# Feed this file to script/make-artificial.go

# This GIF image contains four frames, each of which are outside the nominal
# image's bounds: (0, 0) - (2, 2):
#
#  - frame #0 has bounds (1, 0) - (4, 1).
#  - frame #1 has bounds (0, 1) - (2, 3).
#  - frame #2 has bounds (0, 4) - (1, 5).
#  - frame #3 has bounds (2, 0) - (6, 3).
#
# Here are some (but not all) possible decoding policies:
#
#  0. Reject it.
#  1. Keep   the image width×height at 2×2, clip all of the frames.
#  2. Adjust the image width×height to 3×1, the first frame's size, move
#     frame #0's top-left to be (0, 0), and clip subsequent frames.
#  3. Adjust the image width×height to 4×1, the first frame's bottom-right
#     corner, and clip subsequent frames.
#  4. Expand the image width×height to 4×2, the union of the image and first
#     frame's bounds, and clip subsequent frames.
#  5. Expand the image width×height to 4×5, so that no frames are clipped. This
#     needs to decode all frames before being able to report the image bounds.
#
# The GIF specification (https://www.w3.org/Graphics/GIF/spec-gif89a.txt) calls
# for policy 0, rejection, as it says that "Each image must fit within the
# boundaries of the Logical Screen, as defined in the Logical Screen
# Descriptor". The spec uses "image" and "screen" for what Wuffs calls "frame"
# and "image".
#
# In practice, real world encoders violate the spec, and real world decoders
# have responded by accepting such files anyway. As of May 2018:
#
#  - Chromium chooses policy 4:
#    https://cs.chromium.org/chromium/src/third_party/blink/renderer/platform/image-decoders/gif/gif_image_reader.cc?rcl=5161173c43324da2b13e1aa45bbe69901daa1279&l=740
#
#  - Firefox chooses policy 2:
#    https://dxr.mozilla.org/mozilla-central/rev/b75acf9652937ce79a9bf02de843c100db0e5ec7/image/decoders/nsGIFDecoder2.cpp#814
#
#  - Go chooses policy 0:
#    https://github.com/golang/go/blob/d3beea8c529a20b9eebb5ff8c22684af5ee1c920/src/image/gif/reader.go#L508
#    but a surprised user filed https://github.com/golang/go/issues/20856 when
#    Go rejected some (off-spec) GIF files that other decoders accept.
#
#  - gdk-pixbuf chooses policy 1:
#    https://github.com/GNOME/gdk-pixbuf/blob/c1fd9f5d6592c0183c54efc806b3ca6871e1f496/gdk-pixbuf/io-gif.c#L992
#
#  - giflib enforces no particular policy. It provides image and frame bounds
#    as encoded in the file and leaves it up to the caller how to respond.
#
# Wuffs chooses policy 4 by default, the same as Chromium. Wuffs can be
# configured to choose policy 1, by enabling a 'quirk'.

make gif

header

image {
	imageWidthHeight 2 2
	palette {
		0x00 0x00 0xFF
		0x11 0x00 0xFF
		0x22 0x00 0xFF
		0x33 0x00 0xFF
		0x44 0x00 0xFF
		0x55 0x00 0xFF
		0x66 0x00 0xFF
		0x77 0x00 0xFF

		0x00 0x88 0xFF
		0x00 0x99 0xFF
		0x00 0xAA 0xFF
		0x00 0xBB 0xFF
		0x00 0xCC 0xFF
		0x00 0xDD 0xFF
		0x00 0xEE 0xFF
		0x00 0xFF 0xFF
	}
}

frame {
	frameLeftTopWidthHeight 1 0 3 1
}
lzw 4 0x01 0x02 0x03

frame {
	frameLeftTopWidthHeight 0 1 2 2
}
lzw 4 0x08 0x09 0x0A 0x0B

frame {
	frameLeftTopWidthHeight 0 4 1 1
}
lzw 4 0x0E

frame {
	frameLeftTopWidthHeight 2 0 4 3
}
lzw 4 0x04 0x05 0x06 0x07 0x08 0x09 0x0A 0x0B 0x0C 0x0D 0x0E 0x0F

trailer