ROAST ::

[Edit 2026-04-18: Fixed incorrect byte ordering of colour channels.]
It’s been 25 years — blimey! — since I did any serious ARM coding. The last was fixing some bugs in my Lisp interpreter in 2001, after which my Iyonix died and I moved over to Linux.

Back with RISC OS now, and in the intervening years the ARM processor has had some substantial changes. Originally it was the Acorn RISC Machine, of course, and it first appeared in the Acorn Archimedes computer in 1987. It was a genuine breakthrough at the time, a custom-designed (by Sophie Wilson et al) 32-bit processor running at 8MHz. I’m now using a 4té², a repackaged Raspberry Pi 4b containing an ARM Cortex-A72, running at 1.8GHz — over 200 times faster.

And while the original ARM chips were indeed Reduced Instruction Set Computers, with only about 25 instructions¹, these days it’s something of a misnomer. So what’s been added? SIMD and NEON, mostly. This article is a simple introduction to using some SIMD instructions; we’ll cover NEON² later.
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The previous post in this series covered circular or radial blends and gradients. With a few simple modifications the code can be adapted to create elliptical and annular fills. Or eggs and doughnuts, if you prefer 😉.

Again, all these examples will be using various procedures from the previous posts.

RISC OS provides some graphics primitives to draw ellipses, and from BASIC this is:

ELLIPSE [FILL] centreX,centreY, width,height [, angle] 

We won’t be using this however, and we won’t be including the angle setting, which draws a rotated ellipse. Let’s not make it too complicated…
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The first post in this series covered filling a rectangle with blends and gradients. This time we’ll look at generating circular or radial blends and gradients, which is a bit more complicated. All these examples will be using various procedures from the previous post.

RISC OS provides graphics primitives to draw outline and filled circles. From BASIC:

PROCinit SYS CT_SetGCOL%,&40dd4000 CIRCLE 64,192,60 CIRCLE FILL 192,64,60

The parameters are the centre coordinates and the radius, in OS units. As an aside, here’s the additive RGB triplet as used in the PhotoDesk manual, showing the complementary cyan, magenta and yellow colours. The circles are blended with the OR operation:

PROCinit SYS CT_SetGCOL%,red% CIRCLE FILL 128,160,80 SYS CT_SetGCOL%,blue%,,,,1 CIRCLE FILL 170,88,80 SYS CT_SetGCOL%,green%,,,,1 CIRCLE FILL 92,88,80

So, can we use these primitives to create a radial blend? Here’s a first attempt:
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Coming back to RISC OS has been interesting. I’ve had to re-learn a number of things, like using the WIMP and, in particular, graphics programming. I hope this article will be the first of a series explaining my learning curve, and hopefully providing some useful programs. It’s fairly basic — in all senses! — but does assume some familiarity with RISC OS and BBC BASIC.

We’ll start with some things you can do when drawing rectangles. While writing the Solar application I wanted to provide some better backgrounds for the graphs. A plain background is easy, of course: just use RECTANGLE FILL with some appropriate colour, like this.

GCOL 0,&ff,&dd,&ff RECTANGLE FILL 0,0,256

…which draws a 256×256 square at the graphics origin. More generally, RECTANGLE accepts both width and height, which can be negative.

First up, we’ll generate a blend between two colours, from the bottom to the top of the rectangle. We’re assuming a full-colour display here, which is necessary for displaying blends with any fidelity. The Solar application graphs are built up by redirecting all graphics output to a sprite, then letting the Wimp handle displaying them on screen; this takes care of any mismatch between the screen and graph colour depths.
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