Python half precision floating point
@david3jones
I’ve been working with python for a while to create some addons for Blender which allowing me to import and export various formats that aren’t supported by default. One of these file formats I’ve been working with compresses the vertices/uv/tangent/normal data to reduce the space used. This data is usually stored as 32bit floating point values but in this case it is reduced to 16bit floating point or Half Precision floating point. Halfing the final size of this data. I had a hard time recreating this with python, I’d never used python before starting using Blender so my understanding was still fairly new and most examples online I failed to understand or struggled to implement. Finally I cobbled together something that works from different examples online and got the results I wanted. Like most things on my site I felt the need to share this with anyone else who may be trying a similar thing. Here is my class in python
class Float16Compressor:
def __init__(self):
self.temp = 0
def compress(self,float32):
F16_EXPONENT_BITS = 0x1F
F16_EXPONENT_SHIFT = 10
F16_EXPONENT_BIAS = 15
F16_MANTISSA_BITS = 0x3ff
F16_MANTISSA_SHIFT = (23 - F16_EXPONENT_SHIFT)
F16_MAX_EXPONENT = (F16_EXPONENT_BITS << F16_EXPONENT_SHIFT)
a = struct.pack('>f',float32)
b = binascii.hexlify(a)
f32 = int(b,16)
f16 = 0
sign = (f32 >> 16) & 0x8000
exponent = ((f32 >> 23) & 0xff) - 127
mantissa = f32 & 0x007fffff
if exponent == 128:
f16 = sign | F16_MAX_EXPONENT
if mantissa:
f16 |= (mantissa & F16_MANTISSA_BITS)
elif exponent > 15:
f16 = sign | F16_MAX_EXPONENT
elif exponent > -15:
exponent += F16_EXPONENT_BIAS
mantissa >>= F16_MANTISSA_SHIFT
f16 = sign | exponent << F16_EXPONENT_SHIFT | mantissa
else:
f16 = sign
return f16
def decompress(self,float16):
s = int((float16 >> 15) & 0x00000001) # sign
e = int((float16 >> 10) & 0x0000001f) # exponent
f = int(float16 & 0x000003ff) # fraction
if e == 0:
if f == 0:
return int(s << 31)
else:
while not (f & 0x00000400):
f = f << 1
e -= 1
e += 1
f &= ~0x00000400
#print(s,e,f)
elif e == 31:
if f == 0:
return int((s << 31) | 0x7f800000)
else:
return int((s << 31) | 0x7f800000 | (f << 13))
e = e + (127 -15)
f = f << 13
return int((s << 31) | (e << 23) | f)
and here is how to use it
#read half float from file and print float
h = struct.unpack(">H",file.read(struct.calcsize(">H")))[0]
fcomp = Float16Compressor()
temp = fcomp.decompress(h)
str = struct.pack('I',temp)
f = struct.unpack('f',str)[0]
print(f)
#write half float to file from float
fcomp = Float16Compressor()
f16 = fcomp.compress(float32)
file.write(struct.pack(">H",f16))
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