Code critique please
Peter Otten
__peter__ at web.de
Thu Apr 9 06:26:24 EDT 2015
kai.peters at gmail.com wrote:
> I just wrote this bit (coming from Pascal)
> if (((xdim / 8) * ydim) + header) <> filesize:
Yeah, either Pascal or Barry Warsaw is your uncle ;)
https://www.python.org/dev/peps/pep-0401/
> and am wondering how seasoned
> Python programmers would have done the same? Anything terribly non-python?
> def RenderByte(draw, byte, x, y):
Please read PEP 8 for naming conventions etc.
The function at the core of your code
> def RenderByte(draw, byte, x, y):
>
> blist = list(bin(byte).lstrip('0b')) # turn byte into list with 8
> elements,
> c = 0 # each representing one bit
> for bit in blist:
> if bit:
> draw.point((x + c, y), fcolor)
>
> c += 1
> return
can be fixed/micro-optimised, and if I were to do that I'd precreate a
lookup table that maps every byte (i. e. value in the range 0...255) to a
sequence of offsets
lookup = [
(), # for b"\x00" the inner loop is empty
(0),
(1),
(0, 1),
...
(0, 1, 2, 3, 4, 5, 6, 7), # for b"\xFF" the inner loop comprises
# all 8 bits
]
or to a 8x1 image, but my idea of a "seasoned Python programmer" will try to
keep the big picture in mind -- and that is that handling individual
bits/pixels in Python is inefficient. Therefore many libraries tend to offer
an alternative that is both easier to use and faster.
In this case that's
img = Image.frombytes("1", (width, height), data)
and with the extra comfort of reading the size from the EPD file
from PIL import Image
import struct
EPDFILE = "tmp.epd"
PNGFILE = "tmp.png"
class EPDError(Exception):
pass
with open(EPDFILE, "rb") as f:
header = f.read(16)
width, height, colordepth = struct.unpack("<HHb", header[1:6])
if colordepth != 1:
raise EPDError("Unsupported color depth {}".format(colordepth))
data = f.read()
datasize = width // 8 * height
if len(data) != datasize:
raise EPDError(
"Inconsistent pixel data size. "
"Expected {} but got {} bytes".format(
datasize, len(data)))
img = Image.frombytes("1", (width, height), data)
img.save(PNGFILE)
I'm assuming the header follows the format spec given in this PDF:
http://www.pervasivedisplays.com/_literature_112691/Developer_Guide_for_Demo_Kit
PS: I thought that Image.frombytes() was already pointed out to you, but I
failed to find it in the archives. So there.
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