OpenGL Issue Drawing a Large Image Texture causing Skewing

Question

I'm trying to store a 1365x768 image on a 2048x1024 texture in OpenGL ES but the resulting image once drawn appears skewed. If I run the same 1365x768 image through gluScaleImage() and fit it onto the 2048x1024 texture it looks fine when drawn but this OpenGL call is slow and hurts performance.

I'm doing this on an Android device (Motorola Milestone) which has 256MB of memory. Not sure if the memory is a factor though since it works fine when scaled using gluScaleImage() (it's just slower.)

Mapping smaller textures (854x480 onto 1024x512, for example) works fine though. Does anyone know why this is and suggestions for what I can do about it?

Update

Some code snippets to help understand context...

// uiImage is loaded. The texture dimensions are determined from upsizing the image
// dimensions to a power of two size:
// uiImage->_width = 1365
// uiImage->_height = 768
// width = 2048
// height = 1024

// Once the image is loaded:
// INT retval = gluScaleImage(GL_RGBA, uiImage->_width, uiImage->_height, GL_UNSIGNED_BYTE, uiImage->_texels, width, height, GL_UNSIGNED_BYTE, data);
copyImage(GL_RGBA, uiImage->_width, uiImage->_height, GL_UNSIGNED_BYTE, uiImage->_texels, width, height, GL_UNSIGNED_BYTE, data);

if (pixelFormat == RGB565 || pixelFormat == RGBA4444) 
{
    unsigned char* tempData = NULL;
    unsigned int* inPixel32;
    unsigned short* outPixel16;
    tempData = new unsigned char[height*width*2];
    inPixel32 = (unsigned int*)data;
    outPixel16 = (unsigned short*)tempData;

    if(pixelFormat == RGB565)
    {
        // "RRRRRRRRGGGGGGGGBBBBBBBBAAAAAAAA" --> "RRRRRGGGGGGBBBBB"
        for(unsigned int i = 0; i < numTexels; ++i, ++inPixel32)
        {
            *outPixel16++ = ((((*inPixel32 >> 0) & 0xFF) >> 3) << 11) | 
                            ((((*inPixel32 >> 8) & 0xFF) >> 2) << 5) | 
                            ((((*inPixel32 >> 16) & 0xFF) >> 3) << 0);
        }
    }

    if(tempData != NULL)
    {
        delete [] data;
        data = tempData;
    }
}
// [snip..]

// Copy function (mostly)
static void copyImage(GLint widthin, GLint heightin, const unsigned int* datain, GLint widthout, GLint heightout, unsigned int* dataout)
{
    unsigned int* p1 = const_cast<unsigned int*>(datain);
    unsigned int* p2 = dataout;
    int nui = widthin * sizeof(unsigned int);
    for(int i = 0; i < heightin; i++)
    {
        memcpy(p2, p1, nui);
        p1 += widthin;
        p2 += widthout;
    }
}

In the render code, without changing my texture coordinates I should see the correct image when using gluScaleImage() and a smaller image (that requires some later correction factors) for the copyImage() code. This is what happens when the image is small (854x480 for example works fine with copyImage()) but when I use the 1365x768 image, that's when the skewing appears.

You should post your code, i only have an idea of what you are really doing! — Dr. Snoopy
Did this "skew" look similar to this? stackoverflow.com/q/25921458/2732991 — Halvor Holsten Strand

Nick Gotch Nick Gotch · Accepted Answer · 2011-04-08T17:15:13

Finally solved the issue. First thing to know is what's the maximum texture size allowed for the device:

GLint texSize;
glGetIntegerv(GL_MAX_TEXTURE_SIZE, &texSize);

When I ran this the texture size max for the Motorola Milestone was 2048x2048, which was fine in my case.

After messing with the texture mapping to no end I finally decided to try opening and resaving the image..and voilà it suddenly began working. I don't know what was wrong with the format the original image was stored in but as advice to anyone else experiencing a similar problem: might be worth looking at your image itself.

OpenGL Issue Drawing a Large Image Texture causing Skewing

1 Answers