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Leptonica 1.68
C Image Processing Library
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Leptonica 1.68
C Image Processing Library
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Page orientation detection (on roman text) More...
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Page orientation detection (on roman text)
Page orientation detection (pure rotation by 90 degree increments):
l_int32 pixOrientDetect()
l_int32 makeOrientDecision()
l_int32 pixUpDownDetect()
l_int32 pixUpDownDetectGeneral()
l_int32 pixOrientDetectDwa()
l_int32 pixUpDownDetectDwa()
l_int32 pixUpDownDetectGeneralDwa()
Page mirror detection (flip 180 degrees about line in plane of image):
l_int32 pixMirrorDetect()
l_int32 pixMirrorDetectDwa()
Static debug helper
void pixDebugFlipDetect()
===================================================================
Page transformation detection:
Once a page is deskewed, there are 8 possible states that it
can be in, shown symbolically below. Suppose state 0 is correct.
0: correct 1 2 3
+------+ +------+ +------+ +------+
| **** | | * | | **** | | * |
| * | | * | | * | | * |
| * | | **** | | * | | **** |
+------+ +------+ +------+ +------+
4 5 6 7
+-----+ +-----+ +-----+ +-----+
| *** | | * | | *** | | * |
| * | | * | | * | | * |
| * | | * | | * | | * |
| * | | *** | | * | | *** |
+-----+ +-----+ +-----+ +-----+
Each of the other seven can be derived from state 0 by applying some
combination of a 90 degree clockwise rotation, a flip about
a horizontal line, and a flip about a vertical line,
all abbreviated as:
R = Rotation (about a line perpendicular to the image)
H = Horizontal flip (about a vertical line in the plane of the image)
V = Vertical flip (about a horizontal line in the plane of the image)
We get these transformations:
RHV
000 -> 0
001 -> 1
010 -> 2
011 -> 3
100 -> 4
101 -> 5
110 -> 6
111 -> 7
Note that in four of these, the sum of H and V is 1 (odd).
For these four, we have a change in parity (handedness) of
the image, and the transformation cannot be performed by
rotation about a vertical line out of the page. Under
rotation R, the set of 8 transformations decomposes into
two subgroups linking {0, 3, 4, 7} and {1, 2, 5, 6} independently.
pixOrientDetect*() tests for a pure rotation (0, 90, 180, 270 degrees).
It doesn't change parity.
pixMirrorDetect*() tests for a horizontal flip about the vertical axis.
It changes parity.
The landscape/portrait rotation can be detected in two ways:
(1) Compute the deskew confidence for an image segment,
both as is and rotated 90 degrees (see skew.c).
(2) Compute the ascender/descender signal for the image,
both as is and rotated 90 degrees (implemented here).
The ascender/descender signal is useful for determining text
orientation in Roman alphabets because the incidence of letters
with straight-line ascenders (b, d, h, k, l, <t>) outnumber
those with descenders (<g>, p, q). The letters <t> and <g>
will respond variably to the filter, depending on the type face.
What about the mirror image situations? These aren't common
unless you're dealing with film, for example.
But you can reliably test if the image has undergone a
parity-changing flip once about some axis in the plane
of the image, using pixMirrorDetect*(). This works ostensibly by
counting the number of characters with ascenders that
stick out to the left and right of the ascender. Characters
that are not mirror flipped are more likely to extend to the
right (b, h, k) than to the left (d). Of course, that is for
text that is rightside-up. So before you apply the mirror
test, it is necessary to insure that the text has the ascenders
going up, and not down or to the left or right. But here's
what *really* happens. It turns out that the pre-filtering before
the hit-miss transform (HMT) is crucial, and surprisingly, when
the pre-filtering is chosen to generate a large signal, the majority
of the signal comes from open regions of common lower-case
letters such as 'e', 'c' and 'f'.
All operations are given in two implementations whose results are
identical: rasterop morphology and dwa morphology. The dwa
implementations are between 2x and 3x faster.
The set of operations you actually use depends on your prior knowledge:
(1) If the page is known to be either rightside-up or upside-down, use
either pixOrientDetect*() with pleftconf = NULL, or
pixUpDownDetect*(). [The '*' refers to either the rasterop
or dwa versions.]
(2) If any of the four orientations are possible, use pixOrientDetect*().
(3) If the text is horizontal and rightside-up, the only remaining
degree of freedom is a left-right mirror flip: use
pixMirrorDetect*().
(4) If you have a relatively large amount of numbers on the page,
us the slower pixUpDownDetectGeneral().
We summarize the full orientation and mirror flip detection process:
(1) First determine which of the four 90 degree rotations
causes the text to be rightside-up. This can be done
with either skew confidence or the pixOrientDetect*()
signals. For the latter, see the table for pixOrientDetect().
