ÿÿf1THE BLOBS INVASIONÿÿf0

ò      -------------- Ohh mighty blob - overlapping heaven -------------


We've seen them quite a few times over the past few years, I've used them
myself far too many times.
If you dont know what I mean, keep reading.

Blobs, what are they?
Basicly, they consists of objects that overlap and seem to affect eachother
in a sort of "slime" way. It usually comes in the shape of spheres, sliming
together. To get the theory, read Tats article on blobs in maggie24, here I
will explain how they are actually done and why they are so lovely.



óThe way blobs work
ò-------------------------------

Its all about overlapping and setting a nice palette really. Overlapping
means that for each pixel we calculate how much each blob "contributes".
If we have three blobs we simple add each blobs contribution and use a
lookuptable to find the correct color to use.
This means we are not dealing with the final pixelcolor until all blobs have
been calculated. I say calculated but it doesnt really describe it very good
since all we do is a couple of additions.

To do this we need a picture of a ball, this can be calculated in the code
itself of drawn in an artpackage. Since I am using a falcon I prefer Apex
Media to draw it. The centre of the ball will have the brightest color and
thus needs to have the highest value, say 63. Then the rest of the ball
is filled with a value depending on the pixeldistance from the centre of the
ball. Seems a bit weird maybe but we need a nice gradient over the ball,
all values ranging from 0 to 63 (in this case). Its convinient to use
256colormode on the falcon and color "blue" since this will give us the values
needed.
The size of this ball (or whichever shape) shouldnt be made too big since we
dont want to cover the whole screen when overlapping.
The actual picturesize should be the same as the final output screen.


We also need a palette, this one should hold the final colors of each pixel.
This can range from black to white, with say, 64 entries.


Next thing is the implementation, from here on I will assume we are using a
falcon and truecolormode.
In this example I use a 160*100pixel TC screen and two blobs.


    section text
   
mainloop
    lea.l     blob_data,a0             ; our blobpicture, 160*100 pixels big
    move.l    a0,a1
    add.l     blob_offset1,a0          ; change these two to move the blobs.
    add.l     blob_offset2,a1          ; 
    move.l    screen,a2
    lea.l     blob_palette,a3          ; the TCpalette described above.

    move.w    #160*100-1,d7

blobloop
    move.w    (a0)+,d0                 ; get contribution from blob1
    add.w     (a1)+,d0                 ; add the contribution from blob2
    ..
    [here is the place to add more add.w if you want more than two blobs]

    move.w    (a3,d0.w*2),(a3)+        ; use the lookuptable (palette) to
                                       ; find correct color and set the
                                       ; pixel.
    dbra      d7,blobloop


    jmp       mainloop

    section data

blob_data
    incbin    blobpic.bin              ; the blobdata
    

As you can see, we go thru the screen, plotting every pixel, this means we
dont have to clear the screen.

There are two obvious problems with this implementation:

- Changing the offsets of the blobdata means we will end up reading off the
  actual data so we will have to add some space above and behind the blobdata.
  This also means that the blobs will "wrap" horizontally.

- Overflowing the color palette, this is best solved by adding enough words of
  the topmost color after the palette.


óOptimization
ò-------------------------------

Can it be made faster? Yep!
As always there are as many optimizationtricks as there are coders but I will
show you a couple here.

- Fill the cache. As you can see the innerloop is really small and thus can be
  unrolled a few times to fill up the cache.
  Like this:

    move.w    #160*100/8-1,d7

blobloop
    REPT      8
    move.w    (a0)+,d0                 ; get contribution from blob1
    add.w     (a1)+,d0                 ; add the contribution from blob2
    ..
    [here is the place to add more add.w if you want more than two blobs]

    move.w    (a3,d0.w*2),(a3)+        ; use the lookuptable (palette) to
                                       ; find correct color and set the
                                       ; pixel.
    ENDR
    dbra      d7,blobloop

  I've unrolled the loop eight 8 and divided d7 by 8. This will fill the
  chache some more (not 100% though). Experiment!


- Using movem to read more than one entry at the time. Read maybe 4 entris
  at the time, this will save some bustime.

- Make each blobdataentry only one byte and read more of them that way. We
  dont really need to have 16bit just to store an offset in a palette.



This is the easy way to make blobs, there are more ways ofcourse, including
having pictures in the background that blends with the blobs etc.
If you take a look at our 'Hardcore' demo from SV2000 you will see some blobs
in the last part, these blend with the background, even though itis in
greyscales.
There are numerous ways to use blobs and overlapping in general, hopefully
you will be able to watch some more advanced ones in the near future.

ò/Fredrik Egeberg (deez) - fredrike@smartdog.co.uk - deez@atari.org

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mind-design 2000
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