使用vImageHistogramCalculation计算图像的直方图

Question

使用vImageHistogramCalculation计算图像的直方图

iosiphonemacosaccelerate-frameworkvimage

4

我正在尝试使用vImage的vImageHistogramCalculation_ARGBFFFF计算图像的直方图，但是我收到了一个类型为kvImageNullPointerArgument（错误代码为-21772）的vImage_Error。

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);
}

我猜测这与我的直方图参数有关，根据文档，它应该是“指向四个直方图数组的指针”。我声明正确吗？

谢谢。

- Eric

问题在这里：vImagePixelCount *histogram[4] = {0}; 你正在初始化一个包含直方图结果的四个空指针的数组。如果没有其他人先回答，我稍后会提供更详细的答案。 - Stephen Canon

嘿@StephenCanon，看起来除了你之外没有其他人处理它。如果你能提供上面评论的更多细节就太好了！ - Eric

@Eric...我尝试了Stephen的逻辑，但在两种情况下都出现了EXC_BAD_Access错误。你能建议我可能出了什么问题吗？ - Imran

1个回答

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原文链接

- Stephen Canon · Accepted Answer

麻烦的是，你没有为计算得出的直方图分配空间。如果只在本地使用直方图，可以将它们放在堆栈中，像这样[请注意我使用了八个箱子而不是四个，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.

如果您需要在函数范围之外保留直方图，您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.

希望这能帮到你。