add more bla

docs/usage.md

@@ -11,21 +11,40 @@ initialize, use and terminating.
 ## Initialize
 
 Call the subroutine `genp_init` with two parameters.
-The first is an integer and must be 0. 
+The first is an integer and must be 0. In the futur, it may be used for
+setting some predefined configurations. 
+ 
 The second is the filename for recording the plottings instructions,
-if this filename is an empty string, *stdout* will be used.
+I'm using the `.scratch` extension for naming this file.
+And if this filename is an empty string, *stdout* will be used, so
+you can pipe the data for post-processing purpose.
+ **Be warned:** if you use this option, don't write anything to the
+unit 6 **!**
 
 ## Use it
 
-- real xa, ya, xb, yb
-- integer color
-- call genp_move(xa, ya)
-- call genp_draw(xa, ya, color)
-- call genp_line(xa, ya, xb, yb, color)
+- `real xa, ya, xb, yb`
+- `integer color`
+- `call genp_move(xa, ya)`
+- `call genp_draw(xa, ya, color)`
+- `call genp_line(xa, ya, xb, yb, color)`
 
 ## Terminator
 
-- call genp_end(0)
+- `call genp_end(0)`
+This procedure end the current plotting, add some *bells* to the *WIP*,
+and close the output file, if an output file was specified.
+
+## Rendering
+
+- `subroutine genp_do_render(srcfile, tgafile, xsize, ysize)`
+
+After doing all that useless computing graphics, you can call
+the render, only if you have given a filename to `genp_init`.
+The render engine that I use can only write Targa file, but
+I'm going to add `PNG` to it output in a near futur.
+
+