Pixel span depth buffer

A graphics display system, comprising:

• a reference span including:
• a reference minimum depth value for a reference block of pixels;
• a reference maximum depth value for the reference block of pixels;
• a current span including:
• a current minimum depth value for a current block of pixels;
• a current maximum depth value for the current block of pixels;
• a comparator for comparing at least one of the reference minimum and maximum depth values with at least one of the current minimum and maximum depth values;
• and
• a selector for selecting and saving one of at least a portion of a reference span and at least a portion of a current span based on a result from said comparator.

A method of using a depth buffer, comprising:

• a) providing a reference span having a reference maximum depth value and a reference minimum depth value;
• b) fetching a current span having a current maximum depth value and a current minimum depth value;
• c) comparing at least one of the reference maximum and reference minimum depth values with at least one of the current maximum and current minimum depth values;
• and
• d) saving one of at least a part of the current span and at least a part of the reference span in a span buffer.

A machine readable medium having stored thereon instructions, which when executed by at least one processor cause said at least one processor to perform:

• providing a first pixel group having a first set of depth values;
• providing a second pixel group having a second set of depth values;
• forming a first span having a first minimum value and a first maximum value from the first set;
• forming a second span having a second minimum value and a second maximum value from the second set;
• comparing the first minimum and maximum values with the second minimum and maximum values;
• and
• saving at least a portion of one of the first span and the second span as a result of said comparing.