Basic Radiosity Applications in Photoshop

What is Radiosity?

Radiosity is a concept of bouncing light rays. It is a concept that is used in 3D imaging, and many plug-ins and techniques have been developed to simulate radiosity in 3D applications.

Under intense, you will see that if you put your finger near a white unreflective surface, such as a piece of paper, there will be a slight reflection of your skin colour on the paper. This phenomenon is called radiosity.

What actually happens is that light rays from different surfaces bounce around and the colour of a surface is transferred to a nearby surface, creating an illusion of light bleeding.

Consider this example where three planes are put together to form a room. This is a quick draft I drew up in Photoshop to illustrate a room in perspective view.

By the way, if you haven't done so yet, please download the free Drawing in Perspective ebook to learn how to draw things in perspective view in Photoshop.

Now, there is an intense light source shining down on the ground plane. The ground plane will look lighter because white light rays from the light source is reflected on the ground plane.

The light rays from the lighted ground plane will bounce on the walls.

As a result, a "glow" effect will be seen on the walls. This phenomenon is called radiosity. Now, please go to the second page to see some examples of the application of this concept.

Examples of Using Radiosity in Photoshop

We shall create an example where radiosity is in action with Photoshop. Let's begin with a white canvas in Photoshop. Now, use the Elliptical Marquee Tool to create a circular selection.

Now, select the Gradient Tool and choose a light gray to black gradient. In the Options Bar above, remember to choose a Radial Gradient.

Now, drag the gradient from the top of the selection to the bottom. You will get the image of a basic sphere.

Then, go to Image > Adjustments > Hue/Saturation (CTRL + U) to give the sphere some colour.

Next, add a layer underneath and draw a blurred shadow. The simple procedure can be seen in the animation underneath:

Now, CTRL + Click on the shadow layer to select the shadow. Go to Selection > Modify > Expand and expand the selection by 5 pixels. Create another layer underneath the shadow layer and name it "Radiosity Glow". Fill this layer with the hue (shade) of your sphere.

To minimize loading time, I have created a very low quality version of the animation above. The actual result I achieved is shown here:

As seen in the image, the green shade of the sphere is transferred to the white ground surface. Similarly, the white colour from the ground surface would also be transferred to the bottom of the sphere, which is facing the ground.

This concludes our short lesson on radiosity. This concept can be used to manipulate and create more radiant photographs and more realistic digital art. Try it yourself!