iOS OpenGL ES 2: Obj-C Vertices -> Shader

[Blakeasd]

Hello. I am attempting to learn OpenGL ES 2 with this tutorial. I am having a hard time figuring out when the defined Vertices[] in Objective-C is being passed along to the shaders. Here is my code so far:

#import "OpenGLView.h"

@implementation OpenGLView

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
};


- (GLuint)compileShader:(NSString*)shaderName withType:(GLenum)shaderType {
    
    // 1
    NSString* shaderPath = [[NSBundle mainBundle] pathForResource:shaderName
                                                           ofType:@"glsl"];
    NSError* error;
    NSString* shaderString = [NSString stringWithContentsOfFile:shaderPath
                                                       encoding:NSUTF8StringEncoding error:&error];
    if (!shaderString) {
        NSLog(@"Error loading shader: %@", error.localizedDescription);
        exit(1);
    }
    
    // 2
    GLuint shaderHandle = glCreateShader(shaderType);
    
    // 3
    const char * shaderStringUTF8 = [shaderString UTF8String];
    int shaderStringLength = [shaderString length];
    glShaderSource(shaderHandle, 1, &shaderStringUTF8, &shaderStringLength);
    
    // 4
    glCompileShader(shaderHandle);
    
    // 5
    GLint compileSuccess;
    glGetShaderiv(shaderHandle, GL_COMPILE_STATUS, &compileSuccess);
    if (compileSuccess == GL_FALSE) {
        GLchar messages[256];
        glGetShaderInfoLog(shaderHandle, sizeof(messages), 0, &messages[0]);
        NSString *messageString = [NSString stringWithUTF8String:messages];
        NSLog(@"%@", messageString);
        exit(1);
    }
    
    return shaderHandle;
    
}

- (void)compileShaders {
    
    // 1
    GLuint vertexShader = [self compileShader:@"SimpleVertex"
                                     withType:GL_VERTEX_SHADER];
    GLuint fragmentShader = [self compileShader:@"SimpleFragmentShader"
                                       withType:GL_FRAGMENT_SHADER];
    
    // 2
    GLuint programHandle = glCreateProgram();
    glAttachShader(programHandle, vertexShader);
    glAttachShader(programHandle, fragmentShader);
    glLinkProgram(programHandle);
    
    // 3
    GLint linkSuccess;
    glGetProgramiv(programHandle, GL_LINK_STATUS, &linkSuccess);
    if (linkSuccess == GL_FALSE) {
        GLchar messages[256];
        glGetProgramInfoLog(programHandle, sizeof(messages), 0, &messages[0]);
        NSString *messageString = [NSString stringWithUTF8String:messages];
        NSLog(@"%@", messageString);
        exit(1);
    }
    
    // 4
    glUseProgram(programHandle);
    
    // 5
    _positionSlot = glGetAttribLocation(programHandle, "Position");
    _colorSlot = glGetAttribLocation(programHandle, "SourceColor");
    glEnableVertexAttribArray(_positionSlot);
    glEnableVertexAttribArray(_colorSlot);
}

-(GLuint)setupVBOs{
    GLuint vertexBuffer;
    glGenBuffers(1, &vertexBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
    
    GLuint indexBuffer;
    glGenBuffers(1, &indexBuffer);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);
    
    
    
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        [self setupLayer];
        [self setupContext];
        [self setupRenderBuffer];
        [self setupFrameBuffer];
        [self compileShaders];
        [self setupVBOs];
        [self render];
    }
    return self;
}

+(Class)layerClass{

    return [CAEAGLLayer class];
}

- (void)setupLayer {
    _eaglLayer = (CAEAGLLayer*) self.layer;
    _eaglLayer.opaque = YES;
}


- (void)setupContext {
    EAGLRenderingAPI api = kEAGLRenderingAPIOpenGLES2;
    _context = [[EAGLContext alloc] initWithAPI:api];
    if (!_context) {
        NSLog(@"Failed to initialize OpenGLES 2.0 context");
        exit(1);
    }
    
    if (![EAGLContext setCurrentContext:_context]) {
        NSLog(@"Failed to set current OpenGL context");
        exit(1);
    }
}

-(void)setupRenderBuffer{
    glGenRenderbuffers(1, &_colorRenderBuffer);
    glBindRenderbuffer(GL_RENDERBUFFER, _colorRenderBuffer);
    [_context renderbufferStorage:GL_RENDERBUFFER fromDrawable:_eaglLayer];
}

-(void)setupFrameBuffer{
    GLuint frameBuffer;
    glGenFramebuffers(1, &frameBuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, _colorRenderBuffer);

}

-(void)render{
    glClearColor(0, 104.0/255.0, 55.0/255.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glViewport(0, 0, self.frame.size.width, self.frame.size.height);
    
  
    
    glVertexAttribPointer(_positionSlot, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex),0);
    
    glVertexAttribPointer(_colorSlot, 4, GL_FLOAT, GL_FALSE,
                          sizeof(Vertex), (GLvoid*) (sizeof(float) * 3));
    
  
    
    glDrawElements(GL_TRIANGLES, sizeof(Indices)/sizeof(Indices[0]), GL_UNSIGNED_BYTE, 0);
    
    [_context presentRenderbuffer:GL_RENDERBUFFER];
    
}


@end

This code is a simple OpenGLView (subclass of UIView w/CAEAGLLayer). A rainbow effect is drawn onto the screen. Can someone point out in the code where the definite line is where Objective-C gives the Vertices off to the shaders. I could probably back track if I had the line that says "Hey shaders, here are the Vertices and Indices". I would even like it if someone could just show me a flow-chart type thing (this --> this) of the roadmap from Objc to the shaders -- Not the Cocoa stuff, just gl.

Thanks!!

 

[firewood]

It looks like it happens in the glBufferData() calls in your setupVBOs method.

 

[Blakeasd]

How does this come into play with _positionSlot and _colorSlot?

Thanks

 

[firewood]

It looks like your glGetAttribLocation() and glVertexAttribPointer() calls tell the shader where to find the Position and SourceColor attributes in your Vertices array.

 

[Blakeasd]

Ahh... I get it now. I wish the OpenGL ES 2 API was easier to work with :S

Thanks for All of Your Help!