1 breshenham line draws, 2 pixel step line draws, 4 memory organization for graphics accelerator – Cirrus Logic EP93xx User Manual

Page 268: 4 memory organization for graphics accelerator -4, Table 8-1. screen pixels -4

8-4

DS785UM1

Graphics Accelerator
EP93xx User’s Guide

8.3.1 Breshenham Line Draws

Based on Breshenham's algorithm, this is the fastest of the two lines draws. Patterned lines
drawn are aligned to the major axis. Steps made in the major axis are made on a 4095/4096
pixel step per clock basis. This allows the algorithm to complete the line with the amount of
pixel draws in the major axis. Steps in the minor axis are made in sub pixel increments.

Patterned lines drawn in this mode are aligned to the major axis. A pattern up to 16 bits long
repeats on an interval up to 16 bits. This type of patterning is commonly used.

8.3.2 Pixel Step Line Draws

This is a sub-pixel accumulation line draw that will typically take longer to draw than a
Breshenham line draw. The major advantage of the pixel step line draw is that it provides
angularly corrected patterns. This means that the pattern of the line is applied along the line
at the appropriate angle. The number of algorithm iterations is calculated based on the
calculated pixel length of the line (Pythagorean theorem). A pattern up to 16 bits long repeats
on an interval up to 16 bits. In this mode, visual correctness is emphasized over
completeness. For higher definition patterns, details of the pattern may be lost.

Wide lines are not hardware accelerated, but may be generated by stepping and repeating
single pixel width lines.

If speed is critical, horizontal un-patterned lines may be drawn by single pixel deep block fills.

Note:Line drawing in the negative X or Y directions is not supported by the hardware.

8.4 Memory Organization for Graphics Accelerator

Table 8-1

shows a hypothetical 8 x 6 pixel matrix as it would appear on a display.

P(x,y) is defined as a pixel at location x,y from the upper left corner of the screen.

Table 8-1. Screen Pixels

X-Axis

Y-Axis

P(0,0)

P(1,0)

P(2,0)

P(3,0)

P(4,0)

P(5,0)

P(6,0)

P(7,0)

P(0,1)

P(1,1)

P(2,1)

P(3,1)

P(4,1)

P(5,1) P(6,1)

P(7,1)

P(0,2)

P(1,2)

P(2,2)

P(3,2)

P(4,2)

P(5,2)

P(6,2)

P(7,2)

P(0,3)

P(1,3)

P(2,3)

P(3,3)

P(4,3)

P(5,3) P(6,3)

P(7,3)

P(0,4)

P(1,4)

P(2,4)

P(3,4)

P(4,4)

P(5,4)

P(6,4)

P(7,4)

P(0,5)

P(1,5)

P(2,5)

P(3,5)

P(4,5)

P(5,5) P(6,5)

P(7,5)