start: the object data
the pipeline starts at the object. the object description is made up of the data (points). the basic geometric primitive is the triangle. the corner points of the object's triangles are described using coordinate points (x, y and z), where the 'z' value represents the depth information. depending on the display representation, these points are assigned additional data regarding material, texture, special effects and much more. as a result, an enormous amount of data is sent into the pipeline.
renderingrasterization
at this stage, the 3d scene is filled with color shades and textures are applied. different processes and methods are also applied here.
1 shading—shading takes account of the effects created by different light sources on the 3d object and provide for a very realistic overallimpression. here, too, there are different methods which are more or less processor-intensive:
1 flat shading assigns a color value to each polygon. this results in a faceted representation, which requires only short computation times.
2 in gouraud shading, all the vertices of the polygons are assigned a color value. the remaining pixel information for the polygon is interpolated. this method gives a very gentle color transition, even with fewer polygons than are required for flat shading.
2 texture mapping—at this stage, the 3d object undergoes a sort of ”face lift”. the materials and textures are assigned. different methods are used here to make the textures appear realistic, even when enlarged or reduced. as a first step, the textures are computed:
1 point sampling is the simplest method. a pixel-by-pixel comparison is made between the texture template and the surface to be filled. this method leads to a very coarse representation, especially when enlarged.
2 in linear mapping, a new color value is interpolated from the adjacent pixels (or texels) of a texture. this gives better results than point sampling, as the hard boundary between the coarse pixels is blurred.
3 the mip mapping method stores a large number of enlargement stages for the texture. the depth information of a primitive is then used to determine which enlargement stages of the texture will be used in drawing. information concerning the transparency of the texture is carried in the alpha channel. finally, a distinction is made in mip mapping between bilinear and trilinear filtering. bilinear filtering interpolates between two pixels of two textures, trilinear filtering interpolates between four pixels for each of two textures.
4 bump mapping introduces a new dimension. relief or raised textures can only be generated with the other methods in two dimensions using light and shadow effects.
the staircase effect is corrected by anti-aliasing. this is either done by interpolating mixed pixels, in which a new color value is computed from two adjacent color values.
3 the frame buffer
the finished image will not be written to the frame buffer until this complex sequence of steps is completed. the frame buffer is made up of front and back buffer. the back buffer acts as a buffer page, in which the next image to be displayed is built up. the front buffer is the memory area where the image that appears on the monitor is located. this prevents the process of image drawing being visible. the duplicate storage method is also known as double buffering.
buffer swapping: display on the monitor
two possibilities exist for transferring a completed image from the back to the front buffer, and thus to the display. the obvious method is to copy the contents to the front buffer one byte at a time, a process known as “blitting”. page flipping is significantly faster: in this case the content of the buffer is not rewritten—the addresses of the front and back buffers are simply exchanged. as a result, only tiny data volumes are transferred.
the buffer swapping will only ever be performed once the image drawing process in the back buffer is completed. this procedure should be repeated at least 20 times a second to give a smooth representation of 3d scenarios. in this context, we speak of frames per second (fps). this is a very important value especially for 3d applications. a cinema film runs at 24fps.