UIImageEffects: white image when Gaussian radius a

Question

I'm using the Gaussian blur algorithm found in Apple's UIImageEffects example:

CGFloat inputRadius = blurRadius * inputImageScale;
if (inputRadius - 2. < __FLT_EPSILON__)
    inputRadius = 2.;
uint32_t radius = floor((inputRadius * 3. * sqrt(2 * M_PI) / 4 + 0.5) / 2);

radius |= 1; // force radius to be odd so that the three box-blur methodology works.

NSInteger tempBufferSize = vImageBoxConvolve_ARGB8888(inputBuffer, outputBuffer, NULL, 0, 0, radius, radius, NULL, kvImageGetTempBufferSize | kvImageEdgeExtend);
void *tempBuffer = malloc(tempBufferSize);

vImageBoxConvolve_ARGB8888(inputBuffer, outputBuffer, tempBuffer, 0, 0, radius, radius, NULL, kvImageEdgeExtend);
vImageBoxConvolve_ARGB8888(outputBuffer, inputBuffer, tempBuffer, 0, 0, radius, radius, NULL, kvImageEdgeExtend);
vImageBoxConvolve_ARGB8888(inputBuffer, outputBuffer, tempBuffer, 0, 0, radius, radius, NULL, kvImageEdgeExtend);

free(tempBuffer);

vImage_Buffer *temp = inputBuffer;
inputBuffer = outputBuffer;
outputBuffer = temp;

I'm also working with some fairly large images. Unfortunately, when the radius gets over 280, the blurred image suddenly becomes almost completely blank, regardless of the resolution. What's going on here? Does vImageBoxConvolve_ARGB8888 have an undocumented kernel width/height limit? Or does it have to do with the way the box kernel width is computed from the radius?

---

EDIT:

Found a similar question here: vImageBoxConvolve: errors when kernel size > 255. A Gaussian radius of 280 roughly translates to a 260 size kernel, so that part matches up.

Answer 1

The box and tent convolves can run into a problem where the value modulo overflows the 31-bit accumulator. However 255 seems a bit narrow for that. There should be another 7 bits of headroom at least for 255x255. Certainly, check the error code returned by the function. If it says everything is fine, then this seems bug worthy. Attach some sample code to help Apple reproduce the problem to help ensure it is fixed.