# Python float 32bit to half float 16bit

I'm trying to write a 3D model exporter addon for Blender in Python and need some help.

The spec of the 3D format uses some compression on the vertices, there is a vertex buffer that contains vertices as 32bit floats. When this is compressed it is stored as 16bit float or half precision float.

I've seen lots of examples online of code in C to convert a 32bit float to 16bit float but not much luck with Python.

I was wondering if anyone here can help?

So far I've attempted to take some of the code here conversion of float and put it into Python. I managed to get this working and testing with a triangulated cube rendered only half the cube.

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)

for byte in self._byteBuffer:
f32 = int(byte)
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
file.write(struct.pack(">H",f16))


After some trial and error and trying various different things I finally got something that works for me. I hope this helps someone else - I've posted the code I used on my website. Quoted here:

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
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))

h = ((f32>>16)&0x8000)|((((f32&0x7f800000)-0x38000000)>>13)&0x7c00)|((f32>>13)&0x03ff)


The above should work assuming that none of the below "fatal flaws" are fatal in your situation: "The above expression suffers from a few fatal flaws, however. It doesn't handle zero, Infinity, NaN, or small float numbers which are only representable as subnormal half-floats" Otherwise I would make the table as demonstrated in near the bottom of the paper.

Source

• Thanks i tried this out with the table version but the render result still appears as half the cube. I've been told by the guys that wrote the file spec that the "Float16Compressor" answer here stackoverflow.com/questions/1659440/… should be a good way to do it. Would you say that this Float16Compressor is the same as the table version at your source? – davidejones Apr 25 '12 at 18:45

For some reason none of the other answers worked for me. So I tried to convert the C solution given here. I used the ctypes module to emulate C unions and types. I have no idea how efficient this is, or even if it is crossplatform, but it is working for me. If I haven't made any mistake it should be fully IEEE compliant and branchless.

import ctypes

class Bits(ctypes.Union):
_fields_ = [('f', ctypes.c_float), ('si', ctypes.c_int), ('ui', ctypes.c_uint)]

def compress(f):
SIGN_N = 0x80000000
SHIFT_SIGN = 16
MUL_N = 0x52000000
MIN_N = 0x38800000
INF_N = 0x7F800000
MAX_N = 0x477FE000
SHIFT = 13
INF_C = INF_N >> SHIFT
NAN_N = (INF_C + 1) << SHIFT
MIN_C = MIN_N >> SHIFT
MAX_C = MAX_N >> SHIFT
SUB_C = 0x003FF
MAX_D = INF_C - MAX_C - 1
MIN_D = MIN_C - SUB_C - 1

v = Bits(f=f)
s = Bits(si=MUL_N)
sign = v.si & SIGN_N
v.si ^= sign
sign >>= SHIFT_SIGN
s.f *= v.f
v.si ^= (s.si ^ v.si) & -(MIN_N > v.si)
v.si ^= (INF_N ^ v.si) & -((INF_N > v.si) & (v.si > MAX_N))
v.si ^= (NAN_N ^ v.si) & -((NAN_N > v.si) & (v.si > INF_N))
v.ui >>= SHIFT
v.si ^= ((v.si - MAX_D) ^ v.si) & -(v.si > MAX_C)
v.si ^= ((v.si - MIN_D) ^ v.si) & -(v.si > SUB_C)

return v.ui | sign

• Nice this looks great – davidejones May 11 '17 at 16:34