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diff --git a/external/construct/formats/graphics/png.py b/external/construct/formats/graphics/png.py
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+"""
+Portable Network Graphics (PNG) file format
+Official spec: http://www.w3.org/TR/PNG
+
+Original code contributed by Robin Munn (rmunn at pobox dot com)
+(although the code has been extensively reorganized to meet Construct's
+coding conventions)
+"""
+from construct import *
+import six
+
+
+#===============================================================================
+# utils
+#===============================================================================
+def Coord(name, field=UBInt8):
+    return Struct(name,
+        field("x"),
+        field("y"),
+    )
+
+compression_method = Enum(UBInt8("compression_method"),
+    deflate = 0,
+    _default_ = Pass
+)
+
+
+#===============================================================================
+# 11.2.3: PLTE - Palette
+#===============================================================================
+plte_info = Struct("plte_info",
+    Value("num_entries", lambda ctx: ctx._.length / 3),
+    Array(lambda ctx: ctx.num_entries,
+        Struct("palette_entries",
+            UBInt8("red"),
+            UBInt8("green"),
+            UBInt8("blue"),
+        ),
+    ),
+)
+
+#===============================================================================
+# 11.2.4: IDAT - Image data
+#===============================================================================
+idat_info = OnDemand(
+    Field("idat_info", lambda ctx: ctx.length),
+)
+
+#===============================================================================
+# 11.3.2.1: tRNS - Transparency
+#===============================================================================
+trns_info = Switch("trns_info", lambda ctx: ctx._.image_header.color_type, 
+    {
+        "greyscale": Struct("data",
+            UBInt16("grey_sample")
+        ),
+        "truecolor": Struct("data",
+            UBInt16("red_sample"),
+            UBInt16("blue_sample"),
+            UBInt16("green_sample"),
+        ),
+        "indexed": Array(lambda ctx: ctx.length,
+            UBInt8("alpha"),
+        ),
+    }
+)
+
+#===============================================================================
+# 11.3.3.1: cHRM - Primary chromacities and white point
+#===============================================================================
+chrm_info = Struct("chrm_info",
+    Coord("white_point", UBInt32),
+    Coord("red", UBInt32),
+    Coord("green", UBInt32),
+    Coord("blue", UBInt32),
+)
+
+#===============================================================================
+# 11.3.3.2: gAMA - Image gamma
+#===============================================================================
+gama_info = Struct("gama_info",
+    UBInt32("gamma"),
+)
+
+#===============================================================================
+# 11.3.3.3: iCCP - Embedded ICC profile
+#===============================================================================
+iccp_info = Struct("iccp_info",
+    CString("name"),
+    compression_method,
+    Field("compressed_profile", 
+        lambda ctx: ctx._.length - (len(ctx.name) + 2)
+    ),
+)
+
+#===============================================================================
+# 11.3.3.4: sBIT - Significant bits
+#===============================================================================
+sbit_info = Switch("sbit_info", lambda ctx: ctx._.image_header.color_type, 
+    {
+        "greyscale": Struct("data",
+            UBInt8("significant_grey_bits"),
+        ),
+        "truecolor": Struct("data",
+            UBInt8("significant_red_bits"),
+            UBInt8("significant_green_bits"),
+            UBInt8("significant_blue_bits"),
+        ),
+        "indexed": Struct("data",
+            UBInt8("significant_red_bits"),
+            UBInt8("significant_green_bits"),
+            UBInt8("significant_blue_bits"),
+        ),
+        "greywithalpha": Struct("data",
+            UBInt8("significant_grey_bits"),
+            UBInt8("significant_alpha_bits"),
+        ),
+        "truewithalpha": Struct("data",
+            UBInt8("significant_red_bits"),
+            UBInt8("significant_green_bits"),
+            UBInt8("significant_blue_bits"),
+            UBInt8("significant_alpha_bits"),
+        ),
+    }
+)
+
+#===============================================================================
+# 11.3.3.5: sRGB - Standard RPG color space
+#===============================================================================
+srgb_info = Struct("srgb_info",
+    Enum(UBInt8("rendering_intent"),
+        perceptual = 0,
+        relative_colorimetric = 1,
+        saturation = 2,
+        absolute_colorimetric = 3,
+        _default_ = Pass,
+    ),
+)
+
+#===============================================================================
+# 11.3.4.3: tEXt - Textual data
+#===============================================================================
+text_info = Struct("text_info",
+    CString("keyword"),
+    Field("text", lambda ctx: ctx._.length - (len(ctx.keyword) + 1)),
+)
+
+#===============================================================================
+# 11.3.4.4: zTXt - Compressed textual data
+#===============================================================================
+ztxt_info = Struct("ztxt_info",
+    CString("keyword"),
+    compression_method,
+    OnDemand(
+        Field("compressed_text",
+            # As with iCCP, length is chunk length, minus length of
+            # keyword, minus two: one byte for the null terminator,
+            # and one byte for the compression method.
