HP j6700 OpenGL Implementation Guide for HP-UX 11.x - Page 24

rendering details, default visuals, EXP and EXP2 fogging, bow-tie quadrilaterals - drivers

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overview of OpenGL rendering details rendering details This section provides the details for several of HP's rendering capabilities. These rendering capabilities range from the way HP implements its default visuals to the way HP deals with the decomposition of concave quadrilaterals. default visuals Instead of placing the default visual in the deepest image buffer, HP puts the default visual in the overlay planes. This behavior can be modified using SAM to adjust the Display->X Server Configuration->Screen Options. EXP and EXP2 fogging The Virtual Memory Driver's implementation of fog applies fog per fragment. Hardware devices implement EXP and EXP2 fog per fragment and linear fog per vertex. bow-tie quadrilaterals A quadrilateral has four vertices that are coplanar. When this quadrilateral is twisted and you look at a front view of it on the display, there appears to be a fifth vertex. This fifth vertex which is not a true vertex will have no attributes, therefore, the color at what appears to be the intersection of two lines will in most cases be different from what is expected. HP treats the two parts of the bow tie as two separate triangles that have attributes assigned to their vertices. This special rendering process takes care of the color problem at the non-existent fifth vertex. To learn how other implementations of OpenGL deal with bow-tie quadrilaterals, read the section "Describing Points, Lines, and Polygons" in Chapter 2 of the OpenGL Programming Guide. 22 Chapter 1

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overview of OpenGL
rendering details
Chapter 1
22
rendering details
This section provides the details for several of HP’s rendering
capabilities. These rendering capabilities range from the way HP
implements its default visuals to the way HP deals with the
decomposition of concave quadrilaterals.
default visuals
Instead of placing the default visual in the deepest image buffer, HP puts
the default visual in the overlay planes. This behavior can be modified
using SAM to adjust the Display->X Server Configuration->Screen
Options.
EXP and EXP2 fogging
The Virtual Memory Driver’s implementation of fog applies fog per
fragment. Hardware devices implement EXP and EXP2 fog per fragment
and linear fog per vertex.
bow-tie quadrilaterals
A quadrilateral has four vertices that are coplanar. When this
quadrilateral is twisted and you look at a front view of it on the display,
there appears to be a fifth vertex. This fifth vertex which is not a true
vertex will have no attributes, therefore, the color at what appears to be
the intersection of two lines will in most cases be different from what is
expected. HP treats the two parts of the bow tie as two separate triangles
that have attributes assigned to their vertices. This special rendering
process takes care of the color problem at the non-existent fifth vertex.
To learn how other implementations of OpenGL deal with bow-tie
quadrilaterals, read the section “Describing Points, Lines, and Polygons”
in Chapter 2 of the OpenGL Programming Guide.