4

我正在尝试使用 vImage's 计算图像的直方图vImageHistogramCalculation_ARGBFFFF，但我得到了一个vImage_Error类型kvImageNullPointerArgument（错误代码 a -21772）。

这是我的代码：

- (void)histogramForImage:(UIImage *)image {

    //setup inBuffer
    vImage_Buffer inBuffer;

    //Get CGImage from UIImage
    CGImageRef img = image.CGImage;

    //create vImage_Buffer with data from CGImageRef
    CGDataProviderRef inProvider = CGImageGetDataProvider(img);
    CFDataRef inBitmapData = CGDataProviderCopyData(inProvider);

    //The next three lines set up the inBuffer object
    inBuffer.width = CGImageGetWidth(img);
    inBuffer.height = CGImageGetHeight(img);
    inBuffer.rowBytes = CGImageGetBytesPerRow(img);

    //This sets the pointer to the data for the inBuffer object
    inBuffer.data = (void*)CFDataGetBytePtr(inBitmapData);

    //Prepare the parameters to pass to vImageHistogramCalculation_ARGBFFFF
    vImagePixelCount *histogram[4] = {0};
    unsigned int histogram_entries = 4;
    Pixel_F minVal = 0;
    Pixel_F maxVal = 255;
    vImage_Flags flags = kvImageNoFlags;

    vImage_Error error = vImageHistogramCalculation_ARGBFFFF(&inBuffer,
                                                             histogram,
                                                             histogram_entries,
                                                             minVal,
                                                             maxVal,
                                                             flags);
    if (error) {
        NSLog(@"error %ld", error);
    }

    //clean up
    CGDataProviderRelease(inProvider);
}

我怀疑它与我的histogram参数有关，根据文档，它应该是“指向四个直方图数组的指针”。我是否正确声明了它？

谢谢。

4

1 回答 1

5

问题是您没有分配空间来保存计算的直方图。如果您只在本地使用直方图，您可以像这样将它们放在堆栈中[注意我使用的是八个 bin 而不是四个，以使示例更清晰]：

// create an array of four histograms with eight entries each.
vImagePixelCount histogram[4][8] = {{0}};
// vImageHistogramCalculation requires an array of pointers to the histograms.
vImagePixelCount *histogramPointers[4] = { &histogram[0][0], &histogram[1][0], &histogram[2][0], &histogram[3][0] };
vImage_Error error = vImageHistogramCalculation_ARGBFFFF(&inBuffer, histogramPointers, 8, 0, 255, kvImageNoFlags);
// You can now access bin j of the histogram for channel i as histogram[i][j].
// The storage for the histogram will be cleaned up when execution leaves the
// current lexical block.

如果您需要直方图停留在函数范围之外，则需要在堆上为它们分配空间：

vImagePixelCount *histogram[4];
unsigned int histogramEntries = 8;
histogram[0] = malloc(4*histogramEntries*sizeof histogram[0][0]);
if (!histogram[0]) { // handle error however is appropriate }
for (int i=1; i<4; ++i) { histogram[i] = &histogram[0][i*histogramEntries]; }
vImage_Error error = vImageHistogramCalculation_ARGBFFFF(&inBuffer, histogram, 8, 0, 255, kvImageNoFlags);
// You can now access bin j of the histogram for channel i as histogram[i][j].
// Eventually you will need to free(histogram[0]) to release the storage.

希望这可以帮助。

于 2014-09-09T15:17:46.253 回答