Try replacing:
Code:
dist_to_wall = c_trace(camera.x,my.x,IGNORE_ME);// Khoang cach toi buc tuong, c_trace gui 1 tia tu vi tri nay
//toi vi tri kia và kiem tra tia nay co dung chuong ngai vat tren duong no chieu hay ko


with:
Code:
dist_to_wall = c_trace(vector(my.x,my.y,my.z),vector(my.x+100,my.y,my.z),IGNORE_ME);// Khoang cach toi buc tuong, c_trace gui 1 tia tu vi tri nay
		//toi vi tri kia và kiem tra tia nay co dung chuong ngai vat tren duong no chieu hay ko
draw_text(str_for_num(NULL,dist_to_wall),0,0,vector(255,255,255)); //this just displays the trace result on screen


A bit confused but maybe that helps. Good luck.