GetData from RenderTarget

Hi.
I have an Android/iOS app running with Urho3D and I need to grab a snapshot.
I’ve tried using an aux rendertarget texture and use GetData to get the texture contents, but I’m getting a “Getting texture data not supported” error.
What am I doing wrong? Is there any other way of doing it?
Thank you.

It’s not supported on mobile devices. GetData makes an underlying call to glGetTexImage, which is not supported on GL ES. AFAIK, glBlitFramebuffer is going to be needed but I am not sure if it’s going to be supported here.

glReadPixels, why not?

We could add an OpenGL-only method to Graphics to use glReadPixels to read from the backbuffer or the current rendertarget. Doing it yourself carries a risk that the current rendertarget is not actually current, as it’s being lazily managed before each draw call.

Are guys willing to implement it? We have it working on a custom made engine which is shared between iOS and Android. It was written in c# on top of OpenTK. The function that we have for more than 1 year is the following

    unsafe public byte[] GetTextureBytes()
    {
        byte[] bytes = new byte[Texture.Width * Texture.Height * 4];

        var state = new StateManager ();
        state.Push ();

        FBOManager.Active = FrameBuffer;
        TextureManager.Active = Texture;

        fixed(byte* buffer = &bytes[0]) 
        {
            GL.PixelStore (PixelStoreParameter.PackAlignment, 1);
            GL.ReadPixels (0, 0, Texture.Width, Texture.Height, PixelFormat.Rgba, PixelType.UnsignedByte, (IntPtr)buffer);
        }

        state.Pop ();

        return bytes;
    }

cheers
Manuel

It’s one possibility, another possibility is to just expose a function which makes sure the selected rendertarget (ie. after Graphics::SetRenderTarget) is current, to allow the user to make the glReadPixels() call themselves.The latter could be desirable in case the parameters of glReadPixels() are not obvious, but I suspect in most cases one wants to read 8bit RGB(A) data.

Took a look at the GL rendering code. You can use the following sequence to be able to use glReadPixels from either the backbuffer or a rendertarget:

  • Graphics::SetRenderTarget() <- null ptr for backbuffer
  • Graphics::SetViewport() <- can be anything you like, this ensures that the proper FBO is active
  • Now you’re safe to call glReadPixels()

Also for a fullscreen capture of the backbuffer you’re already able to just use Graphics::TakeScreenShot().

Thank you cadaver, I’ll give it a try.
Nonetheless, it would be great to have a crossplatform way of doing it (we want to support Windows too).

The glReadPixels() GLES workaround was rather easy to add directly to Texture2D::GetData() and TextureCube::GetData(), it’s in the master branch now. It works if the texture in question is a rendertarget. It induces a rendertarget switch when you read the data, so it possibly isn’t very performant.

github.com/urho3d/Urho3D/commit … 17102db709

Tkx!