11 December 2013

Concept of form: The Problem of Visual Form Perception Part 2

The first environmental aspect needed to create this image is the light source. This refers to the direction and intensity of the light coming from the environment. Let’s see and example: imagine being outdoors, in a garden, the main light source would be the light reflected by the sun in most of the objects. Its direction is from up to down and the intensity depends on the time of day. Even though the sun is the main source, there could also be reflections of light from one surface to another. This would provide the secondary sources of illumination. Let’s see another example in an indoor scene. The main light source could be a bulb, or many bulbs in the scenario, and the secondary sources of illumination would be the light reflected in the walls of the room.

The next aspect that is important to understand the concept of form is the reflectance. This refers to the properties various surfaces have (shinny, dull, bright, …) when they come in contact with light. An object absorbs part of the light and reflects the other part, what we perceive is only the light the object reflects. In other words, depending on the surface, some objects absorb more light from one region of the wavelength spectrum than from other regions of the wavelength; depending on the wavelength absorbed we see different colours. For example, to see red, the surface will absorb the short and middle wavelengths and only reflect the longer wavelengths. Furthermore, surfaces differ in the total amount of light they absorb; the ones that absorb lots of light are shinny, glossy and look like a mirror but the others absorb little light and because of this they are dull and matte.


Now we will look at the surface orientation relative to the light source and the viewer.  The surface orientation is set with reference to an imaginary perpendicular line to the surface, which is called the surface normal. So, to have an optimal light reflection in a surface, it is necessary that the angle between the direction of the viewer and the surface normal is exactly the same as the angle between the direction of the light source and the surface. The more different the angles are, the less light is reflected from objects onto the image. To understand this better, lets look at the image below. As we can see, the part of the pyramid that is facing the eye is reflecting much more light than the blocks that are just in front of the pyramid (we can see the pyramid in white and the blocks in grey which means they have a little bit of shadow). This happens because the angle between the pyramid and the light source is mostly the same as the angle between the surface and the eye.


The final environmental aspect we need to look at is the viewing position, in other words, the relationship between the viewer’s eye and the scene (like seen in the image above). Look at figure 10.1 and imagine that Ernst Mach changes his position in the room. The image projected from the room would be completely different despite the fact that all the other environmental aspects (the light source, the reflectance, the surface orientation) had not changed at all relative to one and other.

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