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Forums > C64 Coding > Simple bilinear interpolation in assembler
2018-05-08 16:24

Registered: Apr 2012
Posts: 26
Simple bilinear interpolation in assembler


for some reason, with increasing age, i am no longer interested in spending much time on re-inventing the wheel.

Therefore, i'd like to ask if any of you wizards perhaps know of a close solution to this problem:

Lets say you have a "heatmap" in a matrix of 20x10 cells.

Does anyone perhaps already have a fast and simple routine for interpolating all the values within these cells, so that when you have some cells with very high values and some cells with very low values, that more or less smooth transitions (image blur) can be achieved in very few rasterlines?

Google just spat out hardcore math for me, which i feel unable to wrap my head around, when attempted to translate to a c64 assembler solution. (I code in assembly, directly, no c++ or the like, pretty please).

best regards
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2018-05-09 19:23

Registered: Jul 2007
Posts: 353
Quoting ChristopherJam
Quoting soci
It's ROR and not LSR, right? Otherwise the usable value range is half.

Argh! Yes, you're right of course. Same applies to my code in comment #6, I meant to use ROR there, too.

Thanks for catching that.

Although, if half the value range is acceptable, then one could use ASR #$fe to clear carry in preparation for the next computation.
2018-05-09 19:25

Registered: Apr 2012
Posts: 26
i'd rather have some sort of gamma curve that i can adjust via table. but thanks for clearing up rol vs asl and lsr
2018-05-09 21:35

Registered: Aug 2004
Posts: 786
Nice approach :)

Small optimisation suggestion, which would also give you a little more control:

I notice you have snippets like
    lda gamma,y       ; decay
    sta datafield+2,x ; store
    lda gamma,y       ; decay more

May I suggest instead:
    lda gamma,y       ; decay
    sta datafield+2,x ; store
    lda gamma2,y      ; decay more

where gamma2=gamma[gamma[i]], or some refinement thereof.

You can also drop all but the first CLC if your gamma/gamma2 values are all under 128.
2018-05-09 22:58

Registered: Apr 2012
Posts: 26
that is actually a very nice hint, using stairs of tables.

but i just implemented my first attempt and obviously it just dampens all the values instead of mixing them, so as a conclusion:

after the first load of a cell, it cannot just be destructively set to a fixed translation value without being compared to its neighbours.
the reduction value must be related to the neighbouring cells.

f000 --> destructive --> 8421 - ok
f200 --> destructive --> 8531 - not ok, must be A531
(just arbitrary example values)

workin on that.

by the way, this totally out of the blue, super far fetched noise generator which i utilize for creating data to be blurred, works surprisingly well (IN VICE!):

drawdom ldx #$00
lda $dc01
eor $d800
sta $db00
eor $db00

eor $0400,x
adc $d800,x

sta $0400,x

bne paintle

2018-05-09 23:19

Registered: Apr 2012
Posts: 26
[update 2 on noise generator: works surprisingly well on a real c64, too. actually best performance i had with bus noise to this day. duh. (it needs to run twice for good entropy.)
2018-05-11 00:05

Registered: Dec 2001
Posts: 8520
i strongly advice against using open i/o space for "noise". it is _not_ noise in the first place (what you are reading is what was left on the bus by the previous vic fetch) and on some C64s you will see just zeros or $ff.
2018-05-11 11:15

Registered: Apr 2012
Posts: 26
Youre probably right, i'd also confirm that a mechanism like this should not be used without prior testing on several "chipsets", i must admit that i am quite obsessed with that method, though :-) I only use it for testing purposes, because i am too lazy to setup the SID noise method (if i remember correctly that that was possible).
2018-05-18 23:19

Registered: Apr 2012
Posts: 26
okay. this really seems to be the fastest way:
do_blur clc
lda datafield,x
adc datafield+1,x
sta datafield,x

cpx #$00
bne do_blur

demo: http://dl.dataelephant.net/blur.zip (press space to...)

this is a 2 iterations blur, only thing i am going to add is looking back n pixels plus avoiding backshift.

and yes, i must really learn to think less wishful. took me quite some time to grasp the simpleness of the problem. too many images in my head.

ps: if anyone hints me towards formatting source code nicely in this csdb thing, i will happily comply.
2018-05-18 23:51

Registered: Apr 2012
Posts: 26
i am pretty satisfied already:


4 iterations simple 1 cell blur. i am impressed how easy this stuff is, compared to trying to imagine it.

(prg+src inside zip)
2018-05-19 08:03

Registered: Aug 2004
Posts: 786
Quoting wertstahl
ps: if anyone hints me towards formatting source code nicely in this csdb thing, i will happily comply.

Just follow the Read more link on the comment entry form.

But in short, wrap your code segments in [ code]/[ /code] (only without the spaces after the "["s)
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