Alpha regions

A method for encoding an arbitrary 2-dimensional shape represented as a grid of pixels in a graphics system, comprising:

• subdividing the shape into a series of rectangular areas, at least one rectangular area having an identical pattern of pixels occurring at least twice in the rectangular area in a first direction of the grid;
• for the rectangular areas:
• identifying the starting location and extent of a span of pixels in the rectangular area, wherein the span of pixels is either semi-transparent or opaque;
• recording the starting location and extent of the semi-transparent and opaque spans recording the alpha values of the semi-transparent spans;
• and
• recording the starting location and extent of the rectangular area, wherein the starting location is one corner of the rectangular area, and wherein the extent is a number of lines of pixels in the rectangular area in a second direction of the grid;
• wherein alpha values for identical patterns of pixels in the rectangular area are not re-recorded for adjacently recurring occurrences of the pattern;
• and
• alpha values for opaque spans of pixels are omitted from recording.

A computer-readable medium having stored thereon an alpha image data structure representing an image of an arbitrary shape in a graphics system comprising:

• data representing the starting location and extent of a span of pixels in the image, wherein the span of pixels is semi-transparent;
• and
• data representing the alpha values quantifying the opacity of each pixel of the semi-transparent span;
• wherein the data structure is operable to represent transparent and opaque pixels without storing an alpha value for the transparent or opaque pixels;
• and
• alpha values for a recurring pattern of alpha values are omitted from re-recording in the data structure when the recurring pattern of alpha values recurs adjacently to another, identical pattern of alpha values.

A method for encoding alpha values for an arbitrary 2-dimensional shape represented as a grid of pixels in a graphics system, the method comprising:

• identifying one or more rectangular areas in the grid comprising one or more adjacent rows or columns having identical patterns of pixels;
• for rectangular areas in the grid:
• recording the location of a first row or column and the quantity of rows or columns within the rectangular area;
• identifying in the first row or column spans of one or more contiguous pixels having no pixels of a first predetermined alpha value or a second predetermined alpha value;
• for the row or column spans:
• recording the alpha values of the span;
• and
• recording data identifying the span;
• wherein alpha values for pixels having the first predetermined alpha value or the second predetermined alpha value are not recorded for the rectangular areas and alpha values for adjacent rows or columns having identical patterns are not re-recorded for the rectangular areas.

A computer-implemented method of encoding alpha values of a set of pixels defining a shape, the method comprising:

• storing alpha values for a set of pixel rectangles forming the shape, wherein at least one of the pixel rectangles comprises an adjacently-recurring pattern of alpha values;
• and
• for pixel rectangles having the adjacently-recurring pattern of alpha values, storing the alpha values of the adjacently-recurring pattern once for the pixel rectangle and storing information indicating that the pattern adjacently recurs within the pixel rectangle;
• wherein the stored alpha values omit pixels of a first pre-determined alpha value and indicate the extent of a span of pixels of a second pre-determined alpha value without storing an alpha value for the span of pixels of the second pre-determined value.