Test for images that are too dark

gistfile1.m

uint8_t ComputeMax(const uint8_t* image, size_t width, size_t height, size_t stride) {
    uint8_t m = 0;
    for (int i = 0; i < height; i++) {
        const uint8_t* p = &image[i*stride];
        for (int j = 0; j < width; j++) {
            if (*p > m) {
                m = *p;
            }
            p++;
        }
    }
    return m;
}

- (bool)testDarkness:(CVImageBufferRef)imageBuffer {
    // Look up buffer size
    size_t width = CVPixelBufferGetWidth(imageBuffer);
    size_t height = CVPixelBufferGetHeight(imageBuffer);
    uint8_t *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);
    
    // TODO: confirm that baseAddress is the address of a YUV plane
    CVPlanarPixelBufferInfo_YCbCrBiPlanar *bufferInfo = (CVPlanarPixelBufferInfo_YCbCrBiPlanar *)baseAddress;
    
    uint32_t yOffset = CFSwapInt32BigToHost(bufferInfo->componentInfoY.offset);
    uint32_t yPitch = CFSwapInt32BigToHost(bufferInfo->componentInfoY.rowBytes);
    uint8_t *yBuffer = &baseAddress[yOffset];

    uint8_t max = ComputeMax(yBuffer, width, height, yPitch);
    
    if (max < 100) {
        NSLog(@"Too dark! Max: %d\n", max);
    }
    
    return true;
}