Windows Graphics Device Interface (GDI)

The Windows Graphics Device Interface (GDI) is a core component of the Microsoft Windows operating system that provides a consistent way for applications to interact with graphics hardware and display output. GDI handles the rendering of text, shapes, and images on various output devices such as monitors, printers, and plotters. It offers a rich set of functions for drawing, text manipulation, and device management.

Key Concepts

Device Context (DC): A structure that contains information about a drawing surface and the attributes of the graphics output, such as the drawing pen, brush, font, and background color.
GDI Objects: Resources used for drawing, including Pens (for lines), Brushes (for filling areas), Bitmaps (for images), and Fonts (for text).
Coordinate Systems: GDI uses a logical coordinate system that applications use for drawing, which is then mapped to the physical coordinate system of the device.

Drawing Functions

These functions are used to draw lines, shapes, and curves.

LineTo Draws a line from the current position to a specified point.
Rectangle Draws a rectangle.
Ellipse Draws an ellipse.
Polygon Draws a polygon.
MoveToEx Updates the current position.


// Example: Drawing a red line
HDC hdc = GetDC(hwnd); // Get device context for the window
HPEN hPen = CreatePen(PS_SOLID, 1, RGB(255, 0, 0)); // Create a red pen
HPEN hOldPen = (HPEN)SelectObject(hdc, hPen); // Select the pen into the DC

MoveToEx(hdc, 10, 10, NULL); // Set the starting point
LineTo(hdc, 100, 100); // Draw the line

SelectObject(hdc, hOldPen); // Restore the old pen
DeleteObject(hPen); // Clean up the pen
ReleaseDC(hwnd, hdc); // Release the device context

Text Functions

These functions enable drawing text on the screen.

TextOut Draws a string of characters at a specified location.
SetTextColor Sets the text color.
SetBkColor Sets the background color for text.
CreateFont Creates a logical font.
GetTextExtentPoint32 Calculates the dimensions of a text string.

Object Management

Functions for creating, selecting, and deleting GDI drawing objects.

CreatePen Creates a pen.
CreateSolidBrush Creates a solid brush.
SelectObject Selects an object into a device context.
DeleteObject Deletes a GDI object.

Device Management

Functions for managing device contexts.

GetDC Retrieves a handle to a device context (DC) for a client area of a specified window.
ReleaseDC Releases a device context, freeing it for use by other applications.
CreateCompatibleDC Creates a memory device context that is compatible with the specified device.

For more detailed information and a complete list of functions, please refer to the GDI Programming Guide.