怎么用VB编写放大镜程序？ - VB / 基础类

图像放大镜：在一个图像界面中，通过一个小图像框，图像框可按用户选择的放大率来显示相关区域的内容，小图像框可以移动，显示不同的区域。用户具有选择不同的放大率与设置小图像框大小的功能。

谁能帮帮忙哈
不用给我代码，只要给下主要的设计思路和用的算法什么的。
如果用API函数~请详细说下这个API函数~API还不太懂哈~才学1个月VB
谢谢！

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StreachBlt函数可以放大图片

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新建个form，放一个timer
VB code:
Private Declare Function GetCursorPos Lib "user32" (lpPoint As POINTAPI) As Long
Private Declare Function GetDC Lib "user32" (ByVal hWnd As Long) As Long
Private Declare Function StretchBlt Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal hSrcDC As Long, ByVal xSrc As Long, ByVal ySrc As Long, ByVal nSrcWidth As Long, ByVal nSrcHeight As Long, ByVal dwRop As Long) As Long
Private Type POINTAPI
x As Long
y As Long
End Type
Dim pos As POINTAPI
Const srccopy = &HCC0020
Private Declare Function SetWindowPos Lib "user32" (ByVal hWnd As Long, ByVal hWndInsertAfter As Long, ByVal x As Long, ByVal y As Long, ByVal cx As Long, ByVal cy As Long, ByVal wFlags As Long) As Long
Private Declare Function CreateEllipticRgn Lib "gdi32" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long) As Long