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# Contributed by
# Dany Zatuchna (danzat at gmail)
""" Implementation of the following grammar for the GIF89a file format
<GIF Data Stream> ::= Header <Logical Screen> <Data>* Trailer
<Logical Screen> ::= Logical Screen Descriptor [Global Color Table]
<Data> ::= <Graphic Block> |
<Special-Purpose Block>
<Graphic Block> ::= [Graphic Control Extension] <Graphic-Rendering Block>
<Graphic-Rendering Block> ::= <Table-Based Image> |
Plain Text Extension
<Table-Based Image> ::= Image Descriptor [Local Color Table] Image Data
<Special-Purpose Block> ::= Application Extension |
Comment Extension
"""
from construct_legacy import *
import six
data_sub_block = Struct("data_sub_block",
ULInt8("size"),
String("data", lambda ctx: ctx["size"])
)
gif_logical_screen = Struct("logical_screen",
ULInt16("width"),
ULInt16("height"),
BitStruct("flags",
Bit("global_color_table"),
BitField("color_resolution", 3),
Bit("sort_flag"),
BitField("global_color_table_bpp", 3)
),
ULInt8("bgcolor_index"),
ULInt8("pixel_aspect_ratio"),
If(lambda ctx: ctx["flags"]["global_color_table"],
Array(lambda ctx: 2**(ctx["flags"]["global_color_table_bpp"] + 1),
Struct("palette",
ULInt8("R"),
ULInt8("G"),
ULInt8("B")
)
)
)
)
gif_header = Struct("gif_header",
Const(String("signature", 3), six.b("GIF")),
Const(String("version", 3), six.b("89a")),
)
application_extension = Struct("application_extension",
Const(ULInt8("block_size"), 11),
String("application_identifier", 8),
String("application_auth_code", 3),
data_sub_block,
ULInt8("block_terminator")
)
comment_extension = Struct("comment_extension",
data_sub_block,
ULInt8("block_terminator")
)
graphic_control_extension = Struct("graphic_control_extension",
Const(ULInt8("block_size"), 4),
BitStruct("flags",
BitField("reserved", 3),
BitField("disposal_method", 3),
Bit("user_input_flag"),
Bit("transparent_color_flag"),
),
ULInt16("delay"),
ULInt8("transparent_color_index"),
ULInt8("block_terminator")
)
plain_text_extension = Struct("plain_text_extension",
Const(ULInt8("block_size"), 12),
ULInt16("text_left"),
ULInt16("text_top"),
ULInt16("text_width"),
ULInt16("text_height"),
ULInt8("cell_width"),
ULInt8("cell_height"),
ULInt8("foreground_index"),
ULInt8("background_index"),
data_sub_block,
ULInt8("block_terminator")
)
extension = Struct("extension",
ULInt8("label"),
Switch("ext", lambda ctx: ctx["label"], {
0xFF: application_extension,
0xFE: comment_extension,
0xF9: graphic_control_extension,
0x01: plain_text_extension
})
)
image_descriptor = Struct("image_descriptor",
ULInt16("left"),
ULInt16("top"),
ULInt16("width"),
ULInt16("height"),
BitStruct("flags",
Bit("local_color_table"),
Bit("interlace"),
Bit("sort"),
BitField("reserved", 2),
BitField("local_color_table_bpp", 3)
),
If(lambda ctx: ctx["flags"]["local_color_table"],
Array(lambda ctx: 2**(ctx["flags"]["local_color_table_bpp"] + 1),
Struct("palette",
ULInt8("R"),
ULInt8("G"),
ULInt8("B")
)
)
),
ULInt8("LZW_minimum_code_size"),
RepeatUntil(lambda obj, ctx: obj.size == 0, data_sub_block)
)
gif_data = Struct("gif_data",
ULInt8("introducer"),
Switch("dat", lambda ctx: ctx["introducer"], {
0x21: extension,
0x2C: image_descriptor
})
)
gif_file = Struct("gif_file",
gif_header,
gif_logical_screen,
OptionalGreedyRange(gif_data),
#Const(ULInt8("trailer"), 0x3B)
)
if __name__ == "__main__":
f = open("../../../tests/sample.gif", "rb")
s = f.read()
f.close()
print(gif_file.parse(s))
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