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Resolved In HDRP's compute shader how to correctly translate a World Position to ScreenSpace position?

Discussion in 'High Definition Render Pipeline' started by ryanflees, Apr 11, 2023.

  1. ryanflees

    ryanflees

    Joined:
    Nov 15, 2014
    Posts:
    59
    Hi. I have a compute shader that takes GameObject's world position ,and I want to get the screen space position of it for further operations. (Suppose there's only 1 camera in the scene)

    However I find it frustrating to do it.

    Code (CSharp):
    1. some includes
    2. ...
    3.  
    4. RWStructuredBuffer<float4> _TestData;
    5. float4 _WorldPos;
    6.  
    7. void CalculateScreenSpacePosition()
    8. {
    9.     float4 clipPos = ComputeClipSpacePosition(_WorldPos, UNITY_MATRIX_VP);
    10.     _TestData[0] = clipPos;
    11.  
    12.     float3 ndcPos = ComputeNormalizedDeviceCoordinatesWithZ(_WorldPos, UNITY_MATRIX_VP);
    13.     _TestData[1] = float4(ndcPos, 1);
    14.  
    15.     float4 screenPos = ComputeScreenPos(clipPos, _ProjectionParams.x);
    16.     _TestData[2] = screenPos;
    17. }
    18.  
    19. [numthreads(1,1,1)]
    20. void CSMain (uint3 id : SV_DispatchThreadID)
    21. {
    22.      CalculateScreenSpacePosition();
    23. }
    I did some tests and use a vector buffer to retrieve the results .
    However the positions don't look correct.

    upload_2023-4-11_19-39-18.png
    the elements in the array of the pic: clip pos, ndc pos, screen pos

    And I try to get the native matrices it doesn't feel very correct.

    upload_2023-4-11_19-41-6.png
    Code (CSharp):
    1.  _MatrixData[0] = UNITY_MATRIX_V;
    Above is the UNITY_MATRIX_V matrix I got from compute shader. It sort of lacks of position infomrations.

    However I manually log the Camera's worldToCameraMatrix in c# code, it gives correct result with positions informations.

    I can't find any related resources or guide in google/GPT/source code in HDRP to go further.
    So I need help with how to translate a world pos into screen space pos correctly in compute shader.
     
  2. ryanflees

    ryanflees

    Joined:
    Nov 15, 2014
    Posts:
    59
    OK, I've found that you need to translate the world pos to a relative WS as HDRP use camera as center for rendering.

    Code (CSharp):
    1.     float3 worldPosRelative = GetCameraRelativePositionWS(_WorldPos);
    2.     float4 clipPos = ComputeClipSpacePosition(worldPosRelative, UNITY_MATRIX_VP);
    3.     float3 ndcPos = ComputeNormalizedDeviceCoordinatesWithZ(worldPosRelative, UNITY_MATRIX_VP);
    4.  
    5.     float4 positionSS = 0;
    6.     positionSS.xy = ndcPos.xy / _ScreenSize.zw;