A Picture Viewer for the Pocket PC 2002

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Introduction

This is a very simple picture viewer that illustrates some implementation details that you might want to use in your own Pocket PC 2002 projects. These are:

• Using the imgdecmp.lib for loading BMP, JPEG and GIF image files.
• Implementing a "tap-and-scroll" for scrolling convenience.
• Implementing a full screen view.
• Removing the "New" button from a Document / View toolbar.
• Adding a title bar to the view (new)

Application architecture

This application uses the MFC's Document / View architecture in order to ease implementation. Bitmap pictures are drawn in a CScrollView-derived class, with a special support for the "tap-and-scroll" functionality. This allows the user to scroll the bitmap using the stylus, by tapping and dragging it. The document does not allow saving (after all, this is a viewer), but supports zooming. The zoom in and zoom out commands are supported at the document level although, arguably, they could be implemented at the view level.

Loading images

Images are loaded through a sparsely-documented API, implemented through the imgdecmp.lib. Both this library and its header are distributed with the PPC2002 SDK. Apparently, this is the same mechanism that is used by Pocket IE to load its images from the Internet, and that is why it supports a packet architecture. As for file formats, it is reported to support BMP, JPEG and GIF, but it may support more.

Images are loaded in CPicViewDoc::Serialize(). The process (see source) involves calling the DecompressImageIndirect() function, the DecompressImageInfo structure and two callback functions. The first callback function - GetImageData() - is used to read the image data in chunks, whereas the second - ImageProgress() - is used to report the image loading progress. In this sample, the callback does nothing (but yes, it must be there).

The fields of the DecompressImageInfo structure are quite self-explanatory, with one exception: lParam. This is the parameter that is passed to the callback functions, and I use it to carry a pointer to a structure containing a pair of values (ReadBmp): the CFile pointer retrieved from the open CArchive, and a counter of the image size. This is not strictly necessary for this application because one file has just one image, so passing the CFile* in lParam would be enough. This technique is used because it allows for extensibility, in the case where the file has multiple images, such as a skin file. Here you cannot rely in the end-of-file stop rendering the bitmap.

Tap-And-Scroll

This technique allows scrolling the image with a tap and drag sequence. The image will follow the stylus, scrolling automatically. The scroll bars are also present and active.

In order to achieve this effect, the view handles both OnLButtonDown() and OnMouseMove() (see code). The first stores the tap point, and the second calculates the scroll vector. This vector is then used in the ScrollTo() method, where the new scroll position is calculated. Note that this code was directly adapted from MFC's sources in order to avoid the unpleasant "jagged" scrolling one gets by using CScrollView::ScrollToPosition(). This method does not allow scrolling in both x and y directions simultaneously.

Full screen view

In the Pocket PC 2002, the full screen functionality is achieved through the SHFullScreen API. This function allows the application to position its main window over areas that are usually controlled by the system, such as the SIP and the taskbar. The code that achieves this is in the CMainFrame::OnFullScreen() handler. In order to support full screen toggling, a context menu is available through the tap-and-hold command. This menu allows you to enter and leave the full screen mode, as well as to issue Zoom In and Zoom Out commands.

The New button

Because this is just a viewer, the New button was removed from the main menu using a technique designed by Vassili Philippov. The original article is here: How to remove New button from command bar?

Title bar

Updated on October 27, 2003. The sample application now supports a title bar for displaying the image's file name. This title bar can be hidden or shown via a menu option, under Tools.

This title bar is implemented through the CCeTitleBar class, a CControlBar-derived window. It merely displays the title text in a predefined font and color.

Using this class is very straightforward (see source code). The window is created on the view's OnCreate handler and destroyed in OnDestroy. The view's OnUpdate handler is used to set the title text (document file name). Showing and hiding the title bar is done in the view's OnShowTitle handler.