front or back clipping plane
You are using double buffering but never flip the buffer
You have lighting enabled, but no light
Your color/material settings are set to the same as your background color

There are also many other reasons that you may not be seeing anything. You will have to give more details of your code to be able to determine the exact problem.

About glMultiDrawElements(), I think this is a relatively new method for OpenGL, though I can't tell you what version it was added in. You can check the OpenGL specs, and check the version of your OpenGL implementation with glGetString(GL_VERSION);

hylke · 06-24-2004, 12:57 PM

Quote:

Originally posted by deiussum
Did you try changing that constant? Because that is one definite problem in the small code segment you posted.

Just off the top of my head I can think of a number of possible reasons it's not displaying anything.

OpenGL isn't initialized, thus no context to render to

You have your MODELVIEW and PROJECTION matrices setup wrong

It could be getting clipped by the front or back clipping plane

You are using double buffering but never flip the buffer

You have lighting enabled, but no light

Your color/material settings are set to the same as your background color

There are also many other reasons that you may not be seeing anything. You will have to give more details of your code to be able to determine the exact problem.

About glMultiDrawElements(), I think this is a relatively new method for OpenGL, though I can't tell you what version it was added in. You can check the OpenGL specs, and check the version of your OpenGL implementation with glGetString(GL_VERSION);

neither of the things you came up with was wrong.
I thought glMultiDrawElements came with one of the latest versions.
b.t.w. i've just formatted my pc and forgot to back up my codes.So i'il make a new one and post it here.
Hylke

hylke · 06-24-2004, 01:31 PM

Here's a code that i just wrote and didn't work(just like the other one)

Code:

#include <GL/glut.h>

void init();
void reshape(int w, int h);
void display();

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);
glutMainLoop();
return 0;
}

void init()
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glShadeModel(GL_FLAT);
}

void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-0.5, 5.0 ,-0.5, 5.0, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}

void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glEnableClientState(GL_VERTEX_ARRAY);
GLfloat triangle[] = { 0.0, 0.5, 0.0,
			-0.5, 0.0, 0.0,
			0.5, 0.0, 0.0 };
glVertexPointer(3 ,GL_FLOAT, 0, triangle);
glDrawElements(GL_TRIANGLES, 9, GL_FLOAT, triangle);
glDisableClientState(GL_VERTEX_ARRAY);
glFlush();
}

deiussum · 06-24-2004, 02:26 PM

Ok. I see what the problem is. You are trying to use glDrawElements like glDrawArrays. (I get the 2 mixed up myself all the time too, and should have looked at the man pages first thing to see if that's what the problem was.)

You have 2 options.

1. Just use glDrawArrays like so:

glDrawArrays(GL_TRIANGLES, 0, 3);

2. Create an array of INDICES into your triangles array like so:

unsigned int indices[3] = {0, 1, 2};

glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);

My modified display function looks like so:

Code:

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glEnableClientState(GL_VERTEX_ARRAY);
    GLfloat triangle[] = { 0.0, 0.5, 0.0,
                -0.5, 0.0, 0.0,
                0.5, 0.0, 0.0 };
    unsigned int indices[] = {0, 1, 2};
    glVertexPointer(3 ,GL_FLOAT, 0, triangle);
    // use either of the 2 lines below
    glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);
    //glDrawArrays(GL_TRIANGLES, 0, 3);
    glDisableClientState(GL_VERTEX_ARRAY);
    glFlush();
}

hylke · 06-24-2004, 02:58 PM

So if would add an w value to it would it look like this?

Quote:

void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glEnableClientState(GL_VERTEX_ARRAY);
GLfloat triangle[] = { 0.0, 0.5, 0.0, 1.0,
-0.5, 0.0, 0.0, 1.0,
0.5, 0.0, 0.0, 1.0 };
unsigned int indices[] = {0, 1, 2, 3};
glVertexPointer(4 ,GL_FLOAT, 0, triangle);
// use either of the 2 lines below
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);
//glDrawArrays(GL_TRIANGLES, 0, 3);
glDisableClientState(GL_VERTEX_ARRAY);
glFlush();
}

deiussum · 06-24-2004, 03:41 PM

It could, but that last 3 in "indices" would never be used. What the indices are is in index into the vertex array. Index 0 indicates the first x elements of your float array. (x being 3 for your first example, 4 when you add a w component)

when you also enable things like texcoord arrays, normal arrays, etc. that index is also an index into that array.

What glDrawElements buys you, is that if you have a number of duplicate vertices, you only need the vertex in the array once, and can simply have multiple values in your index array.

deiussum · 06-24-2004, 05:16 PM

Here... I modified your display a bit to add a color array to try and demonstrate what I meant. Think of it in terms of the following:

vertex0 = pos(0.5, 0.5, 0.0) color(1.0, 0.0, 0.0)
vertex1 = pos(-0.5, 0.0, 0.0) color (0.0, 1.0, 0.0)
vertex2 = pos(0.5, 0.0, 0.0) color(0.0, 0.0, 1.0)
vertex3 = pos(-0.5, 0.5, 0.0) color(1.0, 1.0, 1.0)

the first triangle is drawn with vertices 0, 1, 2.
the second triangle is drawn with vertices 3, 1, 2, translated 1 unit to the right

Here's the code: (BTW, you also have to get rid of glShadeModel(GL_FLAT) to see the different colors, flat shading uses only thet color of one of the vertices for the face.

Code:

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);
    GLfloat triangle[] = { 0.5, 0.5, 0.0, // index 0
                -0.5, 0.0, 0.0, // index 1
                0.5, 0.0, 0.0, // index 2
                -0.5, 0.5, 0.0  }; // index 3  
    GLfloat colors[] = { 1.0, 0.0, 0.0, // index 0
        0.0, 1.0, 0.0, // index 1
        0.0, 0.0, 1.0, // index 2
        1.0, 1.0, 1.0 }; // index 3
    unsigned int indices[] = {0, 1, 2};
    unsigned int indices2[] = {3, 1, 2};

    glVertexPointer(3 ,GL_FLOAT, 0, triangle);
    glColorPointer(3, GL_FLOAT, 0, colors);

    // Draw triangle 1 using set of indices
    glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices);

    // Draw triangle 2 using set of indices2, moved to the right a bit
    glPushMatrix();
    glTranslatef(1.0, 0.0, 0.0);
    glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, indices2);
    glPopMatrix();


    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);
    glFlush();
}

hylke · 06-25-2004, 01:55 AM

Thank you very much, i finaly understand how it works.
Hylke