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Vector Displays in OpenGL

From: andrew cooke <andrew@...>

Date: Fri, 8 Jul 2016 18:01:51 -0400

With Hexpilot I am trying to emulate the old vector displays.  You can
see an example and a brief discussion of the issues at
http://ascii.textfiles.com/archives/3786 but most of that page is
about scan lines.

My initial experiements have just hit a wall because modern OpenGL no
longer supports variable line widths.

That may sound odd - how does a plotting sysetem work without line
widths?  Part of the reason is that OpenGL works at either a higher or
a lower level - you describe a scene in terms of triangles, typically,
which are converted into fragments (something like pixels).  Lines get
lost in the middle.

OpenGL does have lines (as well as triangles), but they have a fixed
width and are hardly used (at least, as things actually displayed).

One alternative to using lines is to replace each line with a pair of
very long, thin triangles (arranged as a long thin rectangle).  That
works just fine until you start to worry about 3D images.

Because with 3D images on a vector display, distant lines are (often)
just as thick as nearby lines.  The line itself is not considered a
physical entity.  In contrast, OpenGL's 3D support is consistent:
distant "lines" (more exactly, distant triangles) are thinner.

THis may sound like something you can work round with a little maths.
Just scale the distant lines appropriately so that they end up all the
same size after perspective correction.  But a little thought suggests
that won't play well with hidden line removal.  In short: it's
difficult to bend the rules for some components of the display while
expecting others to stay working correctly.

So what to do?

My current best idea is to render triangles to a depth texture like
you would with shadow masking (except rendered from the real camera
viewpoint).  And then use that depth mask to do hidden line removal
"by hand" for a line-only rendering.

It will have some artefacts, but they are (I hope) the kind of
artefacts you would also see in old school hidden line removal code.

It's going to be a fair amount of work....

Andrew

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