(2) Then, with ascenders pointing up, apply pixMirrorDetect*().
In the normal situation the confidence confidence will be
large and positive. However, if mirror flipped, the
confidence will be large and negative.
Definition in file flipdetect.c.
| static void pixDebugFlipDetect | ( | const char * | filename, |
| PIX * | pixs, | ||
| PIX * | pixhm, | ||
| l_int32 | enable | ||
| ) | [static] |
Definition at line 973 of file flipdetect.c.
References IFF_PNG, pixConvert1To4Cmap(), pixDestroy(), pixMorphSequence(), pixSetMaskedCmap(), and pixWrite().
Referenced by pixMirrorDetect(), and pixUpDownDetectGeneral().
| l_int32 pixOrientDetect | ( | PIX * | pixs, |
| l_float32 * | pupconf, | ||
| l_float32 * | pleftconf, | ||
| l_int32 | mincount, | ||
| l_int32 | debug | ||
| ) |
Input: pixs (1 bpp, deskewed, English text, 150 - 300 ppi) &upconf (<optional return>=""> ; may be null) &leftconf (<optional return>=""> ; may be null) mincount (min number of up + down; use 0 for default) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) See "Measuring document image skew and orientation" Dan S. Bloomberg, Gary E. Kopec and Lakshmi Dasari IS&T/SPIE EI'95, Conference 2422: Document Recognition II pp 302-316, Feb 6-7, 1995, San Jose, CA (2) upconf is the normalized difference between up ascenders and down ascenders. The image is analyzed without rotation for being rightside-up or upside-down. Set &upconf to null to skip this operation. (3) leftconf is the normalized difference between up ascenders and down ascenders in the image after it has been rotated 90 degrees clockwise. With that rotation, ascenders projecting to the left in the source image will project up in the rotated image. We compute this by rotating 90 degrees clockwise and testing for up and down ascenders. Set &leftconf to null to skip this operation. (4) Note that upconf and leftconf are not linear measures of confidence, e.g., in a range between 0 and 100. They measure how far you are out on the tail of a (presumably) normal distribution. For example, a confidence of 10 means that it is nearly certain that the difference did not happen at random. However, these values must be interpreted cautiously, taking into consideration the estimated prior for a particular orientation or mirror flip. The up-down signal is very strong if applied to text with ascenders up and down, and relatively weak for text at 90 degrees, but even at 90 degrees, the difference can look significant. For example, suppose the ascenders are oriented horizontally, but the test is done vertically. Then upconf can be < -MIN_CONF_FOR_UP_DOWN, suggesting the text may be upside-down. However, if instead the test were done horizontally, leftconf will be very much larger (in absolute value), giving the correct orientation. (5) If you compute both upconf and leftconf, and there is sufficient signal, the following table determines the cw angle necessary to rotate pixs so that the text is rightside-up: 0 deg : upconf >> 1, abs(upconf) >> abs(leftconf) 90 deg : leftconf >> 1, abs(leftconf) >> abs(upconf) 180 deg : upconf << -1, abs(upconf) >> abs(leftconf) 270 deg : leftconf << -1, abs(leftconf) >> abs(upconf) (6) One should probably not interpret the direction unless there are a sufficient number of counts for both orientations, in which case neither upconf nor leftconf will be 0.0. (7) Uses rasterop implementation of HMT.
Definition at line 259 of file flipdetect.c.
References DEFAULT_MIN_UP_DOWN_COUNT, ERROR_INT, pixDestroy(), pixGetDepth(), pixRotate90(), pixUpDownDetect(), and PROCNAME.
Referenced by main().
| l_int32 makeOrientDecision | ( | l_float32 | upconf, |
| l_float32 | leftconf, | ||
| l_float32 | minupconf, | ||
| l_float32 | minratio, | ||
| l_int32 * | porient, | ||
| l_int32 | debug | ||
| ) |
Input: upconf (nonzero) leftconf (nonzero) minupconf (minimum value for which a decision can be made) minratio (minimum conf ratio required for a decision) &orient (<return> text orientation enum {0,1,2,3,4}) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) This can be run after pixOrientDetect() (2) Both upconf and leftconf must be nonzero; otherwise the orientation cannot be determined. (3) The abs values of the input confidences are compared to minupconf. (4) The abs value of the largest of (upconf/leftconf) and (leftconf/upconf) is compared with minratio. (5) Input 0.0 for the default values for minupconf and minratio. (6) The return value of orient is interpreted thus: L_TEXT_ORIENT_UNKNOWN: not enough evidence to determine L_TEXT_ORIENT_UP: text rightside-up L_TEXT_ORIENT_LEFT: landscape, text up facing left L_TEXT_ORIENT_DOWN: text upside-down L_TEXT_ORIENT_RIGHT: landscape, text up facing right
Definition at line 318 of file flipdetect.c.
