MSDN Magazine - June 2008 - (Page 105)

ChArles Petzold Foundations Bitmaps And Pixel Bits The retained-mode graphics system of Windows® Presentation constructor that requires the pixel dimensions of the bitmap, the Foundation (WPF) has revolutionized Windows graphics pro- horizontal and vertical resolution in dots per inch, and an object gramming. No longer are programs required to recreate their vi- of type PixelFormat. sual appearance on the screen whenever the system demands. The I’ll have more to say about the PixelFormat structure and the recomposition system retains all graphical figures and arranges them lated static PixelFormats class shortly. For creating objects of type into a total visual presentation. RenderTargetBitmap you must use either PixelFormats.Default or Retained-mode graphics certainly makes life more convenient, PixelFormats.Pbgra32 as the last argument to the RenderTargetbut convenience itself has never been a high priority for Windows Bitmap constructor. In either case, you’re creating a bitmap with programmers. It is really the combination of the retained-mode 32 bits per pixel and transparency. graphics system with notification mechanisms such as depenInitially, the RenderTargetBitmap object is entirely transparent. dency properties that unleashes the flexibility and power of WPF. You can then draw on the bitmap by calling the Render method with Graphical objects such as paths an object of type Visual (includand brushes seem to remain Virtual lab Bitmaps, Pixels, and WPF ing classes that derive from Vi“alive” within the composition sual such as FrameworkElement system and continue to respond WPF bitmaps are responsive to changes via both graphics transand Control). You can restore the forms and changes to pixels within the bitmap. In this Virtual lab, to property changes and graphics all-transparent image by calling transforms, thus allowing these you can experiment with the rendertargetBitmap and WriteClear. If the bitmap is currently ableBitmap classes that make your bitmaps responsive to changobjects to be targets of data bindbeing displayed, these calls are es within your application. the environment is all set up, including ings and animations. reflected in the displayed bitmap Charles Petzold’s sample code, so you can begin coding immediI recently discovered that WPF ately as you read this installment of the Foundations column. without any further ado. bitmaps have a similar dynamic Figure 1 shows a small, comExperiment in the virtual lab: quality. A rendered bitmap replete program that demonstrates go.microsoft.com/fwlink/?LinkId=117105 mains responsive to changes— RenderTargetBitmap. The pronot only to graphics transforms gram creates a bitmap 1,200 pixels (this we all knew) but also to changes in the actual pixel bits with- wide and 900 pixels high. These become the PixelWidth and Pixelin the bitmap. Height properties of the bitmap object. Each pixel is 4 bytes wide, The two bitmap classes that reveal this dynamic responsiveness so the bitmap occupies more than 4 megabytes of memory. are RenderTargetBitmap and WriteableBitmap, which are among The call to the RenderTargetBitmap constructor also specifies a the nine classes that derive from BitmapSource, the abstract class resolution of 300 dots per inch. The combination of the pixel dithat is the basis for all bitmap support in WPF. Regardless of how mension and the resolution results in the creation of a bitmap that a program uses one of these bitmap objects—whether it’s displayed is 4 inches wide and 3 inches tall. The device-independent WPF cowith an Image element, or made into a tiled brush with the Image- ordinate system is 96 units to the inch, so the bitmap has a deviceBrush class, or as part of a larger drawing (perhaps mixed with vec- independent width of 384 units and a height of 288 units. These tor graphics) with the ImageDrawing class—the bitmap is never are the numbers you’ll see if you examine the Width and Height simply rendered and forgotten. The bitmap remains alive within properties defined by BitmapSource. the visual composition system and continues to be responsive to The RenderTargetBitmapDemo program uses the Image element application changes. to display the bitmap without stretching. It also traps MouseDown events. For each mouse click, the program creates a DrawingVi- Using RenderTargetBitmap A RenderTargetBitmap is a bitmap on which you can effectively draw by transferring objects of type Visual to its surface. The only way to create a new object of type RenderTargetBitmap is with a Send your questions and comments to mmnet30@microsoft.com. Code download available at msdn2.microsoft.com/magazine/cc135911. June 2008 105 http://go.microsoft.com/fwlink/?LinkId=117105 http://msdn2.microsoft.com/magazine/cc135911

Table of Contents Feed for the Digital Edition of MSDN Magazine - June 2008

MSDN Magazine - June 2008 Contents Toolbox CLR Inside Out Cutting Edge Patterns In Practice SAAS Concurrency Robotics Form Filler GUI Library Service Station Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - June 2008

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