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Alyce
Oct 6, 2011
**GDI and the Device Context** [[toc|flat]] //Some text below is copied from the Microsoft Developers Network Library.// For an eBook or printed book on using the API with Liberty BASIC, see: [[http://alycesrestaurant.com/apilb/index.htm|APIs for Liberty BASIC]] =GDI= GDI stands for Graphics Device Interface. This is the way that Windows interfaces with the graphics hardware, such as the monitor and printer. =GetDC= GDI communicates with the hardware device using the handle to a Device Context (DC). To get a DC handle, pass the graphicbox handle to the API function from User32.DLL called GetDC. Note that handles are always of type "ulong". The GetDC function retrieves a handle to a device context (DC) for the client area of a specified window or for the entire screen. You can use the returned handle in subsequent GDI functions to draw in the DC. The device context is an opaque data structure, whose values are used internally by GDI. [[code format="lb"]] h = hwnd(#1.graphicbox) calldll #user32, "GetDC",_ h as ulong,_ 'handle of window or graphicbox client area hdc as ulong 'returns handle of Device Context - 0=failure [[code]] =GetWindowDC= The GetWindowDC function retrieves the device context (DC) for the entire window, including title bar, menus, and scroll bars. A window device context permits painting anywhere in a window, because the origin of the device context is the upper-left corner of the window instead of the client area. GetWindowDC assigns default attributes to the window device context each time it retrieves the device context. Previous attributes are lost. [[code format="lb"]] hMain = hwnd(#main) calldll #user32, "GetWindowDC",_ h as ulong,_ 'handle of window hWinDC as ulong 'returns handle of DC for entire window - 0=failure [[code]] The first parameter is the handle to the window with a device context that is to be retrieved. If this value is NULL, GetWindowDC retrieves the device context for the entire screen. =ReleaseDC= At the close of the program, or when the Device Context is no longer needed, call ReleaseDC. The application must call the ReleaseDC function for each call to the GetWindowDC function and for each call to the GetDC function that retrieves a common DC. [[code format="lb"]] calldll #user32, "ReleaseDC",_ h as ulong,_ 'window handle hdc as ulong,_ 'device context ret as long 'nonzero=success [[code]] =Long Color Values= GDI API functions specify colors as a long integer value, made up of the red, green and blue components, each in the range of 0 - 255. 0 is no amount of the specified color, and 255 is complete saturation of the specified color. True red is red=255, green=0, blue=0, for instance. Here is a Liberty BASIC function that creates a long color value from the red, green and blue components. [[code format="lb"]] 'red, green and blue values MUST be in the range 0-255 function MakeRGB(red,green,blue) MakeRGB=(blue*256*256)+(green*256)+red end function [[code]] =SetPixelV= The SetPixelV function sets the pixel at the specified coordinates to the closest approximation of the specified color. SetPixelV is faster than the older SetPixel function. [[code format="lb"]] calldll #gdi32, "SetPixelV",_ hdc as ulong,_ 'device context x as long,_ 'x location of pixel y as long,_ 'y location of pixel color as long,_ 'long color value ret as long 'nonzero=success [[code]] =Demo= [[code format="lb"]] nomainwin winWide=700:winHigh=500 WindowWidth=winWide+50:WindowHeight=winHigh+50 UpperLeftX=1:UpperLeftY=1 graphicbox #1.g, 0,0,winWide,winHigh open "GDI Demo" for window as #1 #1 "trapclose [quit]" #1.g "down" h=hwnd(#1.g) 'graphicbox handle 'get device context for window: calldll #user32, "GetDC",_ h as ulong,_ 'graphicbox handle hdc as ulong 'returns handle to device context color = MakeRGB(255,190,0) 'create a long int color value 'draw lines of pixels in a loop for x = 1 to 500 for y = 40 to 140 step 10 calldll #gdi32, "SetPixelV",_ hdc as ulong,_ 'device context x as long,_ 'x location of pixel y as long,_ 'y location of pixel color as long,_ 'long color value ret as long 'nonzero=success next next wait [quit] calldll #user32, "ReleaseDC",_ h as ulong,_ 'window handle hdc as ulong,_ 'device context ret as long close #1:end function MakeRGB(red,green,blue) MakeRGB=(blue*256*256)+(green*256)+red end function [[code]] ---- [[GDI|GDI Tutorials Home]]