3d线鼠标采摘

Question

I have 3d scene with thousands lines. I want to be able to pick ALL 3d lines in the 10 pixels neighborhood of the mouse cursor (with perspective projection). I've tried to use unique-color based method. But this method is not suitable for me because I can not pick ALL lines - only the closest one. Is there any acceptable solution of my problem ? OpenGL or DirectX - it does not matter.

Answer 1

Why not just compute the distance between those lines and the point in question? It's a 2D line-to-point distance computation. You could probably implement it with a Perl script that calls a Python executable that calls a Lua interpeter and still do 100,000 of them in a second.

This is one of those tunnel-vision "when all I have is a hammer, every problem looks like a nail" issues. You don't have to use rendering to do picking.

Answer 2

In old OpenGL (<= 2.1), you can use Selection Mode to do exactly this. Use gluPickMatrix() to select a small region around the cursor position, initialize a selection buffer, slip into selection mode ( glRenderMode(GL_SELECT) ), and redraw the scene. Then come back out of selection mode and your selection buffer will be full names (really id numbers) of all the drawn objects that appear in your region of interest. You'll have to modify your drawing code a little to push/pop names ( glPushName(objIndex) ) around each object that you render as well.

It's not the most efficient use of modern graphics hardware, but it always works.

Answer 3

Neither OpenGL nor DirectX will do the job for you, because they only draw things. What you must do is projecting all the lines in your scene to the screen and test, if the closest point to the selected position is nearer than your desired max distance. You can accelerate this by keeping the lines in some spatial subdivision structure (like a Kd tree or similar) to discard quickly all those lines which definitely don't match your criteria.