143 lines
3.5 KiB
Python
143 lines
3.5 KiB
Python
"""Read GIF control data.
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http://www.w3.org/Graphics/GIF/spec-gif89a.txt
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"""
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import struct
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from pyglet.image.codecs import ImageDecodeException
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class GIFStream:
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def __init__(self):
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self.images = []
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class GIFImage:
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delay = None
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class GraphicsScope:
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delay = None
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# Appendix A.
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LABEL_EXTENSION_INTRODUCER = 0x21
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LABEL_GRAPHIC_CONTROL_EXTENSION = 0xf9
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LABEL_IMAGE_DESCRIPTOR = 0x2c
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LABEL_TRAILER = 0x3b
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def unpack(fmt, file):
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size = struct.calcsize(fmt)
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data = file.read(size)
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if len(data) < size:
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raise ImageDecodeException('Unexpected EOF')
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return struct.unpack(fmt, data)
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def read_byte(file):
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data = file.read(1)
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if not len(data):
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raise ImageDecodeException('Unexpected EOF')
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return ord(data)
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def read(file):
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"""Read a GIF file stream.
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:rtype: GIFStream
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"""
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# 17. Header
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signature = file.read(3)
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version = file.read(3)
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if signature != b'GIF':
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raise ImageDecodeException('Not a GIF stream')
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stream = GIFStream()
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# 18. Logical screen descriptor
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(logical_screen_width,
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logical_screen_height,
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fields,
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background_color_index,
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pixel_aspect_ratio) = unpack('HHBBB', file)
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global_color_table_flag = fields & 0x80
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global_color_table_size = fields & 0x7
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# 19. Global color table
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if global_color_table_flag:
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global_color_table = file.read(6 << global_color_table_size)
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# <Data>*
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graphics_scope = GraphicsScope()
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block_type = read_byte(file)
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while block_type != LABEL_TRAILER:
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if block_type == LABEL_IMAGE_DESCRIPTOR:
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read_table_based_image(file, stream, graphics_scope)
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graphics_scope = GraphicsScope()
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elif block_type == LABEL_EXTENSION_INTRODUCER:
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extension_block_type = read_byte(file)
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if extension_block_type == LABEL_GRAPHIC_CONTROL_EXTENSION:
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read_graphic_control_extension(file, stream, graphics_scope)
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else:
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skip_data_sub_blocks(file)
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else:
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# Skip bytes until a valid start character is found
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print(block_type)
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pass
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block_type = read_byte(file)
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return stream
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def skip_data_sub_blocks(file):
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# 15. Data sub-blocks
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block_size = read_byte(file)
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while block_size != 0:
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data = file.read(block_size)
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block_size = read_byte(file)
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def read_table_based_image(file, stream, graphics_scope):
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gif_image = GIFImage()
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stream.images.append(gif_image)
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gif_image.delay = graphics_scope.delay
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# 20. Image descriptor
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(image_left_position,
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image_top_position,
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image_width,
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image_height,
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fields) = unpack('HHHHB', file)
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local_color_table_flag = fields & 0x80
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local_color_table_size = fields & 0x7
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# 21. Local color table
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if local_color_table_flag:
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local_color_table = file.read(6 << local_color_table_size)
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# 22. Table based image data
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lzw_code_size = file.read(1)
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skip_data_sub_blocks(file)
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def read_graphic_control_extension(file, stream, graphics_scope):
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# 23. Graphic control extension
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(block_size,
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fields,
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delay_time,
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transparent_color_index,
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terminator) = unpack('BBHBB', file)
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if block_size != 4:
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raise ImageDecodeException('Incorrect block size')
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if delay_time:
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# Follow Firefox/Mac behaviour: use 100ms delay for any delay
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# less than 10ms.
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if delay_time <= 1:
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delay_time = 10
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graphics_scope.delay = float(delay_time) / 100
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