问题
I'm working on a simple 2D Real time strategy game using XNA. Right now I have reached the point where I need to be able to click on the sprite for a unit or building and be able to reference the object associated with that sprite. From the research I have done over the last three days I have found many references on how to do "Mouse picking" in 3D which does not seem to apply to my situation. I understand that another way to do this is to simply have an array of all "selectable" objects in the world and when the player clicks on a sprite it checks the mouse location against the locations of all the objects in the array. the problem I have with this approach is that it would become rather slow if the number of units and buildings grows to larger numbers. (it also does not seem very elegant) so what are some other ways I could do this. (Please note that I have also worked over the ideas of using a Hash table to associate the object with the sprite location, and using a 2 dimensional array where each location in the array represents one pixel in the world. once again they seem like rather clunky ways of doing things.)
回答1:
For up to hundreds of units, it should be fast enough to simply do a linear search O(n) over all the units in the world if the click regions are circles or rectangles. Especially seeing as it will be once per click, not once per frame.
If your units are not circular or rectangular, check against a bounding circle or rectangle first, and if that passes check against the more complicated bounding shape.
For a more detailed answer, here's my answer to a similar question about space partitioning. There I mention bucketed grids and quadtrees as potential structures for performance optimisation.
But you should never do performance optimisation until you have tested and actually do have a performance problem!
回答2:
If you have a class that manages drawabel objects you could have a static int that you increase every time you make a new object, and save the old one as a local instance of Color in the drawabel object. You can then use the .Net type converter to make its to bye arrays and back, dont remember its name and im on my phoneon a train so can't check for you im afraid.
When you build the color from the byte array just remember to max the alpha channel, and if you happen to get too many objects you might overrun the indexes you can use.. not sure what to do then... probably have all your objects reaquire new colors from 0:0:0:255 again since hopefully some old ones are no longer in use :P
Not sure i made alot of sense but since im on a train thats all i can give you, sorry :)
回答3:
You could use pixel perfect picking, which scales very well to massive numbers of objects (and has the advantage of being pixel perfect).
Essentially you render your scene using a unique colour for each object. Then you resolve the backbuffer into a texture and get the texture data back, finally you can simply check the pixel underneath the mouse and find out which object the mouse is on.
You can be clever about the information you get back, you can request just the single pixel the mouse is on top of.
Color[] pixel = new Color[1];
texture.GetData(pixel, mousePosition.Y * texture.Width + mousePosition.x, 1);
//pixel[0] == colour of the item the mouse is over. You can now look this up in a dictionary<Color, item>
You should be careful not to stall the pipeline by doing this (causing the CPU to wait for the GPU to render things). The best way to do this is to swap between 2 render targets, and always GetData from the render target you used last frame, this means the data is a frame out of date, but no human has fast enough reactions to notice.
Addendum in response to your comment.
To assign a unique colour to each object, simply increment a byte for each object. When that byte overflows, increment another, and when that one overflows increment another; Then you can use those three bytes as Red, Green and Blue. Remeber to keep alpha at max value, you don't want any see through objects!
To resolve the backbuffer is slightly changed in XNA4. Now you must render to a rendertarget and resolve that. To do this is pretty simple, and outlined by Shawn Hargreaves here
来源：https://stackoverflow.com/questions/4524710/xna-2d-mouse-picking