# coding: utf-8
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# Copyright (c) 2016-2020 European Synchrotron Radiation Facility
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"""This module provides basic writing Mulitple-image Network Graphics files.
It only supports RGB888 images of the same shape stored as
MNG-VLC (very low complexity) format.
"""
from __future__ import absolute_import
__authors__ = ["T. Vincent"]
__license__ = "MIT"
__date__ = "15/12/2016"
import logging
import struct
import zlib
import numpy
_logger = logging.getLogger(__name__)
def _png_chunk(name, data):
"""Return a PNG chunk
:param str name: Chunk type
:param byte data: Chunk payload
"""
length = struct.pack('>I', len(data))
name = [char.encode('ascii') for char in name]
chunk = struct.pack('cccc', *name) + data
crc = struct.pack('>I', zlib.crc32(chunk) & 0xffffffff)
return length + chunk + crc
[docs]def convert(images, nb_images=0, fps=25):
"""Convert RGB images to MNG-VLC format.
See http://www.libpng.org/pub/mng/spec/
See http://www.libpng.org/pub/png/book/
See http://www.libpng.org/pub/png/spec/1.2/
:param images: iterator of RGB888 images
:type images: iterator of numpy.ndarray of dimension 3
:param int nb_images: The number of images indicated in the MNG header
:param int fps: The frame rate indicated in the MNG header
:return: An iterator of MNG chunks as bytes
"""
first_image = True
for image in images:
if first_image:
first_image = False
height, width = image.shape[:2]
# MNG signature
yield b'\x8aMNG\r\n\x1a\n'
# MHDR chunk: File header
yield _png_chunk('MHDR', struct.pack(
">IIIIIII",
width,
height,
fps, # ticks
nb_images + 1, # layer count
nb_images, # frame count
nb_images, # play time
1)) # profile: MNG-VLC no alpha: only least significant bit 1
assert image.shape == (height, width, 3)
assert image.dtype == numpy.dtype('uint8')
# IHDR chunk: Image header
depth = 8 # 8 bit per channel
color_type = 2 # 'truecolor' = RGB
interlace = 0 # No
yield _png_chunk('IHDR', struct.pack(">IIBBBBB",
width,
height,
depth,
color_type,
0, 0, interlace))
# Add filter 'None' before each scanline
prepared_data = b'\x00' + b'\x00'.join(
line.tobytes() for line in image) # TODO optimize that
compressed_data = zlib.compress(prepared_data, 8)
# IDAT chunk: Payload
yield _png_chunk('IDAT', compressed_data)
# IEND chunk: Image footer
yield _png_chunk('IEND', b'')
# MEND chunk: footer
yield _png_chunk('MEND', b'')
