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POLYGON RENDERING METHODS RENDERING means giving proper intensity at each point in a graphical object to make it look like real world object. Different Rendering Methods are Constant Intensity Shading. Gouraud Shading. Phong Shading. Fast Phong Shading.


CONSTANT INTENSITY SHADING Also Called Flat Shading Fast and Simple Method A single intensity is calculated for each polygon( in the polygon-mesh) All points in the surface of polygon are displayed with the same intensity value. N1 N2 N3 N4

For Getting Good Results Through Constant Intensity Shading:

For Getting Good Results Through Constant Intensity Shading Object is a polyhedron. Light sources illuminating the object are sufficiently far from the surface The viewing position is sufficiently far from the surface.


GOURAUD SHADING Intensity interpolation scheme developed by Gouraud. Represents a polygon surface by linearly interpolating intensity across the polygon surface. Intensity values for each polygon are matched with adjacent polygon along common edges. Eliminates the intensity discontinuity that can occur in flat shading.

Polygon surface is represented by the following calculations::

Polygon surface is represented by the following calculations: Determine the average unit vector at each polygon vertex. Apply an illumination model to each vertex to calculate the vertex intensity. Linearly interpolate the vertex intensities over the polygon surface

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Advantages It removes intensity discontinuities associated with flat shading. Disadvantages Highlights on the surface are sometimes displayed in irregular shapes. Linear intensity interpolation can cause intensity stripe called Mach bands , to appear on the surface.

Phong Shading:

Phong Shading Normal Vector Interpolation Shading Developed by Phong Bui Tuong Accurate method Displays more realistic highlights Reduces Mach band effect Interpolate normal vectors and then apply illumination

Polygon surface is rendered using the following steps ::

Polygon surface is rendered using the following steps : Determine the average unit normal vector at each polygon vertex Linearly interpolate the vertex normals over the surface of the polygon Apply an illumination model along each scan line to calculate projected pixel intensities for the surface points

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Interpolation of surface normals along a polygon edge Scan line N1 N2 N N3

Advantages n Disadvantages:

Advantages n Disadvantages Adv. Intensity calculations produce more accurate results Displays more realistic highlights on the surface Greatly reduces the Mach-band effect. [The Mach band effect exaggerates the change in intensity at any boundary where there is a discontinuity in magnitude or slope of intensity.] Dis adv. Requires more calculations

Fast Phong Shading:

Fast Phong Shading It is a goal of any one writing a 3D engine. Phong shading is still common in 3D system. It is faster and easier way to approximate the intensity calculation.

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Phong shading is more realistic than Gouraud shading, but requires more computation. Phong shading using differences takes about six to seven time longer than Gouraud shading. Limitations in the normal phong shading lead to the development of fast-phong shading technique.

Fast Phong Shading:

Fast Phong Shading Surface Rendering can be speeded up by using approximations in the illumination-model calculations of normal vectors. It approximates the intensity calculations using a Taylor-series expansion and triangular surface patches. Rendering is faster than the normal Phong shading.

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