使用vImageHistogramCalculation计算图像的直方图

我想使用vImage的vImageHistogramCalculation_ARGBFFFF来计算图像的直方图，但我得到一个types为kvImageNullPointerArgument （错误代码-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); } 

我怀疑这与我的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”。 我是否正确地宣布？

谢谢。

Answer 1

麻烦的是，你没有分配空间来保存计算的直方图。 如果你只是在本地使用直方图，你可以像这样把它们放在堆栈上[请注意，我使用了八个bin，而不是四个，以便使示例更清晰]：

 // 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. 

如果你需要直方图在函数的作用域之外，你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。