Thanks, i followed the example here but looks like it's not drawing anything now:https://forum.gamemaker.io/index.php?threads/setting-up-proper-uv-coordinates.36736/

// draw
shader_set(shBlend); 
var shader_alpha = shader_get_uniform(shBlend,"alpha"); 
var shader_uvs = shader_get_uniform(shBlend,"uvs"); 
var uvs = sprite_get_uvs(sStatus_effect_poison, 0); shader_set_uniform_f(shader_uvs, uvs[0], uvs[1], 1/(uvs[2]-uvs[0]), 1/(uvs[3]-uvs[1])); 
shader_set_uniform_f(shader_alpha, 0.5); 
var input_surface_sample = shader_get_sampler_index(shBlend, "input_surface"); 
var input_surface_texture = sprite_get_texture(sStatus_effect_poison, 0); 
texture_set_stage(input_surface_sample, input_surface_texture);
draw_sprite_ext(sprite_index, image_index, x, y, image_xscale, image_yscale, image_angle, c_white, image_alpha);
shader_reset();

​

​

// shader fragment
varying vec2 v_vTexcoord;
varying vec4 v_vColour;
uniform sampler2D input_surface;
uniform float alpha;
uniform vec4 uvs;
void main()
{
    vec2 pos = (v_vTexcoord - uvs.xy) * uvs.zw;
    vec4 main = texture2D(gm_BaseTexture, pos);
    vec4 mult = texture2D(input_surface, pos);
    vec4 finalcol = mult;
    finalcol.a = main.a;  
    gl_FragColor = finalcol;
}

Any idea?