world space texture sampling 2D

// uniforms
uniform mat4 inv_proj;
uniform vec2 screen_resolution;
uniform vec2 uv_scale = vec2(1.0, 1.0);

// calculate world space
vec2 screen_normalized = (gl_FragCoord.xy / screen_resolution) * 2.0 - 1.0;
vec2 world_space = (inv_proj * vec4(screen_normalized, 0.0, 1.0)).xy;

// sample texture
vec3 color = texture(terrain_texture, world_space * uv_scale).rgb;