An image generating method used in a graphics processing unit which generates a graphics image by rendering a polygon according to a rendering instruction from a central processing unit, said image generating method comprising:
- loading, from a memory, coordinate data indicating a position of each of vertices of the polygon and performing coordinate transform on the loaded coordinate data to generate transformed coordinate data, the coordinate data being included in vertex data that indicates information of the vertices of the polygon;
- determining whether or not the polygon is a rendering object, using the transformed coordinate data generated by said performing of the coordinate transform, and, when the polygon is determined to be the rendering object, generating a vertex list indicating the vertices of the polygon;
- storing, for each of the vertices, shape data that is included in the vertex data and other than the coordinate data, the shape data being stored in a shape data cache;
- determining whether or not the shape data for each of the vertices indicated in the generated vertex list is stored in the shape data cache and, when the shape data for each of the vertices indicated in the vertex list is not yet stored in the shape data cache, loading the shape data from the memory and storing the loaded shape data for each of the vertices indicated in the vertex list in the shape data cache in association with the vertices indicated in the vertex list;
- and
- rendering the polygon using the generated transformed coordinate data and the shape data stored in the shape data cache,
- wherein modified shape data generated by modifying the shape data is stored in the shape data cache,
- wherein said determining of whether or not the shape data for each of the vertices indicated in the generated vertex list is stored in the shape data cache further includes:
- judging whether or not the modified shape data for each of the vertices indicated in the vertex list generated by said generating of the vertex list is already stored in the shape data cache;
- loading the shape data for each of the vertices from the memory when said judging judges that the modified shape data is not yet stored;
- and
- modifying the shape data loaded by said loading of the shape data for each of the vertices to generate the modified shape data, and storing the resulting modified shape data into the shape data cache,
- wherein the shape data cache includes a plurality of cache lines,
- wherein each of the cache lines includes:
- a cache tag indicating an ID of one of the vertices;
- a line memory for storing the modified shape data for the one of the vertices indicated by the cache tag;
- and
- a flag indicating whether or not the modified shape data stored in the line memory is valid,
- wherein said judging includes judging, for each of the cache lines, whether or not the ID of the one of the vertices indicated in the vertex list is already stored in the cache tag corresponding to the flag indicating that the modified shape data is valid,
- wherein said loading includes loading, for each of the cache lines, the shape data for the one of the vertices from the memory when it is judged in said judging that the ID of the one of the vertices is not yet stored in the cache tag, and
- wherein each of the cache lines includes a reference counter for counting the number of times the modified shape data stored in the line memory is referred to.