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Resource->ReadPixels is very slow... faster way?

I have set up a 2DSceneCapture component and using c++, I access the render target and read the pixels, like so:

 void MyActor::UpdateBuffer( )

 TArray<FColor> ColorBuffer;

 if (RenderTarget != NULL)

     FTextureRenderTarget2DResource* textureResource = (FTextureRenderTarget2DResource*)RenderTarget->Resource;

     unsigned int ReadPixelsTimer = clock();

     if (textureResource->ReadPixels(ColorBuffer))

     long ReadPixelsDuration = clock() - ReadPixelsTimer;
     GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Red, FString::Printf(TEXT("ReadPixels run time: %d"), ReadPixelsDuration));

I notice that my ReadPixels takes ages to complete... So, I would like to know if there is a faster way that I can do this?

What I really want, is to access the camera view data, but as I can't find a way to do that, I resorted to the RenderTarget method as described here: https://wiki.unrealengine.com/Render_Target_Lookup

I've also read this post: https://answers.unrealengine.com/questions/25594/accessing-pixel-values-of-texture2d.html#answer-73443 but I have no idea how to get MyTexture2D.

Any advice?

Product Version: UE 4.10
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asked Jan 25 '16 at 10:23 PM in C++ Programming

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avatar image flipswitchingmonkey May 27 '16 at 06:18 PM

Have you ever worked something out? Because I have the same problem, ReadPixels() is just crazy slow makes up for 80% of all the time my entire function uses. There must be some way to get to the raw data quickly?

I tried using ReadPixelsPtr() instead and then copy the array around, but that is just as slow.

avatar image rich2020 May 27 '16 at 08:04 PM

Nope. No further. Unfortunately, the support for this engine is not very good so despite my best efforts, I had to give up in the end. Let me know if you figure something out!

avatar image flipswitchingmonkey May 27 '16 at 08:54 PM

Well the only thing I noticed was an absolute massive difference if HDR was activated in the Render Target. That slowed things down a lot.

You could also look into running things asynchronously using Async, but that may not be viable, depending on what you are trying to do.

Otherwise, no, it seems we are stuck with ReadPixels()....

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4 answers: sort voted first

I know this question is getting quite old, but I showed up when I was looking for a solution to the same problem.

I think found a way around this by modifying ReadPixels so it does not block the game thread. I've added a description to the wiki (https://wiki.unrealengine.com/Render_Target_Lookup) in case anyone is interested.

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answered Jan 04 '17 at 12:40 PM

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avatar image Spiris Mar 02 '18 at 01:56 AM

Great article, It was actually a launch point for this class in my code. Which I also wrote an article for. Enjoy!

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I also looking for a faster solution than ReadPixels, this is really important for many tasks.

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answered Jan 12 '17 at 08:30 AM

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I have not found a faster solution than ReadPixels, but, as I explained in my own post here, you can speed ReadPixels up a lot by disabling HDR on the render target and setting the render target's resolution to match the active camera's resolution. Otherwise, the renderer has to re-allocate resources every time it reads from the render target.

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answered Jul 18 '17 at 07:59 PM

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Hey Guys. I share my solution.

Did you guys set the "bGPUSharedFlag"?? This value is belonging to "UTextureRenderTarget2D" class.

In my case, when I set this value to "true", ReadPixels() was faster than before!

This is my code. I sincerely hope it helps.

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answered Mar 03 '19 at 12:50 PM

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