Starfield in Blitz Basic - helpful critique sought

[Anding]

This probably seems totally basic and in the age of the dinosaurs to most of you, but I'm refreshing my Amiga coding by trying to rework some classic algorithms from scratch in Blitz Basic 2. The idea of the exercise, of course, is to learn rather than produce something objectively useful

This routine is for a starfield, except that it is rather slow! I'm sure I'm missing some tricks. I'd be very grateful for any helpful critique or suggestions.

Many thanks indeed

Anding

; Starfield research

DEFTYPE .w ; the default variable type will be 16 bit integers
; to speed up calculations

; Establish the initial starfield on a
; 3D coordinate system with the centre of the screen at (0,0,0)
; and project these stars onto the screen
Dim x(50), y(50), z(50), h(50) ; 3D coordinate and colour of each star
Dim sx(50), sy(50) ; screen position of each star
For i = 1 To 50
h(i) = Rnd*7+1 ; colour
x(i) = Rnd*320-160: y(i) = Rnd*256-128: z(i) = Rnd*512 ; 3D position
; simple projection that scales the star's 3D x and y positions onto
; screen x and y positions according to its depth behind the screen
sx(i) = (x(i) * 512/z(i)) + 160: sy(i) = (y(i) * 512/z(i)) + 128
Next

; Establish the display in lo-res with 3 colours
BLITZ ; BLITZ mode direct access to the hardware
BitMap 0,320,256,3 ; Allocate memory for the bitmap
Slice 0,44,3 ; Set-up the copper list
Use Slice 0 ; Attach the bitmap
Show 0,0,0 ; to the slice

main
; Plot the stars
For i = 1 To 50
;Clear the last position occupied by the star
Plot sx(i),sy(i),0
;Move the stars towards the screen (to simulate forward velocity)
z(i) = z(i) -10
If z(i) <1 ;If the star passes behind the screen, renew it
x(i) = Rnd*320-160: y(i) = Rnd*256-128: z(i) = 512
EndIf
; calculate screen positions using the simple projection as before
sx(i) = (x(i) * 512/z(i)) +160: sy(i) = (y(i) * 512/z(i)) +128
Plot sx(i),sy(i),h(i)
Next

; wait for a mouse button to exit and continue in a loop until then
If Joyb(0)>0 Then End
Goto main

[Codetapper]

I don't know how well Blitz optimises things, but I see a few things straight away:

1. You are calculating 512/z(i) twice in the main loop. Best to calculate that once and buffer the result for the second time, as div is a very expensive operation.

2. If the x co-ordinate is not on the screen, there is no need to calculate the y co-ordinate (or call the Plot() function). Ditto if the y co-ordinate is not on the screen, the call to Plot() might be wasting some CPU cycles. (Then again, Plot() might check the co-ordinates are on-screen and do nothing itself, I have never really used blitz basic so have no idea sorry!)

3. You might see some star flickering as you are not clearing all (old) stars before plotting all the new ones. If a new star position ends up being where an old one was (and a lower position in the array), the new position may well get erased.

Good luck!

[ChrisH]

To follow-on from Codetapper's post...

"Rnd" is likely quite expensive, and is being called twice per loop. With a few tricks you could reduce that to one. And with a LUT (look up table) you could get rid of it entirely.

Not sure about 68k, but multiplications may be slightly expensive (nothing like division though), so if you can make them part of the LUT then so much the better.

It's hard to read the code without indentation, but why do you do "x(i) * 512" & "y(i) * 512" every loop? Why not just do the multiplication once, before entering the loop?

[AlfaRomeo]

Why do you don´t change BlitzBasic2 by AmiBlitz3??.. I read somewhere that it is a great compiler
Someone knows where to find some code to learn BlitzBasic like foruns?

[prowler]

Originally Posted by AlfaRomeo

Someone knows where to find some code to learn BlitzBasic like foruns?

Hi AlfaRomeo,

I've had a look in my Amiga archives and found the following files which may help you in your quest to learn Blitz Basic:

Blitz Support Suite ADFs (3) - RAR archive [1.13MB]
Blitz Basic 2.1 ADFs (3) - RAR archive [1.41MB]
Ultimate Blitz Basic 2.1 CD ISO image - RAR archive [8.61MB]
Blitz Basic 2 Examples ADFs (4) - ZIP archive [1.13MB]

Let me know if you would find any or all of these files useful and I'll upload them for you.

prowler

[Anding]

These are great suggestions and have moved me on quite a bit. Thanks a lot everyone for the helpful ideas.

Merry Christmas and Happy New Year to all Amiga Coders!

[syphus]

Thanks for posting your source, Anding! I've not been having much luck finding good snippets to learn from, but this sort of thing is great for people like me, who are just starting out. It helped me to get stuff displayed on screen for the first time

[KevG]

You can get rid of the 3d calculation completely inside the loop and things would speed up really good!

To do this, simply create an array of x & y values for each star. Calculate all the valid x & y values from 3D space and put them in their own list before entering the main loop. Now all you have to do now is read the lists for each star and plot the x & y values.

This is an ultra fast way of doing it because you are just reading from a list and not performing any calculations at all!

Hope that helps :-)

Kev G.

[AlfaRomeo]

Originally Posted by prowler

Let me know if you would find any or all of these files useful and I'll upload them for you.

Hi prowler,
Thanks for your help and sorry for the delay in my answer but I was out during christmas/new year and so I only saw your message today
I think the BB2 examples ADFs may be useful to me if you could upload to the zone

Thanks in advance

[prowler]

Originally Posted by AlfaRomeo

I think the BB2 examples ADFs may be useful to me if you could upload to the zone

Hi Alfa,

They're in the Zone for you.

Have fun!

prowler

[AlfaRomeo]

Thanks prowler, you´re the best