vb代码关于GetPixel的问题

hdc1 = Picture1.hDC '返回句柄

Dim bBlue As Long, bRed As Long, bGreen As Long

For i = 1 To Picture1.Width '遍历一次图像,用数组存储色点信息
For j = 1 To Picture1.Height

rgb1 = GetPixel(hdc1, i, j)
bGreen = Green(rgb1)
xx(1500, 1500, 1) = bGreen
nfs(bGreen, 1) = nfs(bGreen, 1) + 1

Next j
Next i

上面是别人教我的代码，但是我自己运行会提示说子函数或程序未定义，请问是什么原因。

在代码定义区域添加如下语句，就能使用了

Public Declare Function GetPixel Lib "gdi32" Alias "GetPixel" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long) As Long