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Rectangle argument in surface.LockRectangle(Rectangle, LockFlags) is faulty or do I not understand it?

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Hi all,

First, I know next to nothing about DirectX so thank you in advance for the patience. I want to access RGB pixel data of a rectangular sub-area of the screen. I can successfully access the data for the full screen like this:

var surface = Surface.CreateOffscreenPlain(myDevice, Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height, Format.A8R8G8B8, Pool.Scratch);
myDevice.GetFrontBufferData(0, surface);
var dataRectangle = surface.LockRectangle(LockFlags.None);
DataStream rgbStream = dataRectangle.Data;

My understanding is that to get the byte stream of a 256*256 px square based in the top left the third line needs to be replaced by:

var dataRectangle = surface.LockRectangle(new Rectangle(0, 0, 256, 256), LockFlags.None);

The resulting data stream is however identical to the last byte to the one produced when there was no Rectangle argument. I've tested it with multiple values and the area of the rectangle never changed the stream in any way (the anchoring point of the rectangle on the other hand did).

Am I not understanding the method properly? Thanks for your help.

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The rectangle locked just modifies the dirty region of a texture or surface; i.e. the region that needs to be written back to the GPU after you unlock.  What you're seeing is normal enough; you can test and confirm this by locking a small rect, writing something like all-white to the texture, unlocking and seeing what part of the texture was actually updated when you use it next.

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