Inside Macintosh: Imaging with QuickDraw
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The Monitors control panel is the user interface for changing the pixel depth, color capabilities, and positions of video devices. Since the user can control the capabilities of the video device, your application should be flexible: although it may have a preferred pixel depth, your application should do its best to accommodate less than ideal conditions.
If it is absolutely necessary for your application to draw on a video device of a specific pixel depth, your application can use the SetDepth function to change its pixel depth. Before calling SetDepth , use the HasDepth function to determine whether the available hardware can support the pixel depth you require.
To determine whether a video device supports a specific pixel depth, you can use the HasDepth function.
FUNCTION HasDepth (aDevice: GDHandle; depth: Integer;
whichFlags: Integer; flags: Integer): Integer;
The HasDepth function checks whether the video device you specify in the aDevice parameter supports the pixel depth you specify in the depth parameter, and whether the device is black and white or color, whichever you specify in the flags parameter.
The HasDepth function returns 0 if the device does not support the depth you specify in the depth parameter or the display mode you specify in the flags parameter.
Any other value indicates that the device supports the specified depth and display mode. The function result contains the mode ID that QuickDraw passes to the video driver to set its pixel depth and to specify color or black and white. You can pass this mode ID in the depth parameter for the SetDepth function (described next) to set the graphics device to the pixel depth and display mode for which you tested.
To change the pixel depth of a video device, use the SetDepth function.
FUNCTION SetDepth (aDevice: GDHandle; depth: Integer;
whichFlags: Integer; flags: Integer): OSErr;
The SetDepth function sets the video device you specify in the aDevice parameter to the pixel depth you specify in the depth parameter, and it sets the device to either black and white or color as you specify in the flags parameter. You should use the HasDepth function to ensure that the video device supports the values you specify to SetDepth . The SetDepth returns zero if successful, or it returns a nonzero value if it cannot impose the desired depth and display mode on the requested device.
The SetDepth function does not change the 'scrn' resource; when the system is restarted, the original depth for this device is restored.
Your application should use SetDepth only if your application can run on devices of a particular pixel depth and is unable to adapt to any other depth. Your application should display a dialog box that offers the user a choice between changing to that depth or canceling display of the image before your application uses SetDepth . Such a dialog box saves the user the trouble of going to the Monitors control panel before returning to your application.
The SetDepth function may move or purge blocks of memory in the application heap. Your application should not call this function at interrupt time.