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this article became kinda long... short version:
is there a way to do arithmetically correct parallel subtraction of bytes, ie X R G B are 4 bytes in a row in memory - i have to subtract certain values from each byte a lot of times in an arithmetically correct way... is there a way to do this without having to splice the RGB values out each single loop? like MMX quadword operations, just for bytes? :)
if not - can you tell me how to stretch the 4 bytes into 4 words to get MMX to work and how to operate (parallel subtract) with them, finally, how to get the resulting byte values out of it again?
thank you very much in advance!
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detailed version:

hi there, doing my recent prog, a picture manipulating problem, i ran into the following problem:
for my newest filter, a sharpen operator, i need to do a lot of subtraction:

X1 X2 X3
X4 O X5
X6 X7 X8

my formula for the O(riginal) pixel's new value is:
O = O + 1/16(O-X1) + 1/16(O-X2) + ... + 1/16(O-X8)
which can be simplified to:
O = O + 1/16(O*8 - X1 - X2 - ... - X8)

the *8 multiplication is a nice "shl 3" and /16 becomes "shr 4"... the problem is the many subtractions, because they have to be done for each colour seperately :(

RGB are stored in memory as consecutive bytes. right now, i have to extract and manipulate each color seperately which costs a lot of time and anger ;)
this looks like this:

... now i have a BlueTemp, GreenTemp, RedTemp variable, initialized with 8*the original RGB value i substract the single values from. this has to be done a whola lotta 8(!!!) times for each single pixel :/ splice original RGB, multiply by 8 and store in Temp variables then 8 times the same: each time splice RGB, subtract from Temp RGB values - recombinate RGB after 8 pixels.
this sucks and i want to do the subtraction parallel which would save a wholla lotta time of splicing and single subtraction.

<WHAT I REALLY NEEDED>
the problem is that 8*original value is in the worst case 8*255 = 2040, which means a byte is too less to store it :/ so what i would really need is a function that can do parallel subtraction of 3 BYTE values from 3 WORD values, all in memory :)
</WHAT I REALLY NEEDED>

can you point me the right direction? is the answer MMX? how
could i make use of it concerning my problem?
have you any ideas how i could make it better?

thank you really a lot, have a nice day,

Whoa! VirtualDub is open source!? Thanks for the link bitRAKE, this rocks!