问题
just wondering how someone would go about dragging 4 different objects in openGL. I have very simple code to draw these objects:
glPushMatrix();
glTranslatef(mouse_x, mouse_y, 0);
glutSolidIcosahedron();
glPopMatrix();
glPushMatrix();
glTranslatef(mouse_x2, mouse_y2, 0);
glutSolidIcosahedron();
glPopMatrix();
glPushMatrix();
glTranslatef(mouse_x3, mouse_y3, 0);
glutSolidIcosahedron();
glPopMatrix();
glPushMatrix();
glTranslatef(mouse_x4, mouse_y4, 0);
glutSolidIcosahedron();
glPopMatrix();
I understand how to move an object, but I want to learn how to drag and drop any one of these objects. I've been researching about the Name Stack and Selection Mode, but it just confused the hell out of me. And I also know about having to have some sort of glutMouseFunc. It's just the selection of each shape I'm puzzled on.
回答1:
First thing that you need to do is capturing the position of mouse on the screen when the button is clicked. There are plenty of ways to do it but I believe it's outside of the scope of this question. When you have screen X,Y coords you must detect if any object is selected and which one it is. There are two possible approaches. You can either keep track of a bounding rectangle positions of each object (in screen space) and the test if the cursor is inside any of those rectangles will be quite simple. Or you can cast a ray from eye through cursor position in world space and check intersection of this ray with each object.
The second approach is more versatile for 3D graphics but you seem to be using only X and Y coords so you don't need to worry about Z order of objects.
In case of the first solution the main problem is: how to know how big is your object on the screen. glutSolidIcosahedron() renders an object of radius 1. To calculate it's screen radius you can either use some matrix math or in that case a simple trigonometry. You will need to know the distance from camera to the drawing plane (I believe you're using some glTranslatef(0,0,X) before you render. X is your distance) You also need to know the view angle of the camera. You set it in projection matrix. Now take a piece of paper, draw a cone of angle alpha, intersecting a plane at distance X and knowing that an object has radius 1 you can easily calculate how large area of the screen it occupies. (I'll leave this calculation for you)
Now if you know the radius on screen, simply test the distance from your click position to each object. if the distance is below radius it's selected. If more than one object passes this test just select first one of them.
来源：https://stackoverflow.com/questions/25861374/c-opengl-dragging-multiple-objects-with-mouse