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GuyWithBeard

How to do object "rubber band" mouse selection

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Hey,

 

In my editor you can select multiple objects by dragging a screen space rectangle around several objects using the mouse, aka rubber band selection. The editor view is 3D and uses a perspective camera, basically the game running in a special editor mode. I would want the selection to select the 3D scene objects that appear inside the rectangle, nothing more.

 

My current approach is to transform the corner points of the selection rectangle from screen space to NDC space and do the same for the corners of every world-space bounding box of every potentially hit object. This allows me to do a simple 2D rectangle-to-rectangle collision test to see which objects are inside the selection rectangle. This works, but depending on the shape of the objects, their world transform as well as the camera transform I get a lot of false positives.

 

Any idea how to do this more efficiently? I was thinking the following might work:

 

For each potentially hit object (basically all visible geometry):

  • Iterate over all vertices of the object and transform them to NDC space
  • Create a 2D convex hull out of the vertices
  • Perform a rectangle-to-convex hull collision test to see if the object touches or resides in the selection rectangle.

However, the above seems very complex for such a simple feature, not to mention slow, as the calculations would have to be done every time the mouse moves. I guess the convex hulls could be cached until the camera moves, but that only complicates the system further.

 

I can already perform ray-to-mesh intersection tests perfectly, the problem comes when I want to perform tests with something "fatter" than just a ray...

 

So, how would you do this?

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I've wondered about this myself, from the brief searches online, it looks like most people do it the way you describe, bringing everything to 2d space and doing it there, except usually they use an aabb instead.  

 

I guess the other option would be to create a frustum out of the points of the square being created and then do a frustum check (or maybe a rectangular cube?) against the objects' aabb.

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I couldn't tell from your post (maybe I missed it) if you are happy to select things partially in the area or not. This suggestion assumes you want anything in the area even if it's only partial. It will not work if you want only totally selected objects:

 

How about doing it similar to 'picking', I am a bit rusty now but the idea is you draw the scene but instead of textures/materials etc you draw every object with a separate colour. E.g. one object might have rgb values (0, 0, 1), another (0, 0, 2) and so on, the colour is an ID which maps uniquely to each object. That lets you have a lot of objects if you have 255 per channel (~16.5 million). Once you render that image you just need to look at the pixels in the area you want and work out what they map to. If there are pixels of colour (0, 0, 25) then 'cubeA' (object 25) is in the selection if there are also pixels of colour (21, 5, 0) = 1,377,536 then sphereX is present and so on. You need only render the actual area you have selected and not then entire scene, you can also use a filter of sorts on your object and simply not render them if you don't want them selectable. You also don't need to render this at the normal resolution as often a lower resolution version will suffice and means less pixels to go through when looking for colours.

 

It has it's 'quirks' (some are either positive or negative depending on your use):

It won't let you select totally occluded objects (without modification by some depth peeling type process).

It will select things even if they are only partially in the area.

 

I've only used this for single object selection (mouse click) but it is pretty neat and potentially a lot faster than a more mathematically based approach particularly if you have a lot of objects (it's as quick as it takes to render the area plus a little bit of time to go through the rendered pixels and map them back to objects). It can also be expanded fairly easily to other types of selection without the additional maths. You could do a circle/elliptical selection equally easily and also a more freehand selection tool (user draws the area) which would be an absolute nightmare to do mathematically (breaking the area into triangles then projecting every single triangle to check if an object is in that projected volume).

Edited by Nanoha

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Nanoha, that won't work because I need to be able to select occluded objects. It is quite common in an editor to have a "pile" of objects that you want to select by drawing a rectangle around the whole pile. Other than that, it's a neat way to handle the problem. I am doing picking through ray casting which allows me to select all objects along a ray (which is important for some stuff I am doing) but I have used the technique you mentioned for generating a visible-object-ID-map for occlusion culling in a tile based dungeon crawler I worked on.

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If you pick your location in 3d space using a ray at your initial "click" location and than again at your "release" location, you can using the x, z values to form a 2d box(Directx). If an objects draw location is within those coordinates, select. No need to worry about the y coordinate.

 

Edit: This will also select your partial object picks as long as their draw location is on the inside of the rectangle. No reason to use color id in a shader blah blah blah, unless their is a specific reason you need to have collision perfect object selection which I wouldn't understand why in an editing environment.

Edited by ExErvus

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Multiple way possibles :

- Ray intersection using a spacing value for each ray into the rectangle.

- Convert the AABB of each object in screen space and test this rectangle with the selection rectangle.

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