Re: float to int (kinda OT)

• Subject: Re: float to int (kinda OT)
• From: Chris Cox <email@hidden>
• Date: Thu, 21 Oct 2004 17:58:18 -0700
• Delivered-to: email@hidden
• Delivered-to: email@hidden

On Oct 21, 2004, at 3:09 PM, Chris Cox wrote:

	r = (a * r) >> 8;
	g = (a * g) >> 8;
	b = (a * b) >> 8;

Ouch - No.
255 * 255 / 256 = 254
Even rounding won't fix that mistake.

When you multiply 8 bit components, you have to divide by 255.
Fortunately, there are some tricks to do that without using an actual divide.

Wow, did I ever get hammered on that little "optimization". For those watching at home, the optimization Chris is talking about is probably this one:

tmp = alpha * red;
remainder256 = tmp & 0xFF; // get alpha * red (mod 256)
dividend = tmp >> 8; // compute alpha * red / 256
remainder255 = dividend + remainder256; // compute alpha * red (mod 255)
dividend += ((remainder255 >= (255 + 128)) & 1) + ((remainder255 >= 128) & 1); // add 0, 1, 2 depending on how large the remainder (mod 255) is

This is provably correct and even rounds correctly. And yes, it would be much, much easier in AltiVec.

No, there are faster ways of doing it, without branches.

Chris
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