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!