Re: Speed of horizontal flip

Thu, 02 Apr 2015 04:30:54 -0700 

On Thursday, 2 April 2015 at 09:55:15 UTC, Rikki Cattermole wrote:

On 2/04/2015 10:47 p.m., Rikki Cattermole wrote:

On 2/04/2015 2:52 a.m., tchaloupka wrote:

Hi,
I have a bunch of square r16 and png images which I need to flip 
horizontally.

My flip method looks like this:
void hFlip(T)(T[] data, int w)
{
   import std.datetime : StopWatch;

   StopWatch sw;
   sw.start();

   foreach(int i; 0..w)
   {
     auto row = data[i*w..(i+1)*w];
     row.reverse();
   }

   sw.stop();
   writeln("Img flipped in: ", sw.peek().msecs, "[ms]");
}

With simple r16 file format its pretty fast, but with RGB PNG
files (2048x2048) I noticed its somewhat slow so I tried to
compare it with C# and was pretty surprised by the results.

C#:
PNG load - 90ms
PNG flip - 10ms
PNG save - 380ms

D using dlib (http://code.dlang.org/packages/dlib):
PNG load - 500ms
PNG flip - 30ms
PNG save - 950ms

D using imageformats
(http://code.dlang.org/packages/imageformats):
PNG load - 230ms
PNG flip - 30ms
PNG save - 1100ms

I used dmd-2.0.67 with -release -inline -O
C# was just with debug and VisualStudio attached to process for 
debugging and even with that it is much faster.

I know that System.Drawing is using Windows GDI+, that can be
used with D too, but not on linux.
If we ignore the PNG loading and saving (didn't tried libpng
yet), even flip method itself is 3 times slower - I don't know D enough to be sure if there isn't some more effecient way to make 
the flip. I like how the slices can be used here.

For a C# user who is expecting things to just work as fast as
possible from a system level programming language this can be
somewhat disappointing to see that pure D version is about 3
times slower.

Am I doing something utterly wrong?
Note that this example is not critical for me, it's just a simple hobby script I use to move and flip some images - I can wait. But I post it to see if this can be taken somewhat closer to what can 
be expected from a system level programming language.

dlib:
auto im = loadPNG(name);
hFlip(cast(ubyte[3][])im.data, cast(int)im.width);
savePNG(im, newName);

imageformats:
auto im = read_image(name);
hFlip(cast(ubyte[3][])im.pixels, cast(int)im.w);
write_image(newName, im.w, im.h, im.pixels);

C# code:
static void Main(string[] args)
         {
             var files = Directory.GetFiles(args[0]);

             foreach (var f in files)
             {
                 var sw = Stopwatch.StartNew();
                 var img = Image.FromFile(f);

                 Debug.WriteLine("Img loaded in {0}[ms]",
(int)sw.Elapsed.TotalMilliseconds);
                 sw.Restart();
img.RotateFlip(RotateFlipType.RotateNoneFlipX); 
                 Debug.WriteLine("Img flipped in {0}[ms]",
(int)sw.Elapsed.TotalMilliseconds);
                 sw.Restart();

                 img.Save(Path.Combine(args[0], "test_" +
Path.GetFileName(f)));
                 Debug.WriteLine("Img saved in {0}[ms]",
(int)sw.Elapsed.TotalMilliseconds);
                 sw.Stop();
             }
         }
Assuming I've done it correctly, Devisualization.Image takes around 8ms in debug mode to flip horizontally using dmd. But 3ms for release. 

module test;

void main() {
    import devisualization.image;
    import devisualization.image.mutable;
    import devisualization.util.core.linegraph;

    import std.stdio;

    writeln("===============\nREAD\n===============");
    Image img = imageFromFile("test/large.png");
    img = new MutableImage(img);

    import std.datetime : StopWatch;

    StopWatch sw;
    sw.start();

    foreach(i; 0 .. 1000) {
        img.flipHorizontal;
    }

    sw.stop();
writeln("Img flipped in: ", sw.peek().msecs / 1000, "[ms]"); 
}
I was planning on doing this earlier. But I discovered a PR I pulled 
which fixed for 2.067 broke chunk types reading.
My bad, forgot I decreased test image resolution to 256x256. I'm totally out of the running. I have some serious work to do by the looks.
Have you considered just being able to grab an object with changed iteration order instead of actually doing the flip? The same goes for transposes and 90ยบ rotations. Sure, sometimes you do need actually rearrange the memory and in a subset of those cases you need it to be done fast, but a lot of the time you're better off* just using a different iteration scheme (which, for ranges, should probably be part of the type to avoid checking the scheme every iteration).
*for speed and memory reasons. Need to keep the original and the transpose? No need to for any duplicates
Note that this is what numpy does with transposes. The .T and .transpose methods of ndarray don't actually modify the data, they just set the memory order** whereas the transpose function actually moves memory around.
**using a runtime flag, which is ok for them because internal iteration lets you only branch once on it.

Reply via email to