Usage and Restrictions

Transparency Index

There are four overlay planes in the HP 98765 display. Even though these planes can display 16 colors simultaneously, only 15 are available because one color is reserved for the transparency color. By default, this color is index 7 or 15, depending on the overlay depth. When the transparency color's index is written into the overlay planes, the observed color is that of the image planes.

X Window System See Through Color

The X Window system always uses color 7 (15 for 4-plane devices) as the see-through color. This cannot be changed.

Cursors

The hp98766 device driver implements cursors using either the hardware cursor or overlayed software cursors. If no processes have opened all four overlay planes, then the fourth overlay plane is used for overlayed software cursors.

You can think of the fourth overlay plane used for cursors as a separate cursor plane. Any data in the cursor plane will be displayed over data in the graphics planes. Data in the other three overlay planes will be displayed over data in the graphics planes and the cursor plane. For example, suppose a graphics application is running in the graphics planes while the window manager is running in three of the overlay planes. If the application has a Starbase cursor in the overlay cursor plane, then the cursor will always be visible inside regions of see-thru because the cursor has display priority over the graphics. If the cursor is moved outside of regions of see-thru then it is not visible since the non-see-thru regions in the overlay planes have display priority over the cursor plane.

The HP 98766 Display Station has a hardware cursor that supports all cursor types except rubber-band line and rubber-band box. There is only one hardware cursor available. Usage of the hardware cursor is defined as follows:

If the hardware cursor is already being used by another process, then overlayed software cursors are used by the hp98766 driver. If the fourth overlay plane is not available for cursors, then an error will be generated when any attempts are made to turn on the cursors.

If a process is using the hardware cursor and it switches to using a non-raster cursor, it retains control of the hardware cursor, but the cursor is drawn in the fourth overlay plane using software. If the fourth overlay plane is not available for cursors, an error is generated and non-raster cursors cannot be used. When the process switches back to a raster cursor, it will again use the hardware cursor.

If an application never uses cursors or uses non-raster cursors exclusively, the driver will never attempt to allocate the hardware cursor. However, once the driver has allocated the hardware cursor, the driver does not relinquish control of the hardware cursor until gclose time. While it is not being used, it simply remains inactive, no other process can use the hardware cursor once it has been assigned to a process.

If allocation of the hardware cursor was not successful, then resources for the software cursor area are allocated (that is, offscreen areas for raster echo definitions). The following functions will cause the driver to attempt to allocate either the hardware cursor or software cursor resources:

Once and application has used a rubber-band echo type, it will thereafter use only software cursors, even if the echo type is switched back to a non-rubber-band type.