VB中的GDI编程-1 设备环境DC

设备环境

GDI绘图是在设备环境上实施的，而我们使用GDI的基本步骤如下：

1、获取或建立设备环境

2、在设备环境中绘图

3、释放设备环境

那么接下来我们首先来了解下设备环境的获取、建立以及释放方法。之所以将释放方法放到前面来讲，是因为释放的方法与获取和建立的方法是相互对应的。我们常用的函数有以下一些：

1 Public Declare Function CreateCompatibleDC Lib "gdi32" (ByVal hdc As Long) As Long
2 Public Declare Function CreateDC Lib "gdi32" Alias "CreateDCA" ( _
3     ByVal lpDriverName As String, ByVal lpDeviceName As String, ByVal lpOutput As String, lpInitData As DEVMODE) As Long
4 Public Declare Function DeleteDC Lib "gdi32" (ByVal hdc As Long) As Long
5 Public Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long
6 Public Declare Function GetDCEx Lib "user32" (ByVal hwnd As Long, ByVal hrgnclip As Long, ByVal fdwOptions As Long) As Long
7 Public Declare Function GetWindowDC Lib "user32" (ByVal hwnd As Long) As Long
8 Public Declare Function ReleaseDC Lib "user32" (ByVal hwnd As Long, ByVal hdc As Long) As Long
9 Public Declare Function WindowFromDC Lib "user32" (ByVal hdc As Long) As Long

上述API函数中，CreateDC函数使用到了一个数据结构DEVMODE，下面将该数据结构的定义方式列出：

 1 Public Type DEVMODE
 2         dmDeviceName As String * CCHDEVICENAME
 3         dmSpecVersion As Integer
 4         dmDriverVersion As Integer
 5         dmSize As Integer
 6         dmDriverExtra As Integer
 7         dmFields As Long
 8         dmOrientation As Integer
 9         dmPaperSize As Integer
10         dmPaperLength As Integer
11         dmPaperWidth As Integer
12         dmScale As Integer
13         dmCopies As Integer
14         dmDefaultSource As Integer
15         dmPrintQuality As Integer
16         dmColor As Integer
17         dmDuplex As Integer
18         dmYResolution As Integer
19         dmTTOption As Integer
20         dmCollate As Integer
21         dmFormName As String * CCHFORMNAME
22         dmUnusedPadding As Integer
23         dmBitsPerPel As Long
24         dmPelsWidth As Long
25         dmPelsHeight As Long
26         dmDisplayFlags As Long
27         dmDisplayFrequency As Long
28 End Type

以上函数中以Create开头的将会新创建一个设备环境DC，而Get开头的函数是通过窗体句柄获取相应窗体对应的DC对象。另外对于通过Create创建的设备环境，我们在最后释放时，是调用DeleteDC函数。而对于通过Get函数获取的DC对象需要使用ReleaseDC函数来完成释放。

在VB6中，如果需要在窗体上绘图，我们可以直接使用窗体本身的hDC属性来获取；如果是在VBA或者其他不能直接获取DC对象的情况下，我们就需要使用以上所提供的函数来创建或获取相应的DC对象。