3.3.4 Using a Bitmap on Different Displays

A multiple-resolution bitmap is a single bitmap file that contains one or more bitmaps that have been marked for use with specific displays, such as the CGA, EGA, VGA, or 8514 displays. You use multiple-resolution bitmaps to avoid problems associated with displaying bitmaps designed for a single type of display. Single-resolution bitmaps can have the following problems:

Appear too big or too small on displays having different resolutions

Appear stretched or compressed on displays with different aspect ratios

Lack colors or use unintended colors on displays with different color capabilities.

You create multiple-resolution bitmaps by using MRBC. The compiler, an MS-DOS program, has the following command-line syntax:

mrbc [/s] filename . . .

The filename parameter specifies the name of a Windows bitmap file. Typically, you specify several filenames, one for each type of display. Wildcards can be used. The compiler uses the filename of the first bitmap file as the name of the output file but gives the output file the filename extension .MRB. The following example combines the bitmap files MYBUTTON.EGA, MYBUTTON.VGA, and MYBUTTON.854 into the multiple-resolution bitmap file MYBUTTON.MRB:

mrbc mybutton.ega mybutton.vga mybutton.854

In this example, the compiler checks the biXPelsPerMeter and biYPelsPerMeter members of the BITMAPINFOHEADER structure in each bitmap file to determine the display type for the bitmap. (For a description of the BITMAPINFOHEADER structure, see the Microsoft Windows Programmer's Reference, Volume 3.) If these members are set to zero, the compiler prompts for the display type with a message such as the following:

Please enter the monitor type for the bitmap mybutton.ega:

You must enter at least the first character of one of the following display-type names: CGA, EGA, VGA, or 8514. The compiler sets the display type you specify, but it does not check that the type is valid. For example, if you specify VGA for an EGA bitmap, the compiler marks it as a VGA bitmap. The result may be undesirable.

The /s option, specifying silent mode, speeds up compilation if the names of the bitmap files conform to the MRBC filename convention. If you use the /s option, the compiler uses the first character of the filename extension to determine the display type for the bitmap, as described in the following list:

If the filename extension starts with any other character, MRBC assumes a VGA bitmap. The following example creates the multiple-resolution bitmap file MYBUTTON.MRB, containing bitmaps for EGA, VGA, and 8514 displays:

mrbc /s mybutton.ega mybutton.vga mybutton.854

The compiler never writes over existing multiple-resolution bitmap files. If the output file already exists, the compiler displays an error message.

You insert multiple-resolution bitmaps into your help file by using the same statements as for Windows bitmaps. For example, the following bmc statement inserts the bitmaps from the MYBUTTON.MRB file:

Click the \bmc mybutton.mrb\} button to complete the task and
return to the main window.

Before displaying a multiple-resolution bitmap, Windows Help checks the display type for the computer and then selects the bitmap that has the closest matching resolution, aspect ratio, and color capabilities. Windows Help never displays more than one bitmap from a multiple-resolution bitmap file.