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Re: [Discuss-gnuradio] Persistence causes glAccum exception

From: Jared Boone
Subject: Re: [Discuss-gnuradio] Persistence causes glAccum exception
Date: Thu, 24 May 2012 19:45:52 -0700

On May 24, 2012, at 6:44 PM, Marcus D. Leech wrote:

> Near as I can tell, the "persistence" feature is intended to give a kind of 
> storage-scope effect, or high-persistence phosphor effect.

I achieved a similar effect in the ubertooth-specan-ui Python app, built on top 
of Qt's newer PySide Python library. Perhaps it'll be a useful reference for 
reimplementation in Qt without relying on OpenGL.

The general technique is to draw the graph into an off-screen image. During 
each frame update, black is drawn over the existing image, with a small alpha 
value, which effectively fades the prior image. The new graph is rendered over 
the top at alpha = 1.0, then the image is copied to the screen. It seems to 
perform quite well. I presume most video drivers can push the pixels-with-alpha 
BLTing into the hardware.


The meat of this technique is in RenderArea._draw_graph(). I imagine the code 
would map directly to the C++ Qt API.

I hear the author of Kismet has done something similar in his software, but I 
don't know what graphics API he built it on.

        - Jared

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