Question

I followed along with this tutorial (http://www.bit-101.com/blog/?p=1861) and noticed that upon saving the same image multiple times, the quality slowly degraded.

Aside from the memory leaks, what’s going wrong here? It should be pulling 4 bytes (rgba) for each pixel. Where’s the loss if each pixel is accounted for?

—————– EDIT —————–

I’m saving a new image from the pixel data each time there’s a vertex position transformation, then load this altered image into my texture buffer, and reset the vertex/index buffers. That way I can keep my changes persistent and ultimately make a less choppy warp. See my other SO question: OpenGL ES 2.0 Vertex Transformation Algorithms

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—————– EDIT —————–

Before

After

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Here’s the code from the tutorial:

-(UIImage *) glToUIImage {
    NSInteger myDataLength = 320 * 480 * 4;

    // allocate array and read pixels into it.
    GLubyte *buffer = (GLubyte *) malloc(myDataLength);
    glReadPixels(0, 0, 320, 480, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

    // gl renders "upside down" so swap top to bottom into new array.
    // there's gotta be a better way, but this works.
    GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
    for(int y = 0; y < 480; y++)
    {
        for(int x = 0; x < 320 * 4; x++)
        {
            buffer2[(479 - y) * 320 * 4 + x] = buffer[y * 4 * 320 + x];
        }
    }

    // make data provider with data.
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer2, myDataLength, NULL);

    // prep the ingredients
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;
    int bytesPerRow = 4 * 320;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

    // make the cgimage
    CGImageRef imageRef = CGImageCreate(320, 480, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);

    // then make the uiimage from that
    UIImage *myImage = [UIImage imageWithCGImage:imageRef];
    return myImage;
}

-(void)captureToPhotoAlbum {
    UIImage *image = [self glToUIImage];
    UIImageWriteToSavedPhotosAlbum(image, self, nil, nil);
}

Answer 1

Every time you render the altered image, it is (necessartly) being resampled — that is, converted to a bitmap where the original pixels (texels) align with the screen grid other than on a 1:1 basis. This is necessarily lossy, in that you have lost some of the detail of the original image, so you will get worse results if you distort that image again, compared to transforming the original image with different parameters.