在Java中添加浮点值会减慢512

Question

This line is executing in a loop:

shape.setPosX(shape.getPosX() + (.005f * (float) getDelta()));

getDelta is the milliseconds passed since the last loop as a double, usually averaging at ~0.001.

setPosX accepts and stores the float value.

getPosX , as you can guess, returns the float value

When the shape is drawn I see it steadily move across the screen but once X hits 512 it slows down by what seems like a factor of 10. I can reproduce it 100% of the time so it's a pretty big blocker.

What could be going on here?

Answer 1

The problem is that the float you are using have not enough number of digits of precision to represent the number. You can check this by telling Java to determine if ( 512f + 0.005f * 0.001f == 512f ) (it is true .)

In 32-bit floating point numbers, the least significant bit for the mantissa for values between 256 and 512 is around .00003 which happens to be enough for your getDelta() to push over (which suggests that your assumption that the value averages at 0.001f might be wrong). The least significant bit for the mantissa for values between 512 and 1024 doubles to .00006 (since the exponent part is one more). Your getDelta returns something that rounds up around once every 5 time or so, thus the speed is slowed ten-fold.

You can play around with float s here .

The solution to this is to make your number a double .

Fun trivia: floating point arithmetics is causes Minecraft to behave funny in high coordinates. The effects are described in this page , although the actual bug might just be integer overflow.