var temporary;

draw_enable_alphablend(false);

    // Advect velocity.
    surface_set_target(sf_temporary_0);
        if (maccormack_amount_velocity > 0) {
            shader_set(sh_advect_velocity_maccormack);
                texture_set_stage(shader_get_sampler_index(sh_advect_velocity_maccormack, "texture_world"), surface_get_texture(sf_world));
                texture_set_stage(shader_get_sampler_index(sh_advect_velocity_maccormack, "texture_material"), surface_get_texture(sf_material_0));
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity_maccormack, "texel_size"), sf_texel_width, sf_texel_height);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity_maccormack, "dissipation"), velocity_dissipation);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity_maccormack, "acceleration"), 0, acceleration_y);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity_maccormack, "time_step"), time_step);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity_maccormack, "maccormack_amount"), maccormack_amount_velocity);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity_maccormack, "mode"), mode);
                draw_surface(sf_velocity, 0, 0);
            shader_reset();
        } else {
            shader_set(sh_advect_velocity);
                texture_set_stage(shader_get_sampler_index(sh_advect_velocity, "texture_world"), surface_get_texture(sf_world));
                texture_set_stage(shader_get_sampler_index(sh_advect_velocity, "texture_material"), surface_get_texture(sf_material_0));
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity, "texel_size"), sf_texel_width, sf_texel_height);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity, "dissipation"), velocity_dissipation);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity, "acceleration"), 0, acceleration_y);
                shader_set_uniform_f(shader_get_uniform(sh_advect_velocity, "time_step"), time_step);
                draw_surface(sf_velocity, 0, 0);
            shader_reset();
        }
    surface_reset_target();
    temporary = sf_velocity; sf_velocity = sf_temporary_0; sf_temporary_0 = temporary;

    // Calculates divergence of velocity.
    surface_set_target(sf_temporary_0);
        shader_set(sh_calculate_divergence);
            shader_set_uniform_f(shader_get_uniform(sh_calculate_divergence, "texel_size"), sf_texel_width, sf_texel_height);
            draw_surface(sf_velocity, 0, 0);
        shader_reset();
    surface_reset_target();
    temporary = sf_divergence; sf_divergence = sf_temporary_0; sf_temporary_0 = temporary;

    // Clears pressure.
    surface_set_target(sf_pressure);
        draw_clear_alpha($0000C0, 0);
    surface_reset_target();

    // Calculates pressure with jacobi iteration.
    shader_set(sh_calculate_pressure);
        texture_set_stage(shader_get_sampler_index(sh_calculate_pressure, "texture_divergence"), surface_get_texture(sf_divergence));
        shader_set_uniform_f(shader_get_uniform(sh_calculate_pressure, "texel_size"), sf_texel_width, sf_texel_height);
        repeat (pressure_jacobi_iterations) {
            surface_set_target(sf_temporary_0);
                draw_surface(sf_pressure, 0, 0);
            surface_reset_target();
            temporary = sf_pressure; sf_pressure = sf_temporary_0; sf_temporary_0 = temporary;
        }
    shader_reset();

    // Calculates the gradient of pressure and subtracts it from the velocity.
    surface_set_target(sf_temporary_0);
        shader_set(sh_subtract_pressure_gradient);
            texture_set_stage(shader_get_sampler_index(sh_subtract_pressure_gradient, "texture_pressure"), surface_get_texture(sf_pressure));
            shader_set_uniform_f(shader_get_uniform(sh_subtract_pressure_gradient, "texel_size"), sf_texel_width, sf_texel_height);
            draw_surface(sf_velocity, 0, 0);
        shader_reset();
    surface_reset_target();
    temporary = sf_velocity; sf_velocity = sf_temporary_0; sf_temporary_0 = temporary;

    // Advects material.
    if (mode == 2) {
        surface_set_target_ext(0, sf_temporary_0);
        surface_set_target_ext(1, sf_temporary_1);
            shader_set(sh_advect_material_smoke);
                texture_set_stage(shader_get_sampler_index(sh_advect_material_smoke, "texture_material_1"), surface_get_texture(sf_material_1));
                texture_set_stage(shader_get_sampler_index(sh_advect_material_smoke, "texture_velocity"), surface_get_texture(sf_velocity));
                texture_set_stage(shader_get_sampler_index(sh_advect_material_smoke, "texture_world"), surface_get_texture(sf_world));
                shader_set_uniform_f(shader_get_uniform(sh_advect_material_smoke, "texel_size"), sf_texel_width, sf_texel_height);
                shader_set_uniform_f(shader_get_uniform(sh_advect_material_smoke, "dissipation"), material_dissipation);
                shader_set_uniform_f(shader_get_uniform(sh_advect_material_smoke, "time_step"), time_step);
                shader_set_uniform_f(shader_get_uniform(sh_advect_material_smoke, "maccormack_amount"), maccormack_amount_material);
                draw_surface(sf_material_0, 0, 0);
            shader_reset();
        surface_reset_target();
        temporary = sf_material_0; sf_material_0 = sf_temporary_0; sf_temporary_0 = temporary;
        temporary = sf_material_1; sf_material_1 = sf_temporary_1; sf_temporary_1 = temporary;
    } else {
        surface_set_target_ext(0, sf_temporary_0);
        surface_set_target_ext(1, sf_temporary_1);
            if (maccormack_amount_material > 0) {
                shader_set(sh_advect_material_maccormack);
                    texture_set_stage(shader_get_sampler_index(sh_advect_material_maccormack, "texture_material_1"), surface_get_texture(sf_material_1));
                    texture_set_stage(shader_get_sampler_index(sh_advect_material_maccormack, "texture_velocity"), surface_get_texture(sf_velocity));
                    texture_set_stage(shader_get_sampler_index(sh_advect_material_maccormack, "texture_world"), surface_get_texture(sf_world));
                    shader_set_uniform_f(shader_get_uniform(sh_advect_material_maccormack, "texel_size"), sf_texel_width, sf_texel_height);
                    shader_set_uniform_f(shader_get_uniform(sh_advect_material_maccormack, "dissipation"), material_dissipation);
                    shader_set_uniform_f(shader_get_uniform(sh_advect_material_maccormack, "time_step"), time_step);
                    shader_set_uniform_f(shader_get_uniform(sh_advect_material_maccormack, "maccormack_amount"), maccormack_amount_material);
                    draw_surface(sf_material_0, 0, 0);
                shader_reset();
            } else {
                shader_set(sh_advect_material);
                    texture_set_stage(shader_get_sampler_index(sh_advect_material, "texture_material_1"), surface_get_texture(sf_material_1));
                    texture_set_stage(shader_get_sampler_index(sh_advect_material, "texture_velocity"), surface_get_texture(sf_velocity));
                    texture_set_stage(shader_get_sampler_index(sh_advect_material, "texture_world"), surface_get_texture(sf_world));
                    shader_set_uniform_f(shader_get_uniform(sh_advect_material, "texel_size"), sf_texel_width, sf_texel_height);
                    shader_set_uniform_f(shader_get_uniform(sh_advect_material, "dissipation"), material_dissipation);
                    shader_set_uniform_f(shader_get_uniform(sh_advect_material, "time_step"), time_step);
                    draw_surface(sf_material_0, 0, 0);
                shader_reset();
            }
        surface_reset_target();
        temporary = sf_material_0; sf_material_0 = sf_temporary_0; sf_temporary_0 = temporary;
        temporary = sf_material_1; sf_material_1 = sf_temporary_1; sf_temporary_1 = temporary;
    }

draw_enable_alphablend(true);