漫反射——有方向、有亮度

FragPos = vec3(model * vec4(aPos, 1.0));
vec3 lightDir = normalize(lightPos - FragPos);//于物体在世界坐标的位置而言

float diff = max(dot(norm, lightDir), 0.0);//光方向、法线  
vec3 diffuse = diff * lightColor;//光颜色——
vec3 result = diffuse * objectColor;