Representing Lens Distortion coordinates in 2D I with my custom lens shader - What am I doing wrong?

Hi, I am experimenting with something lens related and I have a 'strange' problem: ---------- ![alt text][1] ---------- **Scenario:** - I have a grid/checkerboard (black/blue) in 3D space to visualize the lens distortion - I want to represent the distortion in 2D space (Canvas) by converting the checkerboard from world space to canvas (red grid). - **The checkerboard and the red grid should match each other.** I have done this by using the **same distortion algorithm with my shader and the C# script that generates the red UI grid.** ---------- The 2D representation of the camera is working great without my shader, but when I enable the shader, it works as if it is inverted (as seen in the gif above). I have simplified the shader algorithm to show the problem easier here. **The shader logic:** ---------- float2 GetSimpleDistortion(float2 uv) { float2 center = float2(0.5, 0.5); // Center of the screen float2 d = uv - center; // Distance vector from center float r = sqrt(d.x * d.x + d.y * d.y); // Distance from center (hypothenusa) float2 newuv = float2(0.0, 0.0); newuv.x = uv.x + d.x * r * r * _simpleK; // move away from the center newuv.y = uv.y + d.y * r * r * _simpleK; return newuv; } float4 LensEffectFragmentProgram(PostProcessVaryings input) : SV_Target { UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input); float2 uv = UnityStereoTransformScreenSpaceTex(input.texcoord); uv = GetSimpleDistortion(uv); float4 color = LOAD_TEXTURE2D_X(_MainTex, uv * _ScreenSize.xy); return color; } ---------- And then **in my C# script,** I am distorting the red UI grid with the same algorithm: ---------- private Vector2 GetSimpleLensDistortion(Vector3 viewPort, MQMLensEffect _lensEffect) { Vector2 uv = new Vector2(viewPort.x, viewPort.y); Vector2 center = new Vector2(0.5f, 0.5f); // Center of the screen Vector2 d = uv - center; // Distance vector from center float r = Mathf.Sqrt(d.x * d.x + d.y * d.y); // Distance from center (hypothenusa) Vector2 newuv = Vector2.zero; newuv.x = uv.x + d.x * r * r * _lensEffect.simpleK.value; // move away from the center newuv.y = uv.y + d.y * r * r * _lensEffect.simpleK.value; return newuv; } ---------- And this is **how I move the UI grid:** for (int x = 0; x < sizex; x++) { for (int y = 0; y < sizey; y++) { Vector3 viewport = cam.WorldToViewportPoint(grid3d_points[x,y].position); Vector2 uv = GetSimpleLensDistortion(viewport, _lensEffect); var newViewport = new Vector3(uv.x, uv.y, viewport.z); var canvas_pos = Vector3.Scale(newViewport, canvasRect.sizeDelta); screen_handles[x,y].anchoredPosition = new Vector2(canvas_pos.x, canvas_pos.y); } } ---------- I am doing this by **converting the 3D grid points to viewport coordinates** - which I think should correspond with the shader's UV-coordinates? - and then to canvas coordinates. But somehow they don't match. The only variable controlling the distortion is a float 'simpleK' - if simpleK == 0, there is no distortion and the grids match perfectly. **It feels like it is working almost, but inverted** - as in I am watching the effect from 'behind' or something. And actually, if I invert the simpleK (simpleK = -simpleK) variable only in the C# script, it almost works, but with little bit strange scaling. **Here is an gif showing what happens if simpleK = -simpleK in the C# script:** ---------- ![alt text][2] ---------- ---------- **So the final question is:** What am I missing in my 2D calculations? Something related to camera frustum or some simple factor or what? I am lost. Help, please. ---------- [1]: https://i.ibb.co/jGF0GZG/Shader-K-normal.gif [2]: https://i.ibb.co/tBQ7Rzy/Shader-K-inverted.gif