Question

I’m loading textures using OpenGL ES, the below is my code.

The graphic is made up of 2 textures of the same size, the stop button and the blue glow behind.

The first graphic below was taken using the iPad simulator in xCode, and the 2nd on the actual device. The 1st graphic is the correct output as I exported the graphics from Illustrator. However, when I loaded the program on the iPad, it gives me the 2nd graphic. It seems somehow that the blue light texture behind the stop button has become smaller. Why is this so?

I can compensate by making the blue light texture bigger, but it wouldn’t be right as the way it is supposed to look in Illustrator is the 1st graphic.

Here’s my code.

//
//  OpenGLES_Ch3_4ViewController.m
//  OpenGLES_Ch3_4
//

#import "OpenGLES_Ch3_4ViewController.h"
#import "AGLKVertexAttribArrayBuffer.h"
#import "AGLKContext.h"

#define Y_POS 1.0
#define ASPECT_RATIO 0.75f
#define SIZE 0.8

@implementation OpenGLES_Ch3_4ViewController

@synthesize baseEffect;
@synthesize vertexBuffer;
@synthesize textureInfo0;
@synthesize textureInfo1;

/////////////////////////////////////////////////////////////////
// This data type is used to store information for each vertex
typedef struct {
   GLKVector3  positionCoords;
   GLKVector2  textureCoords;
}
SceneVertex;

/////////////////////////////////////////////////////////////////
// Define vertex data for a triangle to use in example
//static const SceneVertex vertices[] = 

static const SceneVertex vertices[] =
{
    {{-1.0f*SIZE, -ASPECT_RATIO*SIZE, 0.0f}, {0.0f, 0.0f}},  // first triangle
    {{ 0.0f*SIZE, -ASPECT_RATIO*SIZE, 0.0f}, {1.0f, 0.0f}},
    {{-1.0f*SIZE,          0.0f*SIZE, 0.0f}, {0.0f, 1.0f}},
    {{ 0.0f*SIZE, -ASPECT_RATIO*SIZE, 0.0f}, {1.0f, 0.0f}},  // second triangle
    {{-1.0f*SIZE,          0.0f*SIZE, 0.0f}, {0.0f, 1.0f}},
    {{ 0.0f*SIZE,          0.0f*SIZE, 0.0f}, {1.0f, 1.0f}},
};

/////////////////////////////////////////////////////////////////
// Called when the view controller's view is loaded
// Perform initialization before the view is asked to draw
- (void)viewDidLoad
{
   [super viewDidLoad];

   // Verify the type of view created automatically by the
   // Interface Builder storyboard
   GLKView *view = (GLKView *)self.view;
   NSAssert([view isKindOfClass:[GLKView class]],
      @"View controller's view is not a GLKView");

   // Create an OpenGL ES 2.0 context and provide it to the
   // view
   view.context = [[AGLKContext alloc] 
      initWithAPI:kEAGLRenderingAPIOpenGLES2];

   // Make the new context current
   [AGLKContext setCurrentContext:view.context];

   // Create a base effect that provides standard OpenGL ES 2.0
   // shading language programs and set constants to be used for 
   // all subsequent rendering
   self.baseEffect = [[GLKBaseEffect alloc] init];
   self.baseEffect.useConstantColor = GL_TRUE;
   self.baseEffect.constantColor = GLKVector4Make(
      1.0f, // Red
      1.0f, // Green
      1.0f, // Blue
      1.0f);// Alpha

   // Set the background color stored in the current context 
   ((AGLKContext *)view.context).clearColor = GLKVector4Make(
      0.0f, // Red 
      0.0f, // Green 
      0.0f, // Blue 
      1.0f);// Alpha 

   // Create vertex buffer containing vertices to draw
   self.vertexBuffer = [[AGLKVertexAttribArrayBuffer alloc]
      initWithAttribStride:sizeof(SceneVertex)
      numberOfVertices:sizeof(vertices) / sizeof(SceneVertex)
      bytes:vertices
      usage:GL_STATIC_DRAW];

   // Setup texture0
   CGImageRef imageRef0 = 
      [[UIImage imageNamed:@"stoplight_full.png"] CGImage];

   self.textureInfo0 = [GLKTextureLoader 
      textureWithCGImage:imageRef0 
      options:[NSDictionary dictionaryWithObjectsAndKeys:
         [NSNumber numberWithBool:YES], 
         GLKTextureLoaderOriginBottomLeft, nil] 
      error:NULL];

    self.textureInfo0_2 = [GLKTextureLoader
                         textureWithCGImage:imageRef0
                         options:nil
                         error:NULL];

   // Setup texture1
   CGImageRef imageRef1 = 
      [[UIImage imageNamed:@"stop_button.png"] CGImage];

   self.textureInfo1 = [GLKTextureLoader 
      textureWithCGImage:imageRef1 
      options:[NSDictionary dictionaryWithObjectsAndKeys:
         [NSNumber numberWithBool:YES], 
         GLKTextureLoaderOriginBottomLeft, nil] 
      error:NULL];

    self.textureInfo1_2 = [GLKTextureLoader
                         textureWithCGImage:imageRef1
                           options:nil
                           error:NULL];

   // Enable fragment blending with Frame Buffer contents
   glEnable(GL_BLEND);
   glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
}


/////////////////////////////////////////////////////////////////
// GLKView delegate method: Called by the view controller's view
// whenever Cocoa Touch asks the view controller's view to
// draw itself. (In this case, render into a frame buffer that
// shares memory with a Core Animation Layer)
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{   
   // Clear back frame buffer (erase previous drawing)
   [(AGLKContext *)view.context clear:GL_COLOR_BUFFER_BIT];

   [self.vertexBuffer prepareToDrawWithAttrib:GLKVertexAttribPosition
      numberOfCoordinates:3
      attribOffset:offsetof(SceneVertex, positionCoords)
      shouldEnable:YES];
   [self.vertexBuffer prepareToDrawWithAttrib:GLKVertexAttribTexCoord0
      numberOfCoordinates:2
      attribOffset:offsetof(SceneVertex, textureCoords)
      shouldEnable:YES];

   self.baseEffect.texture2d0.name = self.textureInfo0.name;
   self.baseEffect.texture2d0.target = self.textureInfo0.target;
   [self.baseEffect prepareToDraw];

   // Draw triangles using the vertices in the 
   // currently bound vertex buffer
   [self.vertexBuffer drawArrayWithMode:GL_TRIANGLES
      startVertexIndex:0
      numberOfVertices:sizeof(vertices) / sizeof(SceneVertex)];

   self.baseEffect.texture2d0.name = self.textureInfo1.name;
   self.baseEffect.texture2d0.target = self.textureInfo1.target;
   [self.baseEffect prepareToDraw];

   // Draw triangles using currently bound vertex buffer
   [self.vertexBuffer drawArrayWithMode:GL_TRIANGLES
      startVertexIndex:0
      numberOfVertices:sizeof(vertices) / sizeof(SceneVertex)];
 }

Answer 1

To me, the second texture does not look smaller, just dimmer.
Your laptop and iPad most likely have displays with different gammas.

There are many ways to correct for gamma, a terrific introduction is provided here.

It is worth noting many modern cards can do it for you these days too, but so far unsupported on OpenGL ES as far as I know: sRGB Color Formats.