Discard Pixels from Fragment Shader Based on Screen Pos

I’m trying to write a shader for a UI element with a canvas in world space that discards any pixels to the left of a pair of game objects, so that it looks like the text is revealed by the objects moving from right to left. The objects are also at a (variable) angle, so I’ve used a linear equation in the shader to determine when it is to the left or right.

I’m having trouble with the conversion to screen points in the shader. I read around a little bit and figured out that ComputeScreenPos() gives a range from 0 to 1 when normalized by .w, and then I tried to multiply this by _ScreenParams to get the pixel position that would be equivalent to Unity’s Camera.WorldToScreenPoint(), but I’m still not able to get it working. Here is the code:

Shader "UI/TextShader"
{
	Properties
	{
		[PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
		_Color ("Tint", Color) = (1,1,1,1)
		
		_StencilComp ("Stencil Comparison", Float) = 8
		_Stencil ("Stencil ID", Float) = 0
		_StencilOp ("Stencil Operation", Float) = 0
		_StencilWriteMask ("Stencil Write Mask", Float) = 255
		_StencilReadMask ("Stencil Read Mask", Float) = 255

		_ColorMask ("Color Mask", Float) = 15

		_Point1 ("_Point1", Vector) = (0,0,0,0)
		_Point2 ("_Point2", Vector) = (0,0,0,0)

		[Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
	}

	SubShader
	{
		Tags
		{ 
			"Queue"="Transparent" 
			"IgnoreProjector"="True" 
			"RenderType"="Transparent" 
			"PreviewType"="Plane"
			"CanUseSpriteAtlas"="True"
		}
		
		Stencil
		{
			Ref [_Stencil]
			Comp [_StencilComp]
			Pass [_StencilOp] 
			ReadMask [_StencilReadMask]
			WriteMask [_StencilWriteMask]
		}

		Cull Off
		Lighting Off
		ZWrite Off
		ZTest [unity_GUIZTestMode]
		Blend SrcAlpha OneMinusSrcAlpha
		ColorMask [_ColorMask]

		Pass
		{
		CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"
			#include "UnityUI.cginc"

			#pragma multi_compile __ UNITY_UI_ALPHACLIP
			
			struct appdata_t
			{
				float4 vertex   : POSITION;
				float4 color    : COLOR;
				float2 texcoord : TEXCOORD0;
			};

			struct v2f
			{
				float4 vertex   : SV_POSITION;
				fixed4 color    : COLOR;
				half2 texcoord  : TEXCOORD0;
				float4 screenPos : TEXCOORD1;
				float3 worldPos : TEXCOORD2;
			};
			
			fixed4 _Color;
			fixed4 _TextureSampleAdd;
			float4 _ClipRect;
			uniform float4 _Point1;
			uniform float4 _Point2;


			v2f vert(appdata_t IN)
			{
				v2f OUT;
				OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
				OUT.worldPos = mul(_Object2World, IN.vertex).xyz;
				OUT.screenPos = ComputeScreenPos(OUT.vertex);

				OUT.texcoord = IN.texcoord;
				
				#ifdef UNITY_HALF_TEXEL_OFFSET
				OUT.vertex.xy += (_ScreenParams.zw-1.0)*float2(-1,1);
				#endif

				OUT.color = IN.color * _Color;
				return OUT;
			}

			sampler2D _MainTex;

			fixed4 frag(v2f IN) : SV_Target
			{
				IN.screenPos /= IN.screenPos.w;

				//Equation of the line
				float x1 = _Point1.x;
				float x2 = _Point2.x;
				float y1 = _Point1.y;
				float y2 = _Point2.y;

				float m = (y2 - y1)/(x2 - x1);

				float b = y1 - (m * x1);

				float lineXPosition = (IN.screenPos.y * _ScreenParams.y - b)/m;

				half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;

				color.a *= UnityGet2DClipping(IN.worldPos.xy, _ClipRect);

				if(_ScreenParams.x * IN.screenPos.x > lineXPosition){
					discard;
				}

				#ifdef UNITY_UI_ALPHACLIP
				clip (color.a - 0.001);
				#endif

				return color;
			}
		ENDCG
		}
	}
}

And here is the code for the script that passes the points in to the shader:

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class SetTextline : MonoBehaviour {

	Transform pointOneTrans;
	Transform pointTwoTrans;

	Material textMaterial;

	// Use this for initialization
	void Awake () {
		pointOneTrans = GameObject.Find ("Point").GetComponent<Transform> ();
		pointTwoTrans = GameObject.Find ("Head").GetComponent<Transform> ();

		textMaterial = GameObject.Find ("Poem").GetComponent<Text> ().material;
	}
	
	// Update is called once per frame
	void Update () {
		Vector2 point1 = Camera.main.WorldToScreenPoint (pointOneTrans.position);
		Vector2 point2 = Camera.main.WorldToScreenPoint (pointTwoTrans.position);

		textMaterial.SetVector ("_Point1", point1);
		textMaterial.SetVector ("_Point2", point2);

		Debug.Log (point1);
		Debug.Log (point2);

	}
}

Thanks in advance for any help!

This is actually working now, it seems. I don’t know what I had setup incorrectly in my scene, but it’s all fine now.