[jitter] Re: Re: first shader, first troubles

GLSL doesn't do well with implicit type casting.  You have to always be explicit about moving from one data-type to another.  In this case, gl_FragColor is expecting a vec4.  If you want to pass the alpha channel and set all other channels to 0., you could do the following:
gl_FragColor = vec4(pl,0.,0.,0.);

But it sounds like you are looking to add this alpha channel to another texture, which would be better accomplished using a shader similar to cc.alphaglue.jxs.  You should be able to alter that shader to your needs fairly easily.

AB
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Andrew B.
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Cycling '74 Support