+            lambda ctx: ctx._.length - (len(ctx.keyword) + 2),
+        ),
+    ),
+)
+
+#===============================================================================
+# 11.3.4.5: iTXt - International textual data
+#===============================================================================
+itxt_info = Struct("itxt_info",
+    CString("keyword"),
+    UBInt8("compression_flag"),
+    compression_method,
+    CString("language_tag"),
+    CString("translated_keyword"),
+    OnDemand(
+        Field("text",
+            lambda ctx: ctx._.length - (len(ctx.keyword) + 
+            len(ctx.language_tag) + len(ctx.translated_keyword) + 5),
+        ),
+    ),
+)
+
+#===============================================================================
+# 11.3.5.1: bKGD - Background color
+#===============================================================================
+bkgd_info = Switch("bkgd_info", lambda ctx: ctx._.image_header.color_type, 
+    {
+        "greyscale": Struct("data",
+            UBInt16("background_greyscale_value"),
+            Alias("grey", "background_greyscale_value"),
+        ),
+        "greywithalpha": Struct("data",
+            UBInt16("background_greyscale_value"),
+            Alias("grey", "background_greyscale_value"),
+        ),
+        "truecolor": Struct("data",
+            UBInt16("background_red_value"),
+            UBInt16("background_green_value"),
+            UBInt16("background_blue_value"),
+            Alias("red", "background_red_value"),
+            Alias("green", "background_green_value"),
+            Alias("blue", "background_blue_value"),
+        ),
+        "truewithalpha": Struct("data",
+            UBInt16("background_red_value"),
+            UBInt16("background_green_value"),
+            UBInt16("background_blue_value"),
+            Alias("red", "background_red_value"),
+            Alias("green", "background_green_value"),
+            Alias("blue", "background_blue_value"),
+        ),
+        "indexed": Struct("data",
+            UBInt16("background_palette_index"),
+            Alias("index", "background_palette_index"),
+        ),
+    }
+)
+
+#===============================================================================
+# 11.3.5.2: hIST - Image histogram
+#===============================================================================
+hist_info = Array(lambda ctx: ctx._.length / 2,
+    UBInt16("frequency"),
+)
+
+#===============================================================================
+# 11.3.5.3: pHYs - Physical pixel dimensions
+#===============================================================================
+phys_info = Struct("phys_info",
+    UBInt32("pixels_per_unit_x"),
+    UBInt32("pixels_per_unit_y"),
+    Enum(UBInt8("unit"),
+        unknown = 0,
+        meter = 1,
+        _default_ = Pass
+    ),
+)
+
+#===============================================================================
+# 11.3.5.4: sPLT - Suggested palette
+#===============================================================================
+def splt_info_data_length(ctx):
+    if ctx.sample_depth == 8:
+        entry_size = 6
+    else:
+        entry_size = 10
+    return (ctx._.length - len(ctx.name) - 2) / entry_size
+
+splt_info = Struct("data",
+    CString("name"),
+    UBInt8("sample_depth"),
+    Array(lambda ctx: splt_info_data_length,
+        IfThenElse("table", lambda ctx: ctx.sample_depth == 8,
+            # Sample depth 8
+            Struct("table",
+                UBInt8("red"),
+                UBInt8("green"),
+                UBInt8("blue"),
+                UBInt8("alpha"),
+                UBInt16("frequency"),
+            ),
+            # Sample depth 16
+            Struct("table",
+                UBInt16("red"),
+                UBInt16("green"),
+                UBInt16("blue"),
+                UBInt16("alpha"),
+                UBInt16("frequency"),
+            ),
+        ),
+    ),
+)
+
+#===============================================================================
+# 11.3.6.1: tIME - Image last-modification time
+#===============================================================================
+time_info = Struct("data",
+    UBInt16("year"),
+    UBInt8("month"),
+    UBInt8("day"),
+    UBInt8("hour"),
+    UBInt8("minute"),
+    UBInt8("second"),
+)
+
+#===============================================================================
+# chunks
+#===============================================================================
+default_chunk_info = OnDemand(
+    HexDumpAdapter(Field(None, lambda ctx: ctx.length))
+)
+
+chunk = Struct("chunk",
+    UBInt32("length"),
+    String("type", 4),
+    Switch("data", lambda ctx: ctx.type, 
+        {
+            "PLTE" : plte_info,
+            "IEND" : Pass,
+            "IDAT" : idat_info,
+            "tRNS" : trns_info,
+            "cHRM" : chrm_info,
+            "gAMA" : gama_info,
+            "iCCP" : iccp_info,
+            "sBIT" : sbit_info,
+            "sRGB" : srgb_info,
+            "tEXt" : text_info,
+            "zTXt" : ztxt_info,
+            "iTXt" : itxt_info,
+            "bKGD" : bkgd_info,
+            "hIST" : hist_info,
+            "pHYs" : phys_info,
+            "sPLT" : splt_info,
+            "tIME" : time_info,
+        },
+        default = default_chunk_info,
+    ),
+    UBInt32("crc"),
+)
+
+image_header_chunk = Struct("image_header",
+    UBInt32("length"),
+    Const(String("type", 4), "IHDR"),
+    UBInt32("width"),
+    UBInt32("height"),
+    UBInt8("bit_depth"),
+    Enum(UBInt8("color_type"),
+        greyscale = 0,
+        truecolor = 2,
+        indexed = 3,
+        greywithalpha = 4,
+        truewithalpha = 6,
+        _default_ = Pass,
+    ),
+    compression_method,
+    Enum(UBInt8("filter_method"),
+        # "adaptive filtering with five basic filter types"
+        adaptive5 = 0,
+        _default_ = Pass,
+    ),
+    Enum(UBInt8("interlace_method"),
+        none = 0,
+        adam7 = 1,
+        _default_ = Pass,
+    ),
+    UBInt32("crc"),
+)
+
+
+#===============================================================================
+# the complete PNG file
+#===============================================================================
+png_file = Struct("png",
+    Magic(six.b("\x89PNG\r\n\x1a\n")),
+    image_header_chunk,
+    Rename("chunks", GreedyRange(chunk)),
+)