Looking Good

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	LOOKING GOOD
Using a Graphics-Based LCD Module with C by Bob Perrin and Tak Auyeung Start ı Software Overview ı The Bottom Layer ı Initializing the LCD Display ı Shadow Display ı Drawing Dots and Lines ı Printing Text ı Extensions ı Sources and PDF PRINTING TEXT The interesting aspect of printing text is not how to put the pixels on the screen, but where to get the pixels. Creating binary files of a custom font format byte-by-byte is out of the question. It is equally unreasonable to implement a custom font-editor program. It is more pragmatic to investigate existing font file formats and evaluate their applicability. Borland C++ 4.52 (and earlier versions) includes a Resource Workshop program that, among other abilities, visually edits font files. The Windows font file format for fixed-pitch raster fonts is easy to interpret. Thus, it is reasonable to make the graphics LCD driver able to use Windows fixed-pitch raster font files. There is one efficiency issue, however. Each bitmap in the Windows font file is comprised of one or more eight-pixel-wide vertical strips. The first byte of a font bitmap corresponds to pixels (0,0) to (7,0) of the bitmap. If the font is wider than eight pixels, the second strip, starting with the byte that represents pixels (8,0) to (15,0) of the bitmap, follows the last byte of the first strip. The pixel arrangement in a Windows raster font file is different from the pixel arrangement of the Hantronix HDG12864F-1. One simple solution is to draw the font rotated. If the bitmaps are rotated counter-clockwise by 90ı, the ordering of the bytes in the font bitmaps matches the ordering of bytes on the LCD. However, this approach makes it inconvenient to draw fonts or to use any existing font files. (Note that it is illegal to distribute copyrighted font files without the permission of the owner.) The LCD driver provides a function for loading a Windows font file. The function fntLoad loads the font file into a memory structure. The first argument is a string of the pathname of the file. A DOS-based controller certainly makes it easy to separate fonts as files separated from the application. The second argument is a pointer to a valid struct _fontInfo structure. The function fntLoad examines the font file to ensure it is the right type (version 3.00, fixed pitch, and raster), then allocates memory to store the converted bitmaps of the font. When a font is no longer useful, the application can call fntUnload to free the memory allocated to store the bitmap. Note that fntUnload does not de-allocate the storage used by the struct _fontInfo pointed to by the first argument. Two functions, scrDrawChar and scrDrawString, implement the logic for displaying characters on the screen. Both functions require a pointer to a struct _scrBuffer that represents the shadowed state of the LCD. Both functions also require a pointer to a struct _fontInfo that indicates the font to use for printing. The function scrDrawChar requires a third argument of const char to indicate which character to print. The function scrDrawString requires a third argument of const char * to indicate the string to print. For safety purposes, the scrDrawString function also requires a fourth argument of unsigned type to indicate the maximum length to print. The last argument of both functions is a struct _point that indicates the coordinate of the upper-left corner of the printed text. PUTTING IT TOGETHER After discussing the implementation of various fragments of the driver software, it is important to summarize the API so that a developer can immediately start to use the code. First, it is important to declare or allocate the storage for the shadowed state of the LCD. The simplest method is to declare a local or global variable of struct _scrBuffer. The following sequence initializes both the LCD and shadowed state to a blank screen: lcdInit(); scrSetBrush(&scrBuffer,SCR_BRUSH_OVR); scrFill(&scrBuffer,0); If the application draws text, it should declare or allocate storage of struct _fontInfo for loaded fonts. The simplest method is to declare a local or global variable of struct _fontInfo for each font. The following function call loads a font file named 6x8.fnt to a memory-based variable named fontInfo (of type struct _fontInfo). fntLoad("6x8.fnt",&fontInfo); When a font is no longer useful, fntUnload(&fontInfo) frees the memory allocated. The API presented allows the applications programmer to do a number of common tasks. Setting and clearing points, drawing lines, filling rectangles and displaying text are the most common graphics primitives used. The following list provides a simple set of instructions for using the API found in glcd.c and glcd.h to draw simple graphic and text objects. ı To draw a dot, first initialize a struct _point object that represents the coordinates of the point, then call scrDrawDot. ı To draw a line, initialize two struct _point objects that represent the end-points of the line, then call scrDrawLine. ı To draw a filled rectangle, first initialize two struct _point objects that represent the upper-left and lower-right corners of the rectangle, then call scrFillBox to fill the rectangle. ı To use a pattern to fill the entire screen, call scrFill. ı To display a character, first initialize a struct _point object that represents the upper-left corner of the character, then call scrDrawChar. ı To display a string, first initialize a struct _point object that represents the upper-left corner of the string, then call scrDrawString. PREVIOUS • NEXT Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles. For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ıCircuit Cellar, the Magazine for Computer Applications. Posted with permission.