Question

I need a little help with drawing pixels on screen. The code I have written works fine on simulator but when I deploy on device it output garbage. So here’s my code:

I have GLKViewController setup and here’s the viewDidLoad:

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];

    if (!self.context) {
        NSLog(@"Failed to create ES context");
    }

    GLKView *view = (GLKView *)self.view;
    view.context = self.context;
    view.drawableDepthFormat = GLKViewDrawableDepthFormat24;

    [EAGLContext setCurrentContext:self.context];

    self.effect = [[GLKBaseEffect alloc] init];


    self.effect.useConstantColor = GL_TRUE;

    self.effect.constantColor = GLKVector4Make(
                                               0.0, // Red
                                               0.0, // Green
                                               0.0, // Blue
                                               1.0f);// Alpha

    GLKMatrix4 projectionMatrix = GLKMatrix4MakeOrtho(0, 480, 0, 320, -1024, 1024);
    self.effect.transform.projectionMatrix = projectionMatrix;

}

And here is where I am drawing points/pixels:

- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
    GLfloat points[] =
    {
        110.0f, 110.0f,
        111.0f, 110.0f,
        110.0f, 111.0f,
        111.0f, 111.0f,

        112.0f, 112.0f,
        113.0f, 112.0f,
        112.0f, 113.0f,
        113.0f, 113.0f,
    };

    glClearColor(1.0f, 0.0f, 1.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    GLKMatrix4 modelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, 0.0f);
    self.effect.transform.modelviewMatrix = modelViewMatrix;


    // Prepare the effect for rendering 
    [self.effect prepareToDraw];

    glEnableVertexAttribArray(GLKVertexAttribPosition);
    glVertexAttribPointer(GLKVertexAttribPosition, 2, GL_FLOAT, GL_FALSE, 2*4, points);
    glDrawArrays(GL_POINTS, 0, 8);

}

When I run on simulator it works fine i.e. draw the pixels accordingly, but when I deploy on iPod4 it display some garbage. I am a beginner so need help displaying simple pixels.

Answer 1

I believe the issue here is that you need to set the built-in gl_PointSize variable in the vertex shader, as the default values are different on the simulator and the device.

The answer to this question explains further.