Most effective way to use a single bitmap Graphics.CopyFromScreen in multiple threads?

Question

So I have one thread that will constantly be grabbing a screenshot and placing the bitmap returned from Graphics.CopyFromScreen into a public bitmap object. The idea is to have multiple other threads use this bitmap, but only for reading purposes, no writing to the bitmap. Originally I tried just simply reading from it but quickly found out that it locks the whole bitmap so that it can't be used elsewhere. My second alternative was to just create a new temporary bitmap in each thread that needs to use the public bitmap, and simply tempBitMap = publicBitMap. This is seeming to throw some exceptions and I feel like there's probably a better way anyway. I saw that you can lock specific bits / pixels that you need to use in the multiple threads and then release them once you're done with them, but does that prevent writing to the whole bitmap for the duration of the lock? Or does it simply write to the whole bitmap except for those locked bits / pixels. The methods that are using the public bitmap are only using one pixel at a time so this could probably work, providing that the public bitmap can be wrote to whilst some of its bits / pixels are locked.

Just looking for the most efficient solution, if anyone can give advice on the most effective solution I'd be very grateful, thanks.

EDIT: I am using the BitMap.GetPixel() method which locks the bitmap.

Answer 1

My second alternative was to just create a new temporary bitmap in each thread that needs to use the public bitmap, and simply tempBitMap = publicBitMap

This is the right approach. You can't write and read a bitmap at the same time. You should probably have two bitmaps active at the same time. One for writing, one for the readers. After each write you swap them. Note, that the readers must be done with the bitmap before writing can begin.

If you create a new bitmap for writing each time the hand-off becomes simpler but you now have the problem that you need to dispose each bitmap once it is no longer in use. If you don't do that you can very quickly run out of RAM because the GC does not recognize the existence of unmanaged memory buffers.

This is seeming to throw some exceptions and I feel like there's probably a better way anyway.

You should probably debug them. There's nothing fundamental why this wouldn't work.

I saw that you can lock specific bits / pixels that you need to use in the multiple threads and then release them once you're done with them, but does that prevent writing to the whole bitmap for the duration of the lock?

I don't know. This sounds like a very complex scheme. Having some regions readable and some writable would require quite an amount of coordination and you can never read a consistent full bitmap.

Just looking for the most efficient solution

That would be the two-bitmap model.

Answer 2

I guess you only set another variable to point at the same bitmap. You will need to Clone the bitmap to have a different one in each thread.