Reflective blend
This property allows you to adjust the percentage of reflection blending. At 0%, reflections are simply added to the diffuse and specular components. At 100%, reflections are completely blended with the diffuse and specular components.
Blended reflections more closely mimics how reflections are perceived in reality. Additive reflections are how reflections are generally handled in computer graphics.
Handling reflection in computer graphics is a tricky business. In the real world, everything is reflection.
The diffuse color of a surface is the result of reflection from a perfectly rough surface.
The specular highlight is the result of reflection from a relatively smooth surface.
The mirror type of reflections are the result of reflection from a perfectly smooth surface.
But in computer graphics, both the diffuse and the specular shaders are clever hacks designed to fool the eye. They are based on a very abstract illumination model.
Clearly, a surface cannot be perfectly rough and perfectly smooth at the same time. But in computer graphic, it is very common to set a diffuse color and a reflection on the same surface because simulating the reality would otherwise be way too expensive.
Another real world law is that a surface cannot emit more light than it receives. For this reason, the correct way to handle reflectivity would be to blend it with the diffuse color. That is reduce the diffuse color in proportion to the reflectivity. This is called blended reflectivity.
However, the common way to handle reflectivity is to simply add it to the diffuse component. This is because, in the real world, the reflected image takes on the color of the reflective surface and this effect is better simulated by adding the reflectivity component to the diffuse part. This is called additive reflectivity. The drawback of this method is that without care and manual compensation, a reflective surface can look much brighter than it should.
Without the Fresnel term, the blending can be handled by manually decreasing the intensity of the diffuse color. But with the Fresnel term, which varies the reflectivity, it is not possible to manually handle the blending this way. So a mathematical blending is included in the reflection processing.
Additive reflectivity
Reflective surfaces appear brighter than they should. For instance, the door
frame reflection on the floor is brighter that the actual door frame. And
because the floor is brighter than it should be, it masks the yellow car reflection
on the floor and it cuts short the reflection of the door frame. On the other
hand, the robot, because it is brighter than it should be, is more prominently
visible in the scene.
Blended reflectivity
All objects share a well balanced brightness. The door frame reflection is no
brighter that the actual door frame and the reflection from the car and the
door frame are more visible. On the other hand, the robot could be brighter
because it is designed to be the center of attention. For this reason, the
reflection blend property is a surface property and can be adjusted as required
for each surface.
Additive reflection on the left and blended reflection on the right.
The floor is reflective. The reflection appears way too bright with additive reflections. Even the wall, which is rather dark appears bright in the reflection.
Additive reflection on the left and blended reflection on the right.
The block is reflective, all objects are white and the light have 80% intensity. With additive reflection, the block is completely saturated. With blended reflection, the block appears well illuminated given that it is a specular surface. However, for artistic reason, it could probably benefit from a little blending adjustment.