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Types of Coordinate Systems

GDI+ uses three coordinate spaces: world, page, and device. World coordinates are the coordinates used to model a particular graphic world and are the coordinates you pass to methods in the .NET Framework. Page coordinates refer to the coordinate system used by a drawing surface, such as a form or control. Device coordinates are the coordinates used by the physical device being drawn on, such as a screen or sheet of paper. When you make the call myGraphics.DrawLine(myPen, 0, 0, 160, 80), the points that you pass to the DrawLine method—(0, 0) and (160, 80)—are in the world coordinate space. Before GDI+ can draw the line on the screen, the coordinates pass through a sequence of transformations. One transformation, called the world transformation, converts world coordinates to page coordinates, and another transformation, called the page transformation, converts page coordinates to device coordinates.

Suppose you want to work with a coordinate system that has its origin in the body of the client area rather than the upper-left corner. Say, for example, that you want the origin to be 100 pixels from the left edge of the client area and 50 pixels from the top of the client area. The following illustration shows such a coordinate system.

Coordinate System

When you make the call myGraphics.DrawLine(myPen, 0, 0, 160, 80), you get the line shown in the following illustration.

Coordinate System

The coordinates of the endpoints of your line in the three coordinate spaces are as follows:

World

(0, 0) to (160, 80)

Page

(100, 50) to (260, 130)

Device

(100, 50) to (260, 130)

Note that the page coordinate space has its origin at the upper-left corner of the client area; this will always be the case. Also note that because the unit of measure is the pixel, the device coordinates are the same as the page coordinates. If you set the unit of measure to something other than pixels (for example, inches), then the device coordinates will be different from the page coordinates.

The world transformation, which maps world coordinates to page coordinates, is held in the Transform property of the Graphics class. In the preceding example, the world transformation is a translation 100 units in the x direction and 50 units in the y direction. The following example sets the world transformation of a Graphics object and then uses that Graphics object to draw the line shown in the preceding figure:

myGraphics.TranslateTransform(100, 50);
myGraphics.DrawLine(myPen, 0, 0, 160, 80);

The page transformation maps page coordinates to device coordinates. The Graphics class provides the PageUnit and PageScale properties for manipulating the page transformation. The Graphics class also provides two read-only properties, DpiX and DpiY, for examining the horizontal and vertical dots per inch of the display device.

You can use the PageUnit property of the Graphics class to specify a unit of measure other than the pixel.

Note Note

You cannot set the PageUnit property to World, as this is not a physical unit and will cause an exception.

The following example draws a line from (0, 0) to (2, 1), where the point (2, 1) is 2 inches to the right and 1 inch down from the point (0, 0):

myGraphics.PageUnit = GraphicsUnit.Inch;
myGraphics.DrawLine(myPen, 0, 0, 2, 1);
Note Note

If you don't specify a pen width when you construct your pen, the preceding example will draw a line that is one inch wide. You can specify the pen width in the second argument to the Pen constructor:

Pen myPen = new Pen(Color.Black, 1 / myGraphics.DpiX);

If we assume that the display device has 96 dots per inch in the horizontal direction and 96 dots per inch in the vertical direction, the endpoints of the line in the preceding example have the following coordinates in the three coordinate spaces:

World

(0, 0) to (2, 1)

Page

(0, 0) to (2, 1)

Device

(0, 0, to (192, 96)

Note that because the origin of the world coordinate space is at the upper-left corner of the client area, the page coordinates are the same as the world coordinates.

You can combine the world and page transformations to achieve a variety of effects. For example, suppose you want to use inches as the unit of measure and you want the origin of your coordinate system to be 2 inches from the left edge of the client area and 1/2 inch from the top of the client area. The following example sets the world and page transformations of a Graphics object and then draws a line from (0, 0) to (2, 1):

myGraphics.TranslateTransform(2, 0.5f);
myGraphics.PageUnit = GraphicsUnit.Inch;
myGraphics.DrawLine(myPen, 0, 0, 2, 1);

The following illustration shows the line and coordinate system.

Coordinate System

If we assume that the display device has 96 dots per inch in the horizontal direction and 96 dots per inch in the vertical direction, the endpoints of the line in the preceding example have the following coordinates in the three coordinate spaces:

World

(0, 0) to (2, 1)

Page

(2, 0.5) to (4, 1.5)

Device

(192, 48) to (384, 144)

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