Legacy screens and circles

[CharlieJV]

While working with legacy screens I noticed something odd. Circles are not circles. They need to be forced to an aspect ratio of 1 to become true circles.

I realize these screens were stretched onto CRT monitors with 4:3 aspect ratios. Is that why the default circle is squashed?

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SCREEN 2

CIRCLE (100, 99), 50 '             not a circle

CIRCLE (300, 99), 50, , , , .418 ' this seems to be the default aspect ratio

CIRCLE (500, 99), 50, , , , 1 '    forced to an aspect ratio of 1:1, now a circle

That would be a little bit of legacy from GW-BASIC not implemented in QB64, I think.

Run GW-BASIC at the Internet Archive.  (You might find it interesting to try this with BAM too, because it, as per wwwBASIC, has the same CIRCLE behaviour as GW-BASIC for compatibility.)

Enter the following program and run it.  Notice how the CIRCLE looks like a semi-decent CIRCLE, but the square does not look like a square at all.

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10 screen 2
20 circle (100,100),50
30 line (50,50)-(150,150),,B

Now modify that program and run again.  Notice how it is a somewhat decent CIRCLE, and a somewhat decent square.

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10 screen 7
20 circle (100,100),50
30 line (50,50)-(150,150),,B

Run both of those programs in QB64.

What you are seeing is QB64 not implementing CIRCLE in the same way as CIRCLE was implemented in GW-BASIC to work with different screen modes.

In GW-BASIC, it does its very best to draw a proportionally correct circle no matter the screen mode.  To do that, the CIRCLE implementation adjusts the number of vertical pixels to a number that works (aspect wise) correctly for the number of horizontal pixels.

Screenshots from BAM (which fairly match results from GW-BASIC hosted at the Internet Archive):