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This function is very interesting as it lets you create round rectangle regions which are very extended :

    FUNCTION long CreateRoundRectRgn( long nLeftRect, long nTopRect, long nRightRect, long nBottomRect , long nWidthEllipse, long nHeightEllipse) LIBRARY "gdi32.dll"

    nLeftRect,         // x-coordinate of upper-left corner
    nTopRect,         // y-coordinate of upper-left corner
    nRightRect,       // x-coordinate of lower-right corner
    nBottomRect,    // y-coordinate of lower-right corner
    nWidthEllipse,   // width of ellipse
    nHeightEllipse   // height of ellipse

also take in mind the 'CombineRgn' function (mentioned in the above post) :

    FUNCTION int CombineRgn(ulong hrgnDest, ulong hrgnSrc1, ulong hrgnSrc2, int fnCombineMode) LIBRARY "gdi32.dll"

    hrgnDest,      // handle to destination region
    hrgnSrc1,      // handle to 1st source region
    hrgnSrc2,      // handle to 2nd source region
    fnCombineMode   // region combining mode

where 'fnCombineMode' can take these values :

    CONSTANT RGN_AND   = 1    // Creates the intersection of the two combined regions
    CONSTANT RGN_OR    = 2    // Creates the union of two combined regions
    CONSTANT RGN_XOR   = 3   // Creates the union of two combined regions except for any overlapping areas
    CONSTANT RGN_DIFF   = 4   // Combines the parts of hrgnSrc1 that are not part of hrgnSrc2
    CONSTANT RGN_COPY = 5   // Creates a copy of the region identified by hrgnSrc1

Regions can also be used with objects such PictureButtons or CommandButtons just
taking its handle and aplying them a region as described in the next issue :  https://www.experts-exchange.com/questions/20486020/How-do-I-make-PB-command-buttons-resemble-MS-buttons.html

Anyway, I recommend you to read MS Reference : http://msdn.microsoft.com/library/en-us/gdi/regions_7ab7.asp

Hope it helps
Regards :: Tr1l0b1t