Finding Exact Screen Dimensions - ios

Question

Totally new to iOS OpenGL ES 2.0. I'm trying to precisely identify the boundaries of my screen (iPad) by drawing a simple rectangle. From what I have found, it looks to me like the dimensions of an iPad screen are 768 x 1024. However, my screen is not being covered correctly (note that I am drawing in landscape mode if that matters).
I'm not sure of the interaction between the vertices and how I'm using the projection matrix command.
'self.baseEffect.transform.projectionMatrix = GLKMatrix4MakeOrtho(0, FRAME_WIDTH*2, 0, FRAME_HEIGHT*2, 0, 0);'
If I delete this line, my rectangle renders with the lower left at the origin. But if I leave it in, it seems to render from the lower left, but the dimensions are too large and I can't seem to figure out how to change them predictably.
As you can see, I'm very confused. What is the best way of getting the exact screen dimensions. I need this to properly place other objects on the screen. Thanks!
#import "ViewController.h"
typedef struct {
GLKVector3 positionCoordinates;
GLKVector2 textureCoordinates;
} VertexData;
#define FRAME_HEIGHT 768.0f
#define FRAME_WIDTH 1024.0f
VertexData vertices[] = {
{ { 0.0f, 0.0f, 0.0f}, {0.0f, 0.0f} }, // bottom left
{ {FRAME_WIDTH, 0.0f, 0.0f}, {1.0f, 0.0f} }, // bottom right
{ { 0.0f, FRAME_HEIGHT, 0.0f}, {0.0f, 1.0f} }, // top left
{ { 0.0f, FRAME_HEIGHT, 0.0f}, {0.0f, 1.0f} }, // top left
{ {FRAME_WIDTH, 0.0f, 0.0f}, {1.0f, 0.0f} }, // bottom right
{ {FRAME_WIDTH, FRAME_HEIGHT, 0.0f}, {1.0f, 1.0f} } // top right
};
#interface ViewController ()
#property (nonatomic, strong) EAGLContext *context;
#property (nonatomic, strong) GLKBaseEffect *baseEffect;
#end
#implementation ViewController {
GLuint _vertexBufferID;
}
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
GLKView *view = (GLKView *) self.view;
view.context = self.context;
[EAGLContext setCurrentContext:self.context];
self.baseEffect = [[GLKBaseEffect alloc] init];
self.baseEffect.useConstantColor = YES;
self.baseEffect.constantColor = GLKVector4Make(1.0f, 0.0f, 0.0f, 1.0f);
self.baseEffect.transform.projectionMatrix = GLKMatrix4MakeOrtho(0, FRAME_WIDTH*2, 0, FRAME_HEIGHT*2, 0, 0);
glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
glGenBuffers(1, &_vertexBufferID);
glBindBuffer(GL_ARRAY_BUFFER, _vertexBufferID);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(VertexData), offsetof(VertexData, positionCoordinates));
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#pragma mark - GKLView delegate methods
- (void) glkView: (GLKView *) view drawInRect:(CGRect)rect{
glClear(GL_COLOR_BUFFER_BIT);
[self.baseEffect prepareToDraw];
glDrawArrays(GL_TRIANGLES, 0, 6);
}
- (void) update {
}
#end

Answer 1

You should use FRAME_HEIGHT and FRAME_WIDTH, not twice them.

Answer 2

You can obtain the screen dimensions with:
CGSize screenSize = [UIScreen mainScreen].bounds.size;
however, be wary of using this during application start-up.
To get the orientation of the current view controller:
[UIViewController interfaceOrientation]
To obtain the device orientation:
[[UIDevice currentDevice] orientation]
... or ...
[[UIApplication sharedApplication] statusBarOrientation];
In your case, you can simply use the bounds of EAGLView that you have:
CGSize viewSize = self.view.bounds.size