更快地从图像中提取直方图 [英] Faster way to extract histogram from an image

问题描述

我正在寻找一种从图像中提取直方图数据的更快方法。
我目前正在使用这段代码，需要大约1200毫秒的6mpx JPEG图像：

I'm looking for a faster way to extract histogram data from an image. I'm currently using this piece of code that needs about 1200ms for a 6mpx JPEG image:

        ImageReader imageReader = (ImageReader) iter.next();
        imageReader.setInput(is);
        BufferedImage image = imageReader.read(0);
        int height = image.getHeight();
        int width = image.getWidth();
        Raster raster = image.getRaster();
        int[][] bins = new int[3][256];

        for (int i = 0; i < width; i++) 
            for (int j = 0; j < height; j++) {
                bins[0][raster.getSample(i, j, 0)]++;
                bins[1][raster.getSample(i, j, 1)]++;
                bins[2][raster.getSample(i, j, 2)]++;

            }

您有什么建议吗？

推荐答案

你正在进行很多的 getSamples 方法调用，他们反过来做电话和电话等。

You're doing a lot of getSamples method calls and they're in turn doing calls and calls etc.

我经常使用图片和 获得速度的典型技巧是直接操作底层 int [ ] （在这种情况下，您的BufferedImage必须由int []支持。）

I work often with pictures and the typical trick to gain speed is to manipulate directly the underlying int[] (in this case your BufferedImage must be backed by an int[]).

访问 int [] 然而，做一个 getRGB 可能是巨大的。当我写巨大的时候，我的意思是两个数量级（尝试在OS X 10.4和 int [x] 上做 getRGB ，你会看到性能获得）。

The difference between accessing the int[] and doing, say, a getRGB can be gigantic. When I write gigantic, I mean by as much as two orders of magnitude (try doing a getRGB on OS X 10.4 vs int[x] and you'll see the perf gain).

此外，没有任何电话三次 getSamples 。我只是检索一个与你的ARGB像素相对应的int，然后使用bitshift来获取RGB波段（你每个R，G和B组件做一个直方图吗？）。

Also, there's no call three times getSamples. I'd simply retrieve one int corresponding to your ARGB pixel and then bitshift to get the RGB bands (you're doing one histogram per R, G and B component right?).

您可以通过以下操作访问像素数组：

You can gain access to the pixels array by doing something like this:

final int[] a = ((DataBufferInt) image.getRaster().getDataBuffer()).getData();

此外，您可以使用单个循环执行您想要的操作，循环遍历所有像素。

Also you can do what you want to do with a single loop, looping over all the pixels.

而不是：

for ( int x = 0; x < width; x++ ) {
    for ( int y = 0; y < height; y++ ) {
        ....

你可以这样做：

for ( int p = 0; p < width*height; p++ ) {

现在，如果你想进入更奇怪的优化，不太可能证明你有效：

Now if you want to get into weirder optimizations, not as likely to prove effective you could:

• 使用循环展开（迭代超过600万像素是一种罕见的情况，它可能有所帮助）

• use loop unrolling (iterating over 6 million pixels is one of the rare case where it may help)

反转循环：for（p = width * height - 1; p> = 0; p - ）

invert the loop: for ( p = width*height - 1; p >= 0; p--)

这篇关于更快地从图像中提取直方图的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！