A shot in the dark at how this all works!

Today I decided to implement bullets which meant going back to RE some of the code that calculates the bullet trajectory. TBH it did (and still does) my head in a bit; I understand what it's trying to do and roughly how it's doing it, but some of the finer details still escape me. That's not absolutely critical as long as I can replicate the effect of each instruction (or routine). I freely admit that I had to resort to this approach in a couple of low-level routines in Knight Lore, for example.

The lowest level routine takes the absolute values of a (dX,dY) pair between source and target and calculates the ratio of the smaller to the larger; IOW the tan of the angle in the 1st octant. That gets scaled from 0.0-1.0 (0-256) down to 0-32 which then indexes into a table that in turn returns another value (0-32) from a non-linear but monotonically non-decreasing distribution. Looking at a graph of this distribution (below) it doesn't make a lot of sense to me atm*. Then it adjusts this value for 2nd octant if applicable by examining whether dX or dY was larger, and finally adjusts for the correct quadrant by examing the signs of dX & dY in turn.

The function that is applied to the tan of an angle in an octant

* Does this function correct for pixel aspect ratio, so that bullets splay out at regular angles on the screen (and/or at the (purposeful) expense of accuracy)?

That final value is scaled (again) from 0-31, and used as an index into a table of (dX,dY) pairs for bullet trajectories. IOW, objects can fire at 32 different angles.

It has all been transcoded but not debugged; the trajectories are definitely not correct and the ground-based objects seem to almost always shoot in a SW direction regardless of where the Solvalou is located on the screen.

That Garu Derota is firing way too quickly -
Xevious is not supposed to be a bullet-hell!!!

Some objects also seem to be firing a lot more frequently than they should, but that's another issue.

On the note of firing frequency and AI: none of the gound-based objects' behaviour (for the objects I've completed at least) is contingent on the difficulty level set by the dipswitches, nor the adaptive AI logic that depends on the player's skill. The dipswitches and AI level only seem to affect the flying enemies.

When the RE is complete, I'll do a comprehensive analysis of the difficulty settings and the AI logic.

Until then, I need to debug the bullets and the firing logic of a few objects it seems...