Apparatus and method for managing multiple images in a graphic display system

A method for displaying images stored as encoded picture elements in a plurality of memory buffers, said memory buffers organized to operate in a first state in which a picture element is stored as a plurality of bits at respective locations in each of said plurality of memory buffers or a second state in which a picture element is stored at adjacent locations in a single buffer with each memory buffer containing picture elements representing images for display in a display area of a graphics display device, said method comprising:

• determining whether said buffers are operating in said first or second state;
• if in said first state, accessing a picture element by simultaneously accessing respective locations in each of said plurality of memory buffers and displaying a picture element formed by the accessed bits;
• if in said second state:
• repeating the following steps for each picture element storage location corresponding to a location in said display area;
• simultaneously accessing one picture element from each of said plurality of memory buffers, said picture elements corresponding to a single display area location;
• determining a display priority for each of said plurality of memory buffers;
• examining each corresponding picture element according to said display priority to determine whether said picture element represents a blank or non-blank image portion of said display area;
• displaying the examined picture element if it represents a non-blank image portion of said display area;
• ignoring the other corresponding picture elements once one picture element has been displayed;
• and
• displaying a background color if none of said corresponding picture elements represents a non-blank image portion of said display area.

A method of displaying a movable object with apparent motion relative to a background on a graphics display device, said method comprising the steps of:

• writing encoded images comprising said background to a first memory means with a first display priority;
• writing encoded images representing said moveable object at locations in a second memory means with a second display priority higher than said first display priority;
• combining said encoded images from said memory means, retaining for display the combined encoded image from the memory means with the highest display priority;
• displaying said combined images on a graphics display device;
• erasing said encoded images representing said moveable object and rewriting said encoded images at locations displaced from the previous locations of said encoded images;
• repeating said combining, displaying, and erasing and rewriting steps at a rate sufficient to create the appearance that the encoded images representing said moveable object moves relative to said background on said display device.

An apparatus for combining for display on a display device images stored as encoded picture elements, said apparatus comprising:

• memory means for storing encoded picture elements organized as a plurality of memory buffers;
• state control means for selecting between a first system state in which a plurality of bits representing a picture element are stored at an equivalent relative location in each of a plurality of memory buffers and a second system state in which a plurality of bits representing a picture element are stored at adjacent locations in a single memory buffer;
• selector control means responsive to said state control means and connected to said plurality of memory buffers for controlling selection of picture element bits from said buffers;
• bypass means for bypassing an image mixing operation when said state control means is in said first system state, said bypass means comprising:
• first signal generating means for generating a display control signal from said picture element bits stored in equivalent relative locations in said plurality of memory buffers;
• image mixing means for combining picture elements stored in said memory buffers when said state control means is in said second system state, said image mixing means comprising:
• first selector means connected to a first memory buffer for decoding said encoded picture elements if they are not zero;
• first logic means responsive to said first memory buffer and a second memory buffer for generating an encoded output when said first memory buffer encoded picture element is zero and said second memory buffer encoded picture element is on-zero;
• second selector means connected to said first logic means for decoding said encoded output when said encoded output is non-zero;
• second logic means responsive to said first memory buffer, said second memory buffer and a third memory buffer for generating a second encoded output when said first memory buffer encoded picture element is zero, and said second memory buffer encoded picture element is zero, and said third memory buffer picture element is non-zero;
• third selector means connected to said second logic means for decoding said second encoded output when said second encoded output is non-zero;
• third logic means for generating a third encoded output when said first, second and third memory buffer encoded picture elements are zero;
• fourth selector means connected to said third logical means for decoding said third encoded output when said third encoded output is non-zero;
• second signal generating means for generating a display control signal when a first binary digit of said decoded output of said first, second, third or fourth selector is non-zero;
• third signal generating means for generating a display control signal when a second binary digit of said decoded output of said first, second, third or fourth selector is non-zero;
• fourth signal generating means for generating a display control signal when a third binary digit of said decoded output of said first, second, third, or fourth selector is non-zero;
• and
• enabling means associated with each of said memory buffers of said memory means for enabling said buffers to output data in response to said first, second, or third selector means.