Drawing a triangle with OpenGL ES - objective-c

Question

First of all, sorry for my english,
My problem is :
I'm following a tutorial to learn OpenGL ES and I must draw a triangle in the middle of the screen. In the image they show, the triangle isn't touching the edges of the screen (I think it's because of the z coordinates of the vertices)
The vertices they tell us to make is with those coordinates :
(0.0, 1.0, -3.0)
(1.0, 0.0, -3.0)
(-1.0, 0.0, -3.0)
The tutorial is not uptodate so I did it with a personal code :
//
// EAGLView.m
// OpenGlintro
//
// Created by Arnaud Miguet on 01/12/12.
// Copyright (c) 2012 Tap‘n'Develop. All rights reserved.
//
#import "EAGLView.h"
#implementation EAGLView
+ (Class) layerClass {
return [CAEAGLLayer class];
}
- (void)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) {
// Initialization code
CAEAGLLayer *EAGLLayer = (CAEAGLLayer *) super.layer;
EAGLLayer.opaque = YES;
context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
if (!context || ![EAGLContext setCurrentContext:context]) {
[self release];
return nil;
}
glEnable (GL_DEPTH_TEST);
GLuint framebuffer , renderbuffer;
glGenBuffers(1, &framebuffer);
glGenBuffers(1, &renderbuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
glBindRenderbuffer(GL_RENDERBUFFER, renderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER fromDrawable:EAGLLayer];
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, renderbuffer);
glViewport(10, 0, CGRectGetWidth(frame), CGRectGetHeight(frame));
[self compileShaders];
[self setupVBOs];
[self render];
}
return self;
}
- (void)render {
glClearColor(0.7, 0.7, 0.7, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// 1
glViewport(0, 0, self.frame.size.width, self.frame.size.height);
// 2
glVertexAttribPointer(_positionSlot, 3, GL_FLOAT, GL_FALSE,
sizeof(Vertex), 0);
glVertexAttribPointer(_colorSlot, 4, GL_FLOAT, GL_FALSE,
sizeof(Vertex), (GLvoid*) (sizeof(float) * 3));
// 3
glDrawElements(GL_TRIANGLES, sizeof(Indices)/sizeof(Indices[0]),GL_UNSIGNED_BYTE, 0);
[context presentRenderbuffer:GL_RENDERBUFFER];
}
- (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:#"SimpleFragment"
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);
}
typedef struct {
float Position[3];
float Color[4];
} Vertex;
const Vertex Vertices[] = {
{{0.0, 1.0, -2}, {1, 0, 0, 1}},
{{1.0, 0.0, -2}, {1, 0, 0, 1}},
{{-1.0, 0.0, 0}, {1, 0, 0, 1}}
};
const GLubyte Indices[] = {
0,1,2
};
/*
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
// Drawing code
}
*/
#end
But the vertexes don't appear because of the background drawn at z=-1 coordinates... Can someone help me? I can't find any solution for this...

Answer 1

At first glance, it seems your render function is only being called once, in viewDidLoad. This should instead be implemented in a method with continuous updates, such as drawRect.
OpenGL ES has a very steep learning curve and in iOS you can help yourself a lot by implementing the GLKit framework. It saves you all those calls to different types of buffers and handles most of the boilerplate code for draw/update functions for you.
This tutorial is a very good place to start:
http://www.raywenderlich.com/5223/beginning-opengl-es-2-0-with-glkit-part-1