Windows Graphics Device Interface (GDI)

The Windows Graphics Device Interface (GDI) is a core component of the Microsoft Windows operating system responsible for presenting graphical objects to displays, printers, and other output devices. GDI provides a device-independent way for applications to draw lines, curves, text, and graphical primitives.

This documentation covers the GDI API, its functions, structures, and concepts for developing graphical applications on Windows.

Key Concepts

• Device Context (DC): A data structure that contains information about the drawing attributes of a device. All drawing operations in GDI are performed on a device context.
• Graphical Objects: Resources such as pens, brushes, fonts, bitmaps, and regions that are used to draw on the screen.
• Drawing Functions: A rich set of functions for drawing lines, rectangles, ellipses, polygons, text, and more.
• Coordinate Systems: Understanding the different coordinate systems used by GDI, including logical and physical coordinates.

Core GDI Functions

Here are some of the fundamental GDI functions you will frequently use:

Function Name	Description
CreatePen	Creates a new pen that can be selected into a device context.
CreateSolidBrush	Creates a solid brush with the specified color.
SelectObject	Selects a graphical object into a device context.
MoveToEx	Updates the current position of the device context.
LineTo	Draws a line from the current position to the specified point.
Rectangle	Draws a rectangle.
TextOut	Writes a string of characters at the specified location.
GetDC	Retrieves a handle to a device context for a client area of a specified window.
ReleaseDC	Releases a device context, freeing the memory associated with it.

Structures

GDI uses various structures to define graphical properties and coordinates:

• POINT: Specifies the x- and y-coordinates of a point.
• RECT: Defines the coordinates of the upper-left and lower-right corners of a rectangle.
• LOGPEN: Defines the properties of a pen.
• LOGBRUSH: Defines the properties of a brush.

Example: Drawing a Rectangle

Here's a simple C++ example demonstrating how to draw a rectangle using GDI:

#include <windows.h>

void DrawMyRectangle(HDC hdc)
{
    // Create a blue pen
    HPEN hPen = CreatePen(PS_SOLID, 2, RGB(0, 0, 255));
    // Create a light gray brush
    HBRUSH hBrush = (HBRUSH)GetStockObject(LTGRAY_BRUSH);

    // Select the pen and brush into the device context
    HPEN hOldPen = (HPEN)SelectObject(hdc, hPen);
    HBRUSH hOldBrush = (HBRUSH)SelectObject(hdc, hBrush);

    // Define the rectangle's coordinates
    RECT rect = { 50, 50, 200, 150 }; // { left, top, right, bottom }

    // Draw the rectangle
    Rectangle(hdc, rect.left, rect.top, rect.right, rect.bottom);

    // Restore the original pen and brush
    SelectObject(hdc, hOldPen);
    SelectObject(hdc, hOldBrush);

    // Clean up the created pen and brush
    DeleteObject(hPen);
    DeleteObject(hBrush);
}
            

Further Reading

• Win32 Graphics Overview
• Device-Independent Graphics

Last Updated: October 26, 2023