destructive behaviour when lines overlap

[petaflot]

to reproduce

• take 2 points we'll call a, b
• draw a line between (a,b), with color (ie. bright yellow, (1,1,0,1) )
• display to make sure line is here.
• draw a line between (a,b), with color (ie. dark green with very low alpha, (0,1,0,.1) )
• display again ; only second lime is shown

expected behaviour

display a line with a color somewhere between our 'bright yellow' and our 'dark green', in particular if there if last line added has alpha<1

[djhoese]

I believe this is expected. You'd have to change the GL drawing state to something that works for your use case, but it may produce other artifacts. What you're describing is blending. You can see the LineVisual uses the "translucent" GL state preset here:

vispy/vispy/visuals/line/line.py

Line 299 in 6667599

self.set_gl_state('translucent')

The properties used by this "preset" are defined here:

vispy/vispy/gloo/wrappers.py

Lines 42 to 47 in 6667599

	'translucent': dict(
	depth_test=True,
	cull_face=False,
	blend=True,
	blend_func=('src_alpha', 'one_minus_src_alpha', 'zero', 'one'),
	blend_equation='func_add'),

You can customize this I believe by doing my_visual.set_gl_state(preset="additive") or passing specific property values as keyword arguments to this method. That said, I would have thought translucent would have done what you wanted given the options shown but maybe I'm not thinking about this hard enough. It could also be GPU dependent given you're likely using the "gl" method of drawing the lines. You could try the "agg" drawing method provided by the LineVisual and see if that changes your results.

Let me know what you think and how these goes. Otherwise if @rougier knows this off the top of his head he could maybe answer more concretely.