Computer Graphics - Transformations, Clipping and Rasterization
What is a Transformation?
A transformation is the process of mapping points to other locations. Common transformations are Translation, Scaling and Rotation.
3D rotation sequence:
- Translate the object so that the rotation axis passes through the coordinate origin
- Rotate the object so that the axis of rotation coincides with one of the coordinate axes
- Perform the specified rotation about the selected axis
- Apply inverse rotations to bring the rotation axis back to its original orientation
- Apply the inverse translation to bring the rotation axis back to its original spatial position
What is Clipping?
Clipping is the process of determining the portions of a primitive lying within a region called
the ‘clipping region’. Types of clipping are Point clipping, Line clipping, Polygon clipping.
What is the purpose of Clipping?
It is for preventing:
• Activity in one window from affecting pixels in other windows
• Mathematical overflow and underflow from primitives passing behind the eye point or at great distances (in 3D)
• Clipping a line segment yields at most one line segment
• Clipping a polygon can yield multiple polygons (However, clipping a convex polygon can yield at most one other polygon)
Rasterization (scan conversion) is to determine which pixels that are inside a primitive specified by a set of vertices.
• To convert continuous geometry, inside viewing region, into discrete pixels
• Fragments have a location (pixel location) and other attributes such color and texture coordinates that are determined by interpolating values at vertices
• Pixel colors determined later using color, texture, and other vertex properties
Bresenham’s line algorithm properties:
- Only uses incremental integer calculations
- Can be adapted to display circles and other curves
- Basic idea find next pixel from current one
Start at an inside position and ‘paint’ the interior, pixel by pixel, with the desired colour until the boundary colour is encountered
• Start at an inside position and ‘repaint’ all pixels that are currently set to a certain colour with the desired colour
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