Converting a two layer nested for loop to nested Parallel.for loop in C#

Question

I want to convert the following code in C# to a parallel for. I searched the internet, but I could not find a proper way of doing that. I appreciate your helps.

Bitmap bmp = new Bitmap(1792, 2048);
for (int i = 0; i < 1792; i++)
{
    for (int j = 0; j < 2048; j++)
    {
        bmp.SetPixel(i,j,Color.FromArgb(100, 128, 128));
    }
}

You should probably avoid using multiple threads with `Bitmap`, since it isn't threadsafe. — Matthew Watson, Sep 29 '17 at 12:02
here is the Microsoft documentation : https://msdn.microsoft.com/en-us/library/ff963552.aspx (found by typing `c# replace a for loop by parallel for` in google) — Pac0, Sep 29 '17 at 12:02
I assume that this is just representative code - otherwise you could just use `Graphics.FromImage(Bmp)` and a single call to `Graphics.FillRectangle()` — Matthew Watson, Sep 29 '17 at 12:05
@Matthew Watson: yes, this is not the exact code I am trying to run. I have to retrieve the image data from an embedded device using UDP protocol and refresh a display under 60 ms. — Reza Ghochkhani, Sep 30 '17 at 06:16
What format is the data from the device? If it is a bitmap data (uncompressed) you can just open the data from a memory stream. But based on your original question this duplicate question is still your fastest approach. — Matthew Whited, Sep 30 '17 at 09:41
It looks like you are trying to read a video stream from a device over a network connection. Network latency alone can eat the majority of your time limit. (there is a reason network video stream lag or stutter.) — Matthew Whited, Sep 30 '17 at 09:46
@Matthew Whited: no it is not bitmap, it sends 2 bytes per pixel (from 0 to 56536) and I have to read two consecutive bytes and convert them to this number and then map them to 8-bit, or directly display the int number!!! I don't know. I may be able to use pointers, but they are not very decent in C#. — Reza Ghochkhani, Sep 30 '17 at 21:37
That's 0 to 65536. You are receiving at a 16bit bitmap. Go look at that example from the page i marked as duplicate and change the PixelFormat to 16bit and just use the first two pointers. (of just dump it to a memory stream and open the bitmap using the correct resolution and bit depth. — Matthew Whited, Sep 30 '17 at 21:55

score 0 · Answer 1 · answered Sep 29 '17 at 12:03

0

You should only replace the outer loop:

Bitmap bmp = new Bitmap(1792, 2048);
Parallel.For(0, 1792, i =>
{
    for (int j = 0; j < 2048; j++)
    {
        bmp.SetPixel(i,j,Color.FromArgb(100, 128, 128));
    }
});

This assumes it is safe to call SetPixel concurrently, of which I'm not quite sure.

answered Sep 29 '17 at 12:03

Henrik

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Bitmap is not thread thread safe – Matthew Whited Sep 29 '17 at 12:04
@MatthewWhited Thanks for the info. I already mentioned this posibility in my answer. – Henrik Sep 29 '17 at 12:05
Using `Parallel.For` will cause this to run even slower because of the internal locks in `.SetPixel(...)` – Matthew Whited Sep 29 '17 at 12:09
Sure, in this case you wouldn't use `Parallel.For`. My point is: even if it makes sense to use `Parallel.For`, one would usually only use it for the outer loop. – Henrik Sep 29 '17 at 12:16
1

This is clearly an [x/y problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). The correct solution to the actual problem of OP is in the duplicate-marked answer. – Pac0 Sep 29 '17 at 12:31
https://stackoverflow.com/questions/8807330/nested-parallel-for-loops-speed-and-performance – Matthew Whited Sep 29 '17 at 14:01
Thanks you all guys for giving such good suggestions. Yes you are right, the Parallel.For works even slower. I do not know what to do. I need to refresh a display with a changing image under 60ms. I really do not know how to make it happen. Do you think C# can do that? – Reza Ghochkhani Sep 30 '17 at 05:53

Converting a two layer nested for loop to nested Parallel.for loop in C#

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