ROCoding #2: Circles
Tuesday, January 20, 2026
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:
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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:
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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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