Holographic Sight Reticle

Shader for the reticle of a holographic/red dot sight. Draws the reticle texture where the bullet would go in infinite distance.

Ideally apply it to a quad or other flat mesh. Normal of the mesh is used as forward direction, therefore make sure that the normal of your mesh is parallel to your barrel (when using on a gun).

Shader code

shader_type spatial;
render_mode unshaded;

uniform sampler2D reticle;
uniform vec4 reticle_color : source_color;
uniform float reticle_size = 1.0;

void fragment() {
	vec2 pixelPositionClipSpace = (SCREEN_UV - 0.5) * 2.0; // in clip space top left is (-1,-1) and bottom right is (1,1)
	vec3 normalViewSpaceNormalized = NORMAL / NORMAL.z; // normal is by default in view space; divide by z so we have z=1
	float ratio = VIEWPORT_SIZE.x / VIEWPORT_SIZE.y; // screen ratio
	float slope = -1.0 / PROJECTION_MATRIX[1][1]; // in 1m distance from the camera, how high is the top pixel row above the camera?

	// without multiplying by 'vec2(ratio,1.0)' the width of the reticle would be wrong and dependent on screen width
	vec2 UV_new = ((pixelPositionClipSpace + vec2(normalViewSpaceNormalized.x / (ratio * slope), -normalViewSpaceNormalized.y / slope)) / reticle_size) * vec2(ratio,1.0) + vec2(0.5,0.5);

	UV_new = clamp(UV_new, vec2(0,0), vec2(1,1));

	vec4 col = texture(reticle, UV_new);
	ALBEDO = col.rgb * reticle_color.rgb;
	ALPHA = col.a * reticle_color.a;
}

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