The WM_PAINT Message

Typically, an application draws in a window in response to a WM_PAINT message. Windows sends this message to a window procedure when changes to the window have altered the content of the client area. Windows sends the message only if there are no other messages in the application message queue.

Upon receiving a WM_PAINT message, an application can call BeginPaint to retrieve the display DC for the client area and use it in calls to GDI functions to carry out whatever drawing operations are necessary to update the client area. After completing the drawing operations, the application calls the EndPaint function to release the display DC.

Before BeginPaint returns the display DC, Windows prepares the DC for the given window. It first sets the clipping region for the DC to be equal to the intersection of the portion of the window that needs updating and the portion that is visible to the user. Only those portions of the window that have changed are redrawn. Attempts to draw outside this region are clipped and do not appear on the screen.

Windows can also send WM_NCPAINT and WM_ERASEBKGND messages to the window procedure before BeginPaint returns. These messages direct the application to draw the nonclient area and window background. The nonclient area is the part of a window that is outside of the client area. The area includes features such as the title bar, window menu (also known as the System menu), and scroll bars. Most applications rely on the default window function, DefWindowProc, to draw this area and therefore pass the WM_NCPAINT message to this function. The window background is the color or pattern a window is filled with before other drawing operations begin. The background covers any images previously in the window or on the screen under the window. If a window belongs to a window class having a class background brush, the DefWindowProc function draws the window background automatically.

BeginPaint fills a PAINTSTRUCT structure with information such as the dimensions of the portion of the window to be updated and a flag indicating whether the window background has been drawn. The application can use this information to optimize drawing. For example, it can use the dimensions of the update region, specified by the rcPaint member, to limit drawing to only those portions of the window that need updating. If an application has very simple output, it can ignore the update region and draw in the entire window, relying on Windows to discard (clip) any unneeded output. Because the system clips drawing that extends outside the clipping region, only drawing that is in the update region is visible.

BeginPaint sets the update region of a window to NULL. This clears the region, preventing it from generating subsequent WM_PAINT messages. If an application processes a WM_PAINT message but does not call BeginPaint or otherwise clear the update region, the application continues to receive WM_PAINT messages as long as the region is not empty. In all cases, an application must clear the update region before returning from the WM_PAINT message.

After the application finishes drawing, it should call EndPaint. For most windows, EndPaint releases the display DC, making it available to other windows. EndPaint also shows the caret, if it was previously hidden by BeginPaint. BeginPaint hides the caret to prevent drawing operations from corrupting it.

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