PCSX2 Documentation/What's clamping? And why do we need it?: Difference between revisions

''Originally written by cottonvibes''

Basically there's something called the "The IEEE Standard for Floating-Point Arithmetic" (or IEEE 754), which defines floating point representation; and most systems abide by those rules (of course not our wonderful PS2).

There's 32-bit "Single Precision Floats" (commonly referred to as 'float'), 64-bit "Double Precision Floats" (commonly referred to as 'double'), and there's even 128-bit floats called Quads (most processors don't support these natively).

The one we're concerned about is the Single Precision Float, which is 32bits and has a format like this

S = Sign Bit (0 = Positive Number; 1 = Negative Number)

Example:

NaN + 500 = NaN (generally the same NaN, since it propagates)

INF - 19999999999 = INF

This is how the IEEE 754 standard defined this stuff (I honestly don't like it, but w/e)

For comparison of what your x86-32 CPU does VS the ps2 we have:

1/0 = NaN

sqrt (-4) = NaN

INF - 1000 = INF

1/0 = Greatest Positive Float Number (0x7ffffffff)

sqrt (-4) = sqrt(abs(-4)) = 2

INF - 1000 = Some Large Value - 100

And there's many more combinations, but the point is that the results are not the same between PC and the PS2.

Let me give you a quick example of something clamping can solve:

int main() {

     float value = INF;

         if (value <= 1)  { print("Hello World"); break; }

     } while (true);

If the above code was ran on the ps2, it will eventually print "Hello World", because INF is treated as a regular large number, and will continue to be divided by 2 until its less than 1.

Clamping can solve this problem like this:

int main() {

     float value = INF;

         if (value <= 1)  { print("Hello World"); break; }

     } while (true);

The clamping will convert the INF into a normal (ordered) number. Then that normal number can be divided by 2 repetitively until its less than 1. Then it will eventually satisfy (value less or equal 1), and print "Hello World".

Also clamping is SLOW if done a lot, so different clamp modes are faster than others ('None' is the fastest of course Razz)

sqrt(-4) = NaN; // on your PC

sqrt(-4) = 2; // on the ps2

In this case, we simulate the ps2's sqrt instruction by doing this

float ps2_sqrt(float value) {

     value = clamp(value); // Clamp Value if NaN or Inf to an ordered/normal number

     value = sqrt(value); // Get sqrt of now-positive value

     return value;

so:

ps2_sqrt(-4) = 2;

Now this post is getting really large, and I tried to simplify things a lot (which may have made things more confusing).

Anyways I hope some of this info was useful/interesting

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