AW: [gl4java-usergroup] "destination alpha" in glBlendFunc()

[Thorsten R. <tho...@iv...>]

> > does anybody know, what is meant with "destination alpha" in the docu of
> > glBlendFunc() ? "source alpha" is taken from glColor(), but the
> destination
> > one ?
>
> I think the destination alpha is taken from the frame buffer (front or
> back buffer) if you are using an RGBA frame buffer (32 bpp).

How do I use a RGBA frame buffer ? is it the <GLCanvas.rgba> flag ?
<GLCanvas.getGLContext().isRGBA()> says <true>.

How do I write alpha values in the buffer ?

I thought the following would do the trick:

	glEnable (GL_BLEND);
	glBlendFunc (GL_ONE, GL_ZERO);	// is default for blending
	glColor4f (r,g,b, 0.5f);
	drawPolygon();

but when drawing another Polygon over the first one with

	glBlendFunc (GL_DST_ALPHA, GL_DST_ONE_MINUS_ALPHA);
	glColor3f (r2,g2,b2);			// alpha = 1f
	drawPolygon();

I get a second Polygon with full (r2,g2,b2). So it looks like the alpha
values in the framebuffer are 1's rather than 0.5 ?!

Any ideas ?

Thorsten