jpeg

UNIX下FILE命令的实现源码

#------------------------------------------------------------------------------
# JPEG images
# SunOS 5.5.1 had
#
#	0	string		\377\330\377\340	JPEG file
#	0	string		\377\330\377\356	JPG file
#
# both of which turn into "JPEG image data" here.
#
0	beshort		0xffd8		JPEG image data
>6	string		JFIF		\b, JFIF standard
# The following added by Erik Rossen <[e-mail]> 1999-09-06
# in a vain attempt to add image size reporting for JFIF.  Note that these
# tests are not fool-proof since some perfectly valid JPEGs are currently
# impossible to specify in magic(4) format.
# First, a little JFIF version info:
>11	byte		x		\b %d.
>12	byte		x		\b%02d
# Next, the resolution or aspect ratio of the image:
>13	byte		0		\b, aspect ratio
>13	byte		1		\b, resolution (DPI)
>13	byte		2		\b, resolution (DPCM)
>14	beshort		x		\b, %d x
>16	beshort		x		\b %d
#>4	beshort		x		\b, segment length %d
# Next, show thumbnail info, if it exists:
>18	byte		!0		\b, thumbnail %dx
>>19	byte		x		\b%d
# Here things get sticky.  We can do ONE MORE marker segment with
# indirect addressing, and that's all.  It would be great if we could
# do pointer arithemetic like in an assembler language.  Christos?
# And if there was some sort of looping construct to do searches, plus a few
# named accumulators, it would be even more effective...
# At least we can show a comment if no other segments got inserted before:
>(4.S+5)	byte		0xFE
>>(4.S+8)	string		>\0		\b, "%s"
#>(4.S+5)	byte		0xFE		\b, comment
#>>(4.S+6)	beshort		x		\b length=%d
#>>(4.S+8)	string		>\0		\b, "%s"
# Or, we can show the encoding type (I've included only the three most common)
# and image dimensions if we are lucky and the SOFn (image segment) is here:
>(4.S+5)	byte		0xC0		\b, baseline
>>(4.S+6)	byte		x		\b, precision %d
>>(4.S+7)	beshort		x		\b, %dx
>>(4.S+9)	beshort		x		\b%d
>(4.S+5)	byte		0xC1		\b, extended sequential
>>(4.S+6)	byte		x		\b, precision %d
>>(4.S+7)	beshort		x		\b, %dx
>>(4.S+9)	beshort		x		\b%d
>(4.S+5)	byte		0xC2		\b, progressive
>>(4.S+6)	byte		x		\b, precision %d
>>(4.S+7)	beshort		x		\b, %dx
>>(4.S+9)	beshort		x		\b%d
# I've commented-out quantisation table reporting.  I doubt anyone cares yet.
#>(4.S+5)	byte		0xDB		\b, quantisation table
#>>(4.S+6)	beshort		x		\b length=%d

# HSI is Handmade Software's proprietary JPEG encoding scheme
0	string		hsi1		JPEG image data, HSI proprietary