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This is NOT about how to create and render curves (e.g. Bezeier, NURBS, etc) BUT about how to be able to interactively 'pick' such curves by mouse click when the mouse cursor is hovering over ANY part of such a curve.

After doing the required computing I render a curve, in either 2D or 3D, by subdividing it into lots of individual smaller line segments. The more such line segments there are the smoother and more realistic the curve looks.

I deliberately create each of these GL_LINE segments as separate entities (each being assigned their own unique ID number). These line segments belong to the Curve 'object' (which also has it's own unique ID). Then, using using Ray Casting I can enable mouse-line collsion detection and know when an individual line segment has been 'hit' - and highlight it (e.g. temporarily change its color). BUT at the same time also highlight all the other line segments that belong to the Curve - and so give appearance of the whole curve being selected.

THE PROBLEM is that because each curve is made up of not just the 'core' control points, which effectively define the curve, but also the thousands of points that effectively draw the curve there is quickly a very noticable slowing down of graphics performance.

I am aware that I could more efficiently instead compute the all the subdivision points and use LINE_STRIP instead to render the curve as one graphical object? BUT then that will NOT allow me to use the ray casting technique described to be be able hover the mouse cursor over any part of the curve and 'select' the curve object.

So....how can I more efficiently 'pick' curves in OpenGL?

genpfault
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DavidH
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1 Answers1

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You got 2 obvious options for this:

  1. use ID buffer

    when you render your curve you assign color to the RGBA frame buffer. So simply also assign ID of renderd curve to separate buffer (of the same resolution as view) and then simply pick the pixel position under mouse from this buffer to see exactly which curve or object you select.

    This is pixel perfect and O(1) super fast ... However if your objects are too thin you might have problems using it for mouse picking from single pixel so test up to some distance from mouse (you can glReadPixels rectangle around mouse) end return either all the IDs present or just the most often one.

    See this:

    Do not forget to clear the ID buffer before rendering and if you got too many objects that will not fit into 8bit stencil use different buffer... In case of 2D you can use depth buffer, or you can render to RGBA framebufer in 2 passes first the ID then read into CPU side memory and ten normal render. You can also render to texture ...

  2. compute distance to curve

    its doable for example see:

    as you can see its possible to use this also for the rendering itself (no line approximation just the "perfect" curve) and even with speed ...

    So simply compute the distance of mouse to each of your curves and remember the closest one. If the distance is bigger than some threshold/distance no selection occurs.

Spektre
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  • Thanks Spektre. BUT, just to be clear, are you saying that for suggested option (1) that each curve that is drawn should be deliberately assigned a very slightly different color? This woun't work for me as I need all the curves to be rendered the same color. – DavidH Dec 12 '20 at 15:10
  • @DavidH no you usually have `RGBA` and `Depth` buffers simply add one more ...so when you render any pixel it will set color at `RGBA` buffer, depth at `Depth` buffer and ID of your curve (can be order number in your array or whatever) at the 3th buffer. If you use Stencil for this then OpenGL 1.0 can do this on its own however newer gfx drivers work only with 8 bit stencil limiting to 255 unique IDs and 1 empty space so if that is not enough you need to use either shaders or render in 2 passes (in which case yes you set different ID / color ) to your curves during first pass... – Spektre Dec 12 '20 at 15:53