(very) Brief overview of tnl module

src/mesa/tnl/NOTES

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+INTRODUCTION
+
+A generic, configurable software implementation of GL transformation &
+lighting.
+
+This module provides an implementation of the routines required by the
+'vtxfmt' mechanism of core mesa for tnl functionality in all
+combinations of compile and execute modes.
+
+Most current drivers use the tnl module exclusively to provide this
+functionality, though there is an experimental alternate
+implementation provided by the tnl_dd/t_dd_imm_* files which can
+handle a small subset of GL states in execute mode only.
+
+
+STATE
+
+To create and destroy the module:
+
+	GLboolean _tnl_CreateContext( GLcontext *ctx );
+	void _tnl_DestroyContext( GLcontext *ctx );
+
+The module is not active by default, and must be installed by calling
+_tnl_Wakeup().  This function installs internal tnl functions into all
+the vtxfmt dispatch hooks, thus taking over the task of transformation
+and lighting entirely:
+
+        void _tnl_wakeup_exec( GLcontext *ctx );
+	void _tnl_wakeup_save_exec( GLcontext *ctx );
+
+   
+This module tracks state changes internally and maintains derived
+values based on the current state.  For this to work, the driver
+ensure the following funciton is called whenever the state changes and
+the swsetup module is 'awake':
+
+	void _tnl_InvalidateState( GLcontext *ctx, GLuint new_state );
+
+There is no explicit call to put the tnl module to sleep.  Simply
+install other function pointers into all the vtxfmt dispatch slots,
+and (optionally) cease calling _tnl_InvalidateState().
+
+CUSTOMIZATION
+
+The module provides customizability through several mechanisms.  The
+most important is by allowing drivers to specify the pipeline through
+which vertex data is passed, including its eventual transfer to
+rasterization hardware (or software).
+
+The default pipeline is specified in t_pipeline.c, and is usually a
+starting point for driver pipelines.  Some drivers will remove a stage
+where hardware provides support for the implemented operation (for
+instance fog where per-pixel hardware fog is available, as in the dri
+tdfx driver), or add stages to shortcircuit latter operations (for
+example taking advantage of hardware support for strips and other
+higher-level primitives (for example the radeon driver).
+
+In addition, the following functions provide further tweaks:
+
+extern void
+_tnl_need_projected_coords( GLcontext *ctx, GLboolean flag );
+
+	- Direct the default vertex transformation stage to
+          produce/not produce projected clip coordinates.
+	  
+extern void
+_tnl_need_dlist_loopback( GLcontext *ctx, GLboolean flag );
+      
+        - Direct the display list component of the tnl module to
+          replay display lists as 'glVertex' type calls, rather than
+          passing the display list data directly into the tnl pipeline
+          mechanism.  
+
+	  This allows display lists to be replayed by the tnl module
+	  even when the module is not strictly active.
+
+
+extern void
+_tnl_need_dlist_norm_lengths( GLcontext *ctx, GLboolean flag );
+
+	- Direct the display list component to enable/disable caching
+          1/length values for display list normals.  Doing so is
+          ususally helpful when lighting is performed in software, but
+          wasteful otherwise.
+
+
+DRIVER INTERFACE
+
+The module itself offers a minimal driver interface:
+
+	 void (*RunPipeline)( GLcontext *ctx );
+
+Normally this is set to _tnl_RunPipeline(), however the driver can use
+this hook to wrap checks or other code around this call. 
+
+In addition, the driver interface for the default render pipeline
+stage is housed in the tnl context struct (this could be cleaner).  
+
+
+RENDER DRIVER INTERFACE
+
+See t_context.h for the definition and explanation of this.