Metal api - tex2d ReadPixels() source Y and SystemInfo.graphicsUVStartsAtTop

Unity Engine
1

Hi,

Noticed that under macOS and Metal api when calling texture2d ReadPixels() I have to use source rect coords with origin bottom-left (like OpenGL). Using Unity 5.6. Is this expected?

  • SystemInfo.graphicsUVStartsAtTop is “true” for Metal (same as directx)
  • googling shows that Metal has top-left coords origin (same as directx)

But with ReadPixels() source rect coods (not the destination) Metal behaves like OpenGL (bottom-left).

Aso did a test:

  • filled a render texture top halp white, bottom half black
  • read the topmost pixel into a texture2d and compared it to white
    Let 's call it ReadPixelsYStartsAtTop test.
    Did Debug.Log() of ReadPixelsYStartsAtTop and SystemInfo.graphicsUVStartsAtTop on different plaforms/api:
    3042823--227994--ReadPixelsTest results 2.png
    Only with Metal the results are different.
2

Hi monstercho,

I can confirm with Unity 2018.2.2f1 / iOS Metal SystemInfo.graphicsUVStartsAtTop returns TRUE, yet pixels are read from the bottom.

1 Like
3

does anybody know the solution?? whitch funtion can we use to detect the orition,so we can correctly set ReadPixels’s source rect.

4

This is pulled from a related terrain function, written by Unity, so most likely works for this:

          if (SystemInfo.graphicsDeviceType == GraphicsDeviceType.Metal || !SystemInfo.graphicsUVStartsAtTop)
          {
            alphamapTexture.ReadPixels(new Rect((float) sourceRect.x, (float) sourceRect.y, (float) sourceRect.width, (float) sourceRect.height), dest.x, dest.y);
          }
          else
          {
            alphamapTexture.ReadPixels(new Rect((float) sourceRect.x, (float) (sourceTex.height - sourceRect.yMax), (float) sourceRect.width, (float) sourceRect.height), dest.x, dest.y);
          }
          alphamapTexture.Apply();
1 Like
5

thanks, i’ll try.