iPhone : Texture bigger than 64x64?

Question

Hi,

I took the example of GLPaint... I'm trying to put a background into the "PaintingView", so you could draw over the background and finally save the image as a file..... I'm lost.

I'm loading the PNG (512x512) and try to "paint with it" at the very beginning of the program, but it's painted as 64x64 instead of 512x512...

I tried before to load is as a subview of the painting view... but then, glReadPixels doesn't work as expected (it only take in consideration the PaintingView, not the subview). Also the PaintingView doesn't have a method as initWithImage... I NEED glReadPixels work on the image (and in the modification) but i really don't know why when i load it, the texture has a 64x64 size..

Answer 1

First, glReadPixels() is only going to see whatever framebuffer is associated with your current OpenGL context. That might explain why you're not getting the pixels you expect.

Second, what do you mean by the texture being rendered at a specific pixel size? I assume the texture is rendered as a quad, and then the size of that quad ought to be under your control, code-wise.

Also, check that the loading of the texture doesn't generate an OpenGL error, I'm not sure what the iPhone's limitations on texture sizes are. It's quite conceivable that 512x512 is out of range. You could of course investigate this yourself, by calling glGetIntegerv() and using the GL_MAX_TEXTURE_SIZE constant.

Answer 2

The GLPaint example project uses GL_POINT_SPRITE to draw copies of the brush texture as you move the brush. On the iPhone, the glPointSize is limited to 64x64 pixels. This is a hardware limitation, and in the simulator I think you can make it larger.

It sounds like you're trying to use a GL_POINT_SPRITE method to draw your background image, and that's really not what you want. Instead, try drawing a flat, textured box that fills the screen.

Here's a bit of OpenGL code that sets up vertices and texcoords for a 2D box and then draws it:

const GLfloat verticies[] = {
        0.0f, 0.0f,
        1.0f, 0.0f,
        0.0f, 1.0f,
        1.0f, 1.0f,
    };

    const GLfloat texcoords[] = {
        0, 0,
        1, 0,
        0, 1,
        1, 1,
    };

    glVertexPointer(2, GL_FLOAT, 0, verticies);
    glEnableClientState(GL_VERTEX_ARRAY);
    glTexCoordPointer(2, GL_FLOAT, 0, texcoords);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glEnable(GL_TEXTURE_2D);

    glBindTexture(GL_TEXTURE_2D, texture);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

Hope that helps! Note that you need to specify the vertices differently depending on how your camera projection is set up. In my case, I set up my GL_MODELVIEW using the code below - I'm not sure how the GLPaint example does it.

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glOrtho(0, 1.0, 0, 1.0, -1, 1);