Shader draw lines

Hey !

I'm currently trying to write a shader to draw horizontal lines on the screen (kinda like a scanline shader but not regular).

I managed to write a frag shader but because of the transformation of the screen coordinate from 0,1 to 0,screenSize sometimes the shader isn't precise enought and misses some lines to render.

             struct vertOut {
                 float4 pos:SV_POSITION;
                 float4 src:TEXCOORD;
             };
             
             vertOut vert(appdata_base v) {
                 vertOut o;
                 o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
                 o.src = ComputeScreenPos(o.pos);
                 return o;
             }
             
             fixed4 frag(vertOut i) : COLOR0 {
                 // _ScreenParams.xy => width/height of screen
                 fixed4 output = fixed4(1,1,1,1);
                 
                 float2 wcoord = (i.src.xy / i.src.w);
                 wcoord.y = 1 - wcoord.y;
                 int y = floor(wcoord.y * _ScreenParams.y);

The y here is most of the time the right position in the screen but sometimes it match the previous or next line ...

I guess my question is : What would be the best way to be able to draw horizontal lines of 1px (thickness) at a specific position of the screen ? Maybe there is a better way than shaders ? :D

(Must work on iOS)

Many thanks !