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render vbo not working

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#1 Homunculi   Members   


Posted 20 May 2012 - 11:49 PM

I am new to opengl. I setup a set of classes to manage shaders. I am now trying to debug it and I want to stick to opengl 3.3+. The following is the code I have written to setup a vbo for rendering a triangle.

	Vertex triangle[3];
	triangle[0].x = 0.0f; triangle[0].y = 0.0f; triangle[0].z = 0.0f;
	triangle[0].nx = 0.0f; triangle[0].ny = 0.0f; triangle[0].nz = 1.0f;
	triangle[0].s0 = 0.0f; triangle[0].s1 = 0.0f;

	triangle[1].x = 1.0f; triangle[1].y = 0.0f; triangle[1].z = 0.0f;
	triangle[1].nx = 0.0f; triangle[1].ny = 0.0f; triangle[1].nz = 1.0f;
	triangle[1].s0 = 1.0f; triangle[1].s1 = 0.0f;
	triangle[2].x = 0.0f; triangle[2].y = 1.0f; triangle[2].z = 0.0f;
	triangle[2].nx = 0.0f; triangle[2].ny = 0.0f; triangle[2].nz = 1.0f;
	triangle[2].s0 = 0.0f; triangle[2].s1 = 1.0f;
	glBufferData(GL_ARRAY_BUFFER, sizeof(Vertex)*3,&triangle[0].x,GL_STATIC_DRAW);
	glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(unsigned short)*3,indices,GL_STATIC_DRAW);

I then call this function in my main loop of the program:

	glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_SHORT, indices);

The shaders which I am using are just a basic fragment and vertex shader. I have checked and they have been compiled and loaded succesfully.

What happens is the screen blinks. The entire screen... rapidly... when running my program.


I just noticed the triangle 0's for all verts thing and changed it... I also noticed the double bind buffer and removed that --_-

Edited by Homunculi, 21 May 2012 - 12:03 AM.

#2 Vorel512   Members   


Posted 21 May 2012 - 02:31 AM



glBufferData(GL_ARRAY_BUFFER, sizeof(Vertex)*3,&triangle[0].x,GL_STATIC_DRAW);
You should pass a pointer to "&triangle" and with sizeof(triangle) now you allocate memory for only the first position. And then try to set opengl where to search for specified data like normals. It would be a great start to read and try out some example."
#define BUFFER_OFFSET(i) ((char *)NULL + (i))
    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 24, BUFFER_OFFSET(0));
    glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE, 24, BUFFER_OFFSET(12));
    glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 24, BUFFER_OFFSET(24));

sizeof() gives you the size of the variable in bytes, not the amount of data stored in them. In your case all vertexes has 8 elements (3+3+2 = 8) and 3 vertexes (3*8=24).

#3 mhagain   Members   


Posted 21 May 2012 - 03:51 AM

sizeof (Vertex) should be fine, and is preferable to hard-coding vertex-sizes in your glVertexAttribPointer calls - what happens if you ever need to add a second pair of texcoords to your vertex definition?

For your glBufferData call it's sufficient to use "glBufferData(GL_ARRAY_BUFFER, sizeof(Vertex)*3,triangle,GL_STATIC_DRAW);" - triangle is an array and this will work. &triangle will - if you're lucky - crash the program.

If you're getting crazy flickering the most likely explanation is that you originally followed an old GLUT tutorial that created a single-buffered context. Changing that to double-buffered and adding the appropriate SwapBuffers call (glutSwapBuffers I believe) should resolve that.

It appears that the gentleman thought C++ was extremely difficult and he was overjoyed that the machine was absorbing it; he understood that good C++ is difficult but the best C++ is well-nigh unintelligible.

#4 Homunculi   Members   


Posted 21 May 2012 - 04:26 PM

I am not using GLUT I am using SDL to create an opengl context. I have SDL_GL_DOUBLEBUFFER enabled so that isn't the issue. I even put glEnable(GL_DOUBLEBUFFER) as well.

I changed &triangle[0].x to just triangle as suggested.

I am fully aware of what sizeof() does. Anyways. After making the recommended changes it still doesn't work.

#5 Homunculi   Members   


Posted 21 May 2012 - 06:12 PM

Ok, well I have been placing glGetError around my code and doing a bit of debugging. I have narrowed it down a bit. The error I am getting is "GL_INVALID_ENUM" and I you were onto something with the double buffer perhaps... the spot where I get the error is on the call to


I commented it out because it should be doublebuffered anyways since on SDL_SetVideoMode() I put the flag SDL_GL_DOUBLEBUFFER

I no longer get a gl error at any point in my code that I can get with glGetError()

But I am still having the same problem with it making my screen blink and not rendering my triangle.


Ok, so I just tested my shaders using fixed pipeline functions (glVertex3f() etc). The shaders do work fine. It just doesn't work when trying to render the VBO way >.<


Ok, I fixed the issue last night. It was my call to glDrawElements that was incorrect. I was passing in the address of the array that held the indices when I should have really been passing BUFFER_OFFSET(0)

Edited by Homunculi, 22 May 2012 - 12:15 PM.

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