Tricks to blit noise or corrupted data on purpose

[KONEY]

I'm calling all Amiga HW experts to share any known trick which would allow the blitter to copy on purpose random noise or corrupted data to a destination.


Borderline techniques also accepted and welcome. For example something like updating a register just after the blitter operation has started and without checking DMAF_BLTDONE bit, would be interesting



Thanks!

[DanScott]

That's a bit of a broad question.

An old trick...Random noise can be generated by blitter filling an area of memory, and writing (XOR or OR) some random data in there too.

Ie.. if you have a screen buffer, and draw a line from top (random x start position) to bottom (random x end position), and fill, then keep repeating this each frame, you end up with what looks like TV static

[KONEY]

The draw trick sounds interesting, I'll give it a try! Thanks!

[paraj]

There is at least one demo (from TEK IIRC) that I think just lets one bitplane pointer run wild to achieve noisy effect for free. Doesn't fit your requirements but food for thought.

[koobo]

Not exactly noise but maybe worth a mention. I remember tinkering with some blits where the input was 16 white pixels on the top left of the screen, back in the day.

I don't remember what exactly I was after or what the initial parameters were, but after turning on the line mode and running it, the screen was filled with sierpinski triangles. I almost fell off the chair from amazement! I still have the source somewhere.

[KONEY]

Would be nice if you share the source

[Cyprian]

Originally Posted by paraj

There is at least one demo (from TEK IIRC) that I think just lets one bitplane pointer run wild to achieve noisy effect for free. Doesn't fit your requirements but food for thought.

do you mean this one?
[ Show youtube player ]

[mc6809e]

Crazy idea:
Set up line mode to draw a vertical line from top to bottom.

Put random in BLTADAT. Put random in BLTBDAT. Set minterms for D= A xor B xor C.

Use CPU to change BLTADAT and BLTBDAT while line draw is running.

Do this 20 times to cover each vertical strip lowres.

Idea is that blitter will be rotating texture in BDAT and mixing with A and C giving some apparent randomness while CPU is working to generate new random seed values.

Optimization: set C and D mod to two and set line length to 780. Blitter thinks it's moving down one line at a time but instead is moving sequentially through bp. Only 8 line draws required for a bp.

Another idea for black and white noise: use three interleaved bps. Now draw your 20 strips with line length of 936. Use a color pallet filled with black and white values like WWBWBBWB. Now every three writes by blitter picks random indexes for 16 pixels. Gray values might be even better to produce retro TV look.

[paraj]

Originally Posted by koobo

Not exactly noise but maybe worth a mention. I remember tinkering with some blits where the input was 16 white pixels on the top left of the screen, back in the day.

I don't remember what exactly I was after or what the initial parameters were, but after turning on the line mode and running it, the screen was filled with sierpinski triangles. I almost fell off the chair from amazement! I still have the source somewhere.

Yes, if start just with a couple of bits set and do what DanScott said you will get these patterns. Very nice


bltcon0=BC0F_SRCA | BC0F_SRCB | BC0F_DEST | 0x3c, bltcon1 = BC1F_BLITREVERSE | BC1F_FILL_XOR, bltapt pointing to last row, b/dpt pointing to line above

Originally Posted by Cyprian

do you mean this one?
[ Show youtube player ]

Yes, exactly.

[KONEY]

As crazy as I was hoping for!

Originally Posted by mc6809e

Crazy idea:
Set up line mode to draw a vertical line from top to bottom.
Put random in BLTADAT. Put random in BLTBDAT. Set minterms for D= A xor B xor C.

[KONEY]

Nice one! In fact I've already some random bpl pointers trick in place

Originally Posted by paraj

There is at least one demo (from TEK IIRC) that I think just lets one bitplane pointer run wild to achieve noisy effect for free. Doesn't fit your requirements but food for thought.

[mc6809e]

Originally Posted by KONEY

As crazy as I was hoping for!

Hope you get it working!

Noticed you use octamed. You trying to generate noise for percussive sounds?

[KONEY]

Actually no, but if you're suggesting to feed real-time noise data into a SynthSound waveform... it's an idea

[pandy71]

Probably even moderately compressed data has sufficient level of entropy so they can be used as noise source. I assume you are using compression for exe/data so reusing compressed data as noise source can be option.