Published April 2000

LOOKING GOOD

Using a Graphics-Based LCD
Module with C
by Bob Perrin and Tak Auyeung

Start • Software Overview • The Bottom Layer • Initializing the LCD Display • Shadow Display • Drawing Dots and Lines • Printing Text • Extensions • Sources and PDF

DRAWING DOTS AND LINES

Given the arrangement of pixels on the LCD, drawing dots is boring. It’s simply a matter of computing the page address and column address, modifying the shadow display data at the correct bit, and sending the new display data to the LCD. However, drawing a line is a bit more complicated.

The most intuitive method is to use the equation y = mx + c, in which m is the slope and c is the intercept. However, this method involves at least one multiplication for each pixel on the line. A less intuitive, but more effective method is presented in pseudo code as follows.

Let (x1, y1) and (x2, y2) be the end points of the line, furthermore, x1 <= x2, y1 <= y2. In addition, let limit = x2 – x1 and increment = y2 – y1. Let us assume limit >= increment, then the following code draws a straight line from (x1, y1) to (x2, y2):

sum = 0
y = y1
for x = x1 to x2
sum = sum + increment
if (sum >= limit) then
sum = sum – limit
y = y + 1
end if
draw (x,y)
end for

Note that there are no multiplication or division operations in the above code. Only integer operations are used. The computation for each pixel involves one increment (for x), one addition (for sum), one comparison (for sum), maximum one subtraction (for sum), and maximum one more increment (for y).

The restrictions of x1 <= x2, y1 <= y2, (x2 – x1) >= (y2 – y1) are handled by making provisions for each case so that the algorithm can be applied.

FILLING

Two functions fill either the whole screen or a rectangle. The function scrFill fills the entire screen with a pattern, whereas scrFillBox fills a rectangle in the screen. The first argument of both functions is a pointer to a struct _scrBuffer that represents the shadowed state of the screen. The second argument of scrFill is a byte that specifies the pattern to fill the screen. The second and third arguments of scrFillBox are of type struct _point, which indicate the upper-left corner and lower-right corners of the rectangle.

Note that the brush type applies to both functions. In other words, scrFill or scrFillBox can set, clear, flip, or overwrite the pixels on the LCD. The overwrite mode (SCR_BRUSH_OVR) is not recommended for rectangles that are not on page boundaries, because overwrite affects all eight pixels of a page regardless of the boundary of the rectangle. The following call sequence efficiently clears the screen:

scrSetBrush(SCR_BRUSH_OVR);
scrFill(&scrBuffer, 0x00);

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