A WIP sample on how to measure the distance from a given object in viewport to mouse - in pixels.

mouseProximity.cpp

// This is not entirely working as it crashes MODO when an item with this drawing
// is selected and transform tool is enable. So something somewhere has to return bad value at some point
// and this sample is not handling that.
// This code needs to be executed from within item's Draw method.
// Drawing is aborted if mouse pointer is more then 120 pixels from the item to be drawn.
// The idea is to only draw the item when mouse pointer is close enough to it.
#include <lx_vp.hpp>

    CLxUser_View view(stroke);

    CLxUser_Matrix itemWorldMatrixRaw;
    chan.Object(item, "worldMatrix", itemWorldMatrixRaw);
    CLxMatrix4 itemWorldMatrix(itemWorldMatrixRaw);
    CLxVector item3dpos = itemWorldMatrix.getTranslation();

    CLxUser_View3DportService viewService;
    int mx, my;
    int viewIndex = viewService.Mouse(&mx, &my);
    if (viewIndex >= 0)
    {
        CLxVector mousePos(mx, my, 0.0);

        double ix, iy;
        int result = view.ToScreen(item3dpos.v, &ix, &iy);
        if(result == 0)
        {
            return; // this 3d location is not on screen, bail out.
        }

        if (ix < 0.0 || iy < 0.0 || mx < 0 || my < 0)
        {
            return;
        }

        CLxVector itemPos(ix, iy, 0.0); // item pos in screen coords

        CLxVector screenOffset = mousePos - itemPos;
        double distance = screenOffset.length();

        if (distance > 120.0)
        {
            return;
        }
    }