2D Motion Blur Techniques
Motion blur can also be accomplished using 2D techniques, which are computationally simpler than but not as accurate as the 3D method. 2D motion blur evaluates the change in position of pixels on the picture plane, and does not take into account the actual 3D distance covered by objects. This method works fairly well for motion across the picture plane, but is less successful when objects rotate, deform or fly directly toward or away from the camera. 2D blur evaluates the difference between two images and blurs differing pixels by an amount proportionate to the distance (in 2D screen space) they have changed. Some 3D software packages incorporate the 2D motion blur technique, with the shutter setting defining times for two renders, much the same as with the 3D method. If 2D motion blur is added, the difference between the two renders required for the blur can only be measured in the x and y directions. No movement in the z direction is taken into account. As with 3D motion blur, though, the edges will still be semi-transparent.

[Figure 5] Illustration of two block renders to be combined for motion blur.

This is a result of combining images in which the block exists in different positions. If the two renders (one at frame 22, for instance, and one at frame 22.1) are overlaid, the transparency is defined by the areas where one image extends beyond the other (see Figure 5). In this figure, one block is semi-transparent and the other darkened to clearly illustrate the areas in which one extends beyond the other. In the interior areas of the block, in which each render has red, green, blue and alpha values, the two images are blurred together. In the areas in which one block extends beyond the other, one block is blurred to fill the space of the second block’s alpha or each block is blurred into the other block’s alpha and then those images are averaged. The blurring of color and alpha channels creates semi-transparency. There are several variations in 2D motion blur methods implemented by different software packages or used for different situations. They all come down mixing, blurring and repositioning a combination of images.

Reflections
Reflections help clearly integrate computer graphics objects into scenes. They are most noticeable in very reflective objects, but are also helpful in providing subtle details to many elements in a scene. A reflection is, in essence, the same phenomenon that provides a specular highlight on any surface. It is the light reflecting directly from a surface into the camera lens. If the surface is very smooth, as with a perfect mirror, then the reflected light provides a clear reproduction of the elements in the scene. As the roughness of the surface increases, the clarity of the reflection is reduced due to scattering of light. The brightest light sources and objects reflect off almost any surface causing specular highlights.

Collecting Textures
A surface reflects the world around it, but unfortunately the camera only records a very narrow view of this world. To create an accurate reflection in computer graphics, additional footage is required of the surrounding scene. One way to collect this information is to take several shots of the surrounding area using a wide-angle lens. With the camera placed in the position of the subject, shots are taken of the surrounding area to the left and right, as well as above and below. The sky and the ground in most scenes are the simplest part of this process, because they can be generic textures. The viewer expects to see a blue sky reflected in the top of a shiny element, and grass, concrete or any generic ground texture as the bottom portion of the reflection (see Figures 6 and 7).

[Figure 6] Possible ground textures.

[Figure 7] Possible sky textures.