frisk@rhi.hi.is (Fridrik Skulason) (05/29/90)
I have noticed a minor problem when drawing circles in MSC 5.00, namely that
the circle sometimes seems be one pixel too large.
Take the following program for example:
#include <stdio.h>
#include <graph.h>
main()
{
_setvideomode(_ERESCOLOR);
_moveto(50,90);_lineto(80,90);
_ellipse(_GBORDER,61,90,69,95); /* upper limit of circle = 90 */
getch();
_setvideomode(_TEXTC80);
}
It is supposed to produce something like this:
line 90 *******************************
** **
* *
* *
** **
***
Instead I get something like this:
***
line 90 *******************************
* *
* *
** **
***
where the top of the circle seems to be one pixel too high. This effect
only happens in certain cases, which seems to indicate a minor bug in the
library. My questions:
I only have MSC 5.00 - does this effect also appear in the latest versions ?
I could solve this by using Turbo C, (which should have been used
to start with), but then I would have to rewrite a considerable part of the
application. Does anyone know of another C compiler with MSC-like
graphics functions, without this problem ?
-frisk
--
Fridrik Skulason University of Iceland |
Technical Editor of the Virus Bulletin (UK) | Reserved for future expansion
E-Mail: frisk@rhi.hi.is Fax: 354-1-28801 | everett@hpcvlx.cv.hp.com (Everett Kaser) (05/30/90)
The problem with the _ellipse function in Microsoft C giving you an ellipse that is actually one dot too wide (or high) is this: the function was written to accept the corners of the bounding rectangle as the arguments, or ellipse specifying parameters. However, the actual code (or algorithm) used is based on knowing the "center" of the ellipse and the lengths of the two axis. Hence, the _ellipse function must first find the "center" of the rectangle and the distance from the "center" to the two sides, before beginning the actual drawing. This works fine if the "ellipse" (or the bounding rectangle) is an odd number of dots wide (and high), because then you can find the EXACT center of the figure. But when the ellipse is an even number of dots wide (or high), the center is at a half dot position, and you can't plot "half-dots" on a CRT, so the center has to be adjusted a half-dot one way or the other. The algorithm for drawing the ellipse assumes that the "quarters" of the ellipse are symetrical about the center, so this forces the ellipse to be drawn one dot larger (or smaller) than the user actually requested. Yes, you could come up with an algorithm that would do a best fit to the bounding rectangle that the user requested, but the REAL problem here is that Microsoft chose to have the user specify the parameters from a different frame (or point of view) than that which the algorithm actually uses, and accuracy is lost in the conversion. Everett Kaser Hewlett-Packard Company ...hplabs!hp-pcd!everett work: (503) 750-3569 Corvallis, Oregon everett%hpcvlx@hplabs.hp.com home: (503) 928-5259 Albany, Oregon