CODEWITHSUNDEEP
copenglglewglfw
steave
steave

Reputation: 395

Simple openGL app with wierd vertex position after call glBindBuffer

I wrote this to test how to draw with opengl buffer,here is the function to initialize:

#include "GL/glew.h"
#include "GL/glfw.h"

    int main ( int argc, char* argv[] )
    {
        //init glfw
        glfwInit();

        glfwOpenWindow ( 1024, 768, 8, 8, 8, 8, 24, 0, GLFW_WINDOW );

        //init extension
        glewInit();

        //i
        glGenBuffers ( 1, &buffID );
        //init test buffer
        for ( int i = 0 ; i < 15; i ++ ) {
            testBuffer[i][0] += 1 + i * i;
            testBuffer[i][1] += 1 + i + i * i;
            testBuffer[i][2] = 0;
        }

        do {
            render_loop();
        } while ( glfwGetWindowParam ( GLFW_OPENED ) );

        glfwTerminate();
    }

and this is the render_loop function:

GLuint buffID;
static GLfloat testBuffer[15][3];

void render_loop()
{
    glClearColor ( .7, .1, .1, 1.0f );
    glClear ( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
    glViewport ( 0, 0, 1024, 768 );
    glMatrixMode ( GL_PROJECTION );

    glLoadIdentity();
    glOrtho ( 0, 1024, 768, 0, 0, -1 );
    glMatrixMode ( GL_MODELVIEW );
    glLoadIdentity();

    //draw a test point
    glPointSize ( 10 );
    glBegin ( GL_POINTS );
    glColor4f ( 1, 1, 1, 1 );
    glVertex3f ( 512, 384, 0 );
    glEnd();

     glBindBuffer(GL_ARRAY_BUFFER,buffID);
     glBufferData(GL_ARRAY_BUFFER,3 * sizeof(GLfloat) * 15, testBuffer,GL_DYNAMIC_DRAW);

    glEnableClientState(GL_VERTEX_ARRAY);
    glVertexPointer(3,GL_FLOAT,0,testBuffer);
    glDrawArrays(GL_POINTS,0,15);
    glBindBuffer(GL_ARRAY_BUFFER,0);
    glDisableClientState(GL_VERTEX_ARRAY);

    glfwSwapBuffers();

}

all 15 vertices are at 0,0 position which is top left corner of the window,I can see white points there,after I remove these 3 lines from loop:

glBindBuffer(GL_ARRAY_BUFFER,buffID);
glBufferData(GL_ARRAY_BUFFER,sizeof(GLfloat) * 15, testBuffer,GL_DYNAMIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER,0);

everything works as I expected, I have a few questions:

1.what changed the vertex buffer position?

2.by the way how much perfomance gain using these calls?

Upvotes: 0

Views: 324

Answers (1)

steave
steave

Reputation: 395

have to call this :

glBindBuffer(GL_ARRAY_BUFFER,0);

after glBufferData function.

also I found I need to change this:

//added this line before glVertexPointer:
glBindBuffer(GL_ARRAY_BUFFER,buffID);
//glVertexPointer(3,GL_FLOAT,0,testBuffer); change last param to 0:
glVertexPointer(3,GL_FLOAT,0,0);

after added that,everything seems working well.

Upvotes: 1

Related Questions