DepthMask shader with gradient alpha

Hi there,

I have a depth mask shader to mask out certain areas on a texture.

This works fine, but I would want it to gradually go from no masking to full masking. So I thought I could just add an overlay texture to get this effect.

As you see here it works fine as long as they're not overlapping

So my question, is there a way to do the gradient alpha through the shader instead, to avoid the overlapping problem? Thanks!

And here's the shader I'm using:

 Shader "Alpha Test Depth Mask" {
      
     Properties {
         _MainTex ("Texture", 2D) = ""
     }
      
     Category {
         Tags {Queue = Background}
         ColorMask 0
      
         SubShader {Pass {
             GLSLPROGRAM
             varying lowp vec2 uv;
      
             #ifdef VERTEX
             void main() {
                 gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
                 uv = gl_MultiTexCoord0.xy;
             }
             #endif
      
             #ifdef FRAGMENT
             uniform lowp sampler2D _MainTex;
             void main() {
                 if (texture2D(_MainTex, uv).a > 0.99) 
                     discard;
             }
             #endif      
             ENDGLSL 
         }}
     }
      
     }