逐像素进行图像处理需要很长时间

Question

I'm writing an Android application which processes an image pixel by pixel. To read and set the pixels I use two nested loops.

This process is very time-consuming. Even if I delete the instructions in the loops, it takes about a minute to run on an 8 megapixel image.

Why is it taking so long? Are there any other ways to get and set pixels?

    long mn = img_w*img_h;
    long o = 0;
    for(int i =0;i<img_w;i++) {
        for(int j=0;j<img_h;j++) {

                System.out.println(mn-o);

        o++;

        }
    }

This code simulate processing image, and this code take very long time

logcat (section):

    02-12 19:01:40.815: I/System.out(332): 4999.0
02-12 19:01:40.815: I/System.out(332): 4998.0
02-12 19:01:40.815: I/System.out(332): 4997.0
02-12 19:01:40.825: I/System.out(332): 4996.0
02-12 19:01:40.825: I/System.out(332): 4995.0
02-12 19:01:40.825: I/System.out(332): 4994.0
02-12 19:01:40.825: I/System.out(332): 4993.0
02-12 19:01:40.825: I/System.out(332): 4992.0
02-12 19:01:40.825: I/System.out(332): 4991.0
02-12 19:01:40.825: I/System.out(332): 4990.0
02-12 19:01:40.835: I/System.out(332): 4989.0
02-12 19:01:40.835: I/System.out(332): 4988.0
02-12 19:01:40.835: I/System.out(332): 4987.0
02-12 19:01:40.835: I/System.out(332): 4986.0
02-12 19:01:40.845: I/System.out(332): 4985.0
02-12 19:01:40.845: I/System.out(332): 4984.0
02-12 19:01:40.845: I/System.out(332): 4983.0
02-12 19:01:40.845: I/System.out(332): 4982.0
02-12 19:01:40.845: I/System.out(332): 4981.0
02-12 19:01:40.845: I/System.out(332): 4980.0
02-12 19:01:40.845: I/System.out(332): 4979.0
02-12 19:01:40.845: I/System.out(332): 4978.0
02-12 19:01:40.855: I/System.out(332): 4977.0
02-12 19:01:40.855: I/System.out(332): 4976.0
02-12 19:01:40.855: I/System.out(332): 4975.0
02-12 19:01:40.855: I/System.out(332): 4974.0
02-12 19:01:40.855: I/System.out(332): 4973.0
02-12 19:01:40.865: I/System.out(332): 4972.0

Answer 1

The way you have arranged your loops is very cache unfriendly. Make the inner loop pass over the width and the outer loop pass over the height.

Answer 2

I don't think you can solve your problem in Java in this manner. There are overheads involved in executing Java statements (and method calls in particular) and doing this 8 million times - even if the instructions do almost nothing, like setting a value - will always be slow.

The standard answer would be to implement this function in C++ which is compiled instead of interpreted by the Java Virtual Machine, and hence should be faster. It still probably won't be fast enough for a real time application; you are still doing something 8 million times, just less each time.

Doing peek and poke operations in Java across an entire 8 Mb pixel buffer isn't ever going to be fast. However, the designers of Java were aware of this issue, which is why they provide the extended 2D graphics API to often eliminate the need for for loops that execute a million times. I would recommend having a closer look at the 2D API library, which is designed to set and read bits in a bitmap. Depending on what you are really trying to do, there may be much faster ways. You can set up a bitmap with transparent background, and write your changes to this bitmap, and write the bitmap on top of the previous image. When you write bitmaps on top of bitmaps, you can set up a Paint class which will manipulate the impressed image (eg by applying variable transparency to it). The Android 2d Graphics API also supports "Porter-Duff" compositing which provides calls which allow images to be combined in novel ways. All of these are essentially performed in hardware, and hence are extremely fast.

Without knowing what you are trying to do (exactly), I can't tell you how to do it more efficiently, but in a lot of cases there is a far more efficient implementation than the brute force approach you are using. If there isn't, and you want this real time, I suspect you are stuffed.