2011

Luma channel offset value of column is 0.704 and matches that of the modulation
image retaining focus.
Although the Modulation Blur tool can simulate depth of field, artifacts may
appear for a number of reasons and are inherent 2.5D motion blur and depth
of field.
Depth of field is a process that occurs in 3D. Out-of-focus objects blur on top
of objects that stand further away. They never blur on top of objects in front
of them. The modulated blur does not make this distinction about objects
that are in front or behind thus causing artifacts every time a silhouette edge
appears in an image. Rendering depth of field therefore requires segmentation
of the image along the silhouette edges. Please note that it is not always
possible to partition a scene into separate objects using silhouette edges.
Anti-aliased images also cause some problems in that the Z-channel cannot
be used to determine which portion of the pixel belongs to the background
object and which part belongs to the foreground. When an object becomes
out of focus, one starts to see parts of the scene that were not visible before
through the blurred edge. To replicate that effect as a 2D post-process, the
depth of field tool must guess what the background pixels might look like
behind foreground objects. To do this, segmenting and matting out foreground
objects and then reconstructing the background using in-filling techniques
become necessary, which can be a difficult task.
To correct some of the Z-channel problems, the 3D elements of a scene can
be rendered as separate layers. The scene would be manually segmented into
objects all having approximately the same Z. A single layer would not have
any significant Z discontinuities. Each layer can be properly anti-aliased. The
Z-information of the foreground element and the background element would
be available at partially covered edge pixels and the color of the background
objects behind foreground elements would also be available. However, the
Z-information is not defined at the transparent pixels of each layer. A renderer
is likely to assign them some very far-away value. To the modulated blur tool,
394 | Chapter 17 Image Processing Tools