This is not the rudimentary turtle-graphics system I wrote as a teenager

[mmcirvin]

This is only the tribute.

I was fascinated by turtle graphics, as embodied in the computer language LOGO, which was intended for kids. This is a system where your drawing cursor, the "turtle", has a position and a movement direction associated with it, and takes relative angular rotation and forward movement instructions. It's good for drawing things like simple fractals and certain mathematical curves.

The first thing I implemented, I recall, was an interactive turtle graphics system where the user chose commands directly from a menu by number. But that was pretty limited for anything other than simple geometric doodling. What I really wanted was a turtle-graphics library I could call from BASIC. Well, in a language that doesn't even really have functions that is a kind of awkward thing to manage, but I did figure out that I could have a kind of named GOSUB by assigning a line number to a variable, so that was nice. I just had to put all my "arguments" in special global variables and then call GOSUB TURN or GOSUB DRAW, etc. Then I had a sort of BASIC program skeleton I could save on a floppy, load whenever I wanted and put my little application at the beginning. It was sort of a mess, but it worked. It's long-gone, but today I reconstructed something like it in a couple of hours.

Comments

[jwgh]

The color artifacting you were talking about recently is here in spades and it definitely brings me back.

[mmcirvin]

Here is a version with simulated artifacting turned off, for more detail:

[mmcirvin]

Also, to make these images really accurate to the original experience, I should be stretching them vertically in the image embeddings to make the pixels slightly non-square.

[jwgh]

This is a good point. And even without that, the letters in 'READY' are so boxy and wide!

[mmcirvin]

The horizontal resolution limits of NTSC composite video meant that any text display intended for a television really had to be that chunky. It was the dividing line in the early 80s between things primarily intended as home computers and things primary intended as productivity machines: was there an 80-column text mode?

[mmcirvin]

In this old Compute! magazine article, a correspondent describes his extension to Applesoft BASIC to provide named GOSUB with passable parameters (though I think they are still really global variables). He wrote it so he could more easily translate stuff from FORTRAN:

https://www.atarimagazines.com/compute/issue21/Named_GOSUB_With_Variable_Passing.php

Apparently Applesoft BASIC didn't allow the trick of GOSUB to a variable name out of the box (I think a lot of BASICs didn't, but Atari BASIC did).
I didn't get that ambitious.

[secretagentmoof]

Applesoft did have ON A GOTO 10,20,30,40,50 so you could just GOSUB to a statement that had that. Speaking of Applesoft, it also had "shape tables" which encoded a series of vector graphics instructions. Slow as the dickens, though.

[mmcirvin]

I remember reading about those shape tables in an Applesoft manual and coveting them. But I imagined they would be fast sprite-like things.

[secretagentmoof]

Yeah, that's what you'd think from reading the description; in practice, they were all semi-interpreted for each and every render. The Apple II's graphics model (Bizarro vertical layout; and then 1 color mode bit per byte, and then two bits at a time selecting which color is rendered) made individual pixel plotting slow and painful. Everybody who cared about performance precalculated masks and pre-shifted graphics blocks.

[dewline]

Looks okay, given the plan as described there.

[thewayne]

IIRC, Python has a LOGO/Turtle library if you ever really want to play with it again in something slightly more modern....

[mmcirvin]

It does, but I've had some trouble getting it to work in the environments I've got (seems to be X11-based graphics, that's certainly doable but I haven't done the necessary fiddling with configurations). Might try rolling my own in sagemath using its plot functions, which would allow access to the mathematical goodies there.

[mmcirvin]

...Yeah, I just basically replicated my little BASIC library in Sage Python, and it's a lot more readable there.