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Asked: 2026-05-28T08:15:40+00:00

I am new to OpenGL ES programming. I found a good tutorial to start

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I am new to OpenGL ES programming. I found a good tutorial to start and the project displayed a square. I then tried to move the most OpenGL code out of the View Controller to a Model object which would store vertices, color, etc and then in the View Controller I would just call:

[model update];
[model load];

This way it would be more practice if I had several models to display. And since I did this the cube no longer displays on screen. I think I pinpointed to error to the update method where all the model view matrix calculations occur, because when I comment out this method the cube displays but it just fills the screen.

-(void)update: (int)width And:(int)height{
float aspect = (width/height);
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(
                                      GLKMathDegreesToRadians(65.0f), aspect,
                                      4.0f, 10.0f);
self.effect.transform.projectionMatrix = projectionMatrix;

GLKMatrix4 modelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, 6.0f);   
rotation += 90;
modelViewMatrix = GLKMatrix4Rotate(modelViewMatrix,
                                       GLKMathDegreesToRadians(rotation), 0, 0, 1);
self.effect.transform.modelviewMatrix = modelViewMatrix;
}

And here is the loading method:

- (void)load{
self.effect = [[GLKBaseEffect alloc] init];

NSDictionary * options = [NSDictionary dictionaryWithObjectsAndKeys:
                          [NSNumber numberWithBool:YES],
                          GLKTextureLoaderOriginBottomLeft, 
                          nil];

NSError * error;    
NSString *path = [[NSBundle mainBundle] pathForResource:@"tile_floor" ofType:@"png"];
GLKTextureInfo * info = [GLKTextureLoader textureWithContentsOfFile:path options:options error:&error];
if (info == nil) {
    NSLog(@"Error loading file: %@", [error localizedDescription]);
}
self.effect.texture2d0.name = info.name;
self.effect.texture2d0.enabled = true;

glGenVertexArraysOES(1, &vertexArray);
glBindVertexArrayOES(vertexArray);

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);

glEnableVertexAttribArray(GLKVertexAttribPosition);        
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, Position));
glEnableVertexAttribArray(GLKVertexAttribColor);
glVertexAttribPointer(GLKVertexAttribColor, 4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, Color));
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, TexCoord));

glBindVertexArrayOES(0);
}

And here is the rendering method:

- (void)render{
[self.effect prepareToDraw];
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);

glBindVertexArrayOES(vertexArray);
glDrawElements(GL_TRIANGLES, sizeof(Indices)/sizeof(Indices[0]), GL_UNSIGNED_BYTE, 0);
}

Any help would be appreciated. Thanks.

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1 Answer

  1. Editorial Team

    There’s not really an ‘error’ here. The problem is that you’re not really setting up the projection and modelview matrices so that it frames your model nicely. There’s no magic solution. You should consider the vertices in the model that you are drawing (do they go up to 100? 1000? Just 0.5?) and choose appropriate settings for GLKMatrix4MakePerspective that put them squarely in the screen.

    As a short cut, (since you said that without the translation and rotation it fills the whole screen), you can replace those transformations with a scale command to scale down the size of the object.

    Something like:

    GLKMatrix4 modelViewMatrix = GLKMatrix4MakeTranslation(0.1f, 0.1f, 0.1f);

    As general advice, try to consider the effect of each command individually. If you’re looking for an introduction to projection/transformation concepts, I found this link: http://duriansoftware.com/joe/An-intro-to-modern-OpenGL.-Chapter-3:-3D-transformation-and-projection.html Skip down to the section called Projection and world space.

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