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RobinvanDijke

OpenGL Vertex/IndexBuffer setup not working

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RobinvanDijke    100
I'm building a simple OpenGL-based Graphics Library for iOS. Just for fun and for learning purposes. At this time I have a few simple classes which are able to initialize an OpenGL context, can clear the screen with a specified UIColor and can draw a simple square with linear-interpolated colors.
I went a bit further and specified a GLTriangle class which, obviously, needs to draw a triangle on the screen.
This is my code:

[code]
#import "GLTriangle.h"

typedef struct _GLRawVertex {
GLVector4 position;
GLVector4 color;
} GLRawVertex;

typedef struct _GLVector4 {
float w;
float x;
float y;
float z;
} GLVector4;


@implementation GLTriangle
@synthesize program;

- (void)dealloc {
[vertices release];
[program release];

// release rawVertexData
free(rawVertexData);

// free rawIndicesData
free(rawIndicesData);

[super dealloc];
}

// FYI: GLVertex is a simple Obj-C class with position and color properties of type GLVector4
- (id)initWithVertices:(NSArray *)_vertices andIndices:(NSArray *)_indices {
self = [super init];
if (self){
if ([_vertices count] != 3 || [_indices count] != 3){
NSLog(@"GLTriangle initialization: more of less than 3 vertices are used");
return nil;
}

vertices = [_vertices retain];
indices = [_indices retain];

// check of the vertices array holds GLVertex objects
if (![[vertices objectAtIndex:0] isKindOfClass:[GLVertex class]]){
NSLog(@"GLTriangle initialization: Please use GLVertex instances in the vertices array");
return nil;
}


// check of the indices array holds NSNumber objects
if (![[indices objectAtIndex:0] isKindOfClass:[NSNumber class]]){
NSLog(@"GLTriangle initialization: Please use NSNumber instances in the indices array");
return nil;
}

// convert vertices array to raw data
rawVertexData = malloc(sizeof(GLRawVertex) * 3);
int i = 0;
for (GLVertex *currentVertex in vertices){
rawVertexData[i].position = currentVertex.position;
rawVertexData[i].color = currentVertex.color;

NSLog(@"POSITION: %f, %f, %f", rawVertexData[i].position.x, rawVertexData[i].position.y, rawVertexData[i].position.z);
NSLog(@"COLOR: %f, %f, %f", rawVertexData[i].color.x, rawVertexData[i].color.y, rawVertexData[i].color.z);
i++;
}

// create vertexbuffer
glGenBuffers(1, &vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(GLRawVertex) * 3, rawVertexData, GL_STATIC_DRAW);

// convert indices array to raw data
rawIndicesData = malloc(sizeof(int) * 3);
i = 0;

for (NSNumber *currentIndex in indices){
rawIndicesData[i] = [currentIndex intValue];
i++;
}

// create indexBuffer
glGenBuffers(1, &indexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(int) * 3, rawIndicesData, GL_STATIC_DRAW);

// unbind any buffer
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
}

return self;
}

- (void)render {
// use correct program
glUseProgram(self.program.programHandle);

// bind buffers
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);

// set position and color
glVertexAttribPointer([self.program identifierForAttribute:GLShaderVariablePosition], 4, GL_FLOAT, GL_FALSE, sizeof(GLRawVertex), 0);
glVertexAttribPointer([self.program identifierForAttribute:GLShaderVariableColor], 4, GL_FLOAT, GL_FALSE, sizeof(GLRawVertex), (GLvoid *)(4*sizeof(float)));

// draw triangle
glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_BYTE, 0);

// unbind any buffer
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
}

@end
[/code]

As you can see it looks pretty straightforward.
Explanation: I'm initializing this class with 3 proper vertices which are positioned in the viewport. The shaders are working (according to the non-buffer based drawings) and I logged almost each variable to see if it contains the correct value. Attribpointers are correct (0 and 1).

The error message: sometimes I get a bad access error in the glDrawElements function which means that something is pointing to the wrong memory-location. Also sometimes I see a strange black drawing and the next time it is not visible which confirms the memory-issue. My guess therefore: the buffers are not initialized properly or the glDrawElements function has no valid reference to them.

Some log output:


[code]
2011-06-11 23:24:41.418 TWGL[50069:207] POSITION: 0.000000, 0.000000, 0.000000
2011-06-11 23:24:41.419 TWGL[50069:207] COLOR: 1.000000, 0.000000, 0.000000
2011-06-11 23:24:41.419 TWGL[50069:207] POSITION: 0.500000, 0.000000, 0.000000
2011-06-11 23:24:41.420 TWGL[50069:207] COLOR: 1.000000, 0.000000, 0.000000
2011-06-11 23:24:41.420 TWGL[50069:207] POSITION: 0.000000, 0.500000, 0.000000
2011-06-11 23:24:41.421 TWGL[50069:207] COLOR: 1.000000, 0.000000, 0.000000
[/code]


The rawIndices contains {0,1,2}.

I hope somebody can help me with this. I've already looked into several webpages and books for a week and now it starts to drive me crazy :-)

Maybe also interesting

[code]

#import "GLTestRender.h"

typedef struct {
float Position[3];
float Color[4];
} Vertex;
const Vertex Vertices[] = {
{{1, -1, 0}, {1, 0, 0, 1}},
{{1, 1, 0}, {0, 1, 0, 1}},
{{-1, 1, 0}, {0, 0, 1, 1}},
{{-1, -1, 0}, {0, 0, 0, 1}}
};

const GLubyte Indices[] = {
0, 1, 2,
2, 3, 0
};

@implementation GLTestRender

- (id)init {
self = [super init];
if (self){
glGenBuffers(1, &vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);

glGenBuffers(1, &indexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);

// unbind any buffer
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
}
return self;

}

- (void)render {
// bind buffers
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);

// 2
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), 0);
glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*) (sizeof(float) * 3));
// 3
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, 0);

// unbind any buffer

glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

}

@end
[/code]

This is the part of the test render stuff that is working.. also with buffers, but initialized with static data.

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