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How to quickly draw a curved shape?

  • by "quickly" I presume one should use hardware facilities as much as possible

  • by "curved" I mean boundaries defined by either quadratic or cubic Bezier curves

  • by "shape" I mean either a "fat" stroke (i.e. more than 1px wide) or even-odd/non-zero filled "2D curved polygon", possibly with holes (i.e. letter "O")

I'm asking because the options I know of have several drawbacks:

  • triangulating the shape and sending it to OpenGL - does the most difficult work on CPU and might use too many/few triangles (i.e. wasteful/coarse)

  • texture atlas - has to recompute/upload the texture on every change (shape, scale, rotation, ...)

  • Signed distance field - on large scales the details don't look pretty or has to recompute/upload the texture

  • NV_path_rendering - could be it, if it was not working only on Nvidia's cards

  • OpenVG - could be it, if it was not working only on mobile

  • ?


* It seems to me that OpenVG is not exactly moving forward, to put it mildly. Does anyone know anything about its future prospects? Is it worth at all to keep an eye on in the present day?

** OpenGL 4+ provides means of on-fly tessellation of polygons. Could it be somehow used to refine the the mesh from the "triangulating" option so that the shape boundary at least wont look "angled"?

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    $\begingroup$ This GPU Gems article rasterizes quadratic curves by identifying parts of the hull that are curved and analytically computing the coverage in the pixel shader, might be worth a look: developer.nvidia.com/gpugems/GPUGems3/gpugems3_ch25.html $\endgroup$ – yuriks Aug 10 '15 at 20:41
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    $\begingroup$ @yuriks Yeah, Loop & Blinn, totally forgot about it. But isn't it patented? $\endgroup$ – Ecir Hana Aug 10 '15 at 22:12
  • $\begingroup$ See also Massively-Parallel Vector Graphics, published in SIGGRAPH Asia 2014. $\endgroup$ – lhf Aug 20 '15 at 1:24
  • $\begingroup$ Between the options you've listed in your question and the Loop and Blinn paper, I think you've pretty much exhausted all the possibilities. $\endgroup$ – Nathan Reed Aug 20 '15 at 5:41
  • $\begingroup$ You can tessellate a line, like described here. Or you can triangulate in a compute shader. $\endgroup$ – nikitablack Aug 20 '15 at 13:28
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You can use OpenGl 4.x tessellation shaders to convert Bezier control points into polygons.

A google search for "tessellation shader bezier" found this outline describing the tessellation of Bezier surfaces and curves:

http://web.engr.oregonstate.edu/~mjb/cs519/Handouts/tessellation.1pp.pdf

This offloads the Bezier evaluation from the CPU to the GPU and reduces the data flow across the bus.

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    $\begingroup$ You could improve this answer by elaborating a bit, or even just linking to something that explains further. $\endgroup$ – Nathan Reed Nov 12 '15 at 20:41
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    $\begingroup$ One way of expanding this answer would be to address the drawbacks the author mentions, and explain how your approach helps with those. $\endgroup$ – trichoplax Nov 12 '15 at 20:46
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One can do curved drawing with hardware. There is a method described in GPU Gems 3 that describes how to do this. User @yuriks actually comments this. I have actually made a quick a dirty demo for you to take a look at.

curve

Image 1: HW accelerated curve shapen (drawn usung a triangle) and webgl source

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