References DEFAULT_MIN_UP_DOWN_CONF, DEFAULT_MIN_UP_DOWN_RATIO, ERROR_INT, L_ABS, L_TEXT_ORIENT_DOWN, L_TEXT_ORIENT_LEFT, L_TEXT_ORIENT_RIGHT, L_TEXT_ORIENT_UNKNOWN, L_TEXT_ORIENT_UP, and PROCNAME.
Referenced by main().
Input: pixs (1 bpp, deskewed, English text, 150 - 300 ppi) &conf (<return> confidence that text is rightside-up) mincount (min number of up + down; use 0 for default) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) Special (typical, slightly faster) case, where the pixels identified through the HMT (hit-miss transform) are not clipped by a truncated word mask pixm. See pixOrientDetect() and pixUpDownDetectGeneral() for details. (2) The returned confidence is the normalized difference between the number of detected up and down ascenders, assuming that the text is either rightside-up or upside-down and not rotated at a 90 degree angle.
Definition at line 391 of file flipdetect.c.
References pixUpDownDetectGeneral().
Referenced by pixOrientDetect().
| l_int32 pixUpDownDetectGeneral | ( | PIX * | pixs, |
| l_float32 * | pconf, | ||
| l_int32 | mincount, | ||
| l_int32 | npixels, | ||
| l_int32 | debug | ||
| ) |
Input: pixs (1 bpp, deskewed, English text, 150 - 300 ppi) &conf (<return> confidence that text is rightside-up) mincount (min number of up + down; use 0 for default) npixels (number of pixels removed from each side of word box) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) See pixOrientDetect() for other details. (2) is the normalized difference between the number of detected up and down ascenders, assuming that the text is either rightside-up or upside-down and not rotated at a 90 degree angle. (3) The typical mode of operation is == 0. If > 0, this removes HMT matches at the beginning and ending of "words." This is useful for pages that may have mostly digits, because if npixels == 0, leading "1" and "3" digits can register as having ascenders or descenders, and "7" digits can match descenders. Consequently, a page image of only digits may register as being upside-down. (4) We want to count the number of instances found using the HMT. An expensive way to do this would be to count the number of connected components. A cheap way is to do a rank reduction cascade that reduces each component to a single pixel, and results (after two or three 2x reductions) in one pixel for each of the original components. After the reduction, you have a much smaller pix over which to count pixels. We do only 2 reductions, because this function is designed to work for input pix between 150 and 300 ppi, and an 8x reduction on a 150 ppi image is going too far -- components will get merged.
Definition at line 437 of file flipdetect.c.
References boxaDestroy(), boxaGetBox(), boxaGetCount(), boxDestroy(), boxGetGeometry(), DEFAULT_MIN_UP_DOWN_CONF, DEFAULT_MIN_UP_DOWN_COUNT, ERROR_INT, IFF_PNG, L_CLONE, L_MAX, NULL, PIX_SET, pixAnd(), pixConnComp(), pixCountPixels(), pixCreateTemplate(), pixDebugFlipDetect(), pixDestroy(), pixHMT(), pixMorphCompSequence(), pixMorphSequence(), pixOr(), pixRasterop(), pixReduceRankBinaryCascade(), pixWrite(), PROCNAME, selCreateFromString(), selDestroy(), textsel1, textsel2, textsel3, and textsel4.
Referenced by main(), and pixUpDownDetect().
| l_int32 pixOrientDetectDwa | ( | PIX * | pixs, |
| l_float32 * | pupconf, | ||
| l_float32 * | pleftconf, | ||
| l_int32 | mincount, | ||
| l_int32 | debug | ||
| ) |
Input: pixs (1 bpp, deskewed, English text) &upconf (<optional return>=""> ; may be null) &leftconf (<optional return>=""> ; may be null) mincount (min number of up + down; use 0 for default) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) Same interface as for pixOrientDetect(). See notes there for usage. (2) Uses auto-gen'd code for the Sels defined at the top of this file, with some renaming of functions. The auto-gen'd code is in fliphmtgen.c, and can be generated by a simple executable; see prog/flipselgen.c. (3) This runs about 2.5 times faster than the pixOrientDetect().
Definition at line 574 of file flipdetect.c.
References DEFAULT_MIN_UP_DOWN_COUNT, ERROR_INT, pixDestroy(), pixGetDepth(), pixRotate90(), pixUpDownDetectDwa(), and PROCNAME.
Referenced by main().
Input: pixs (1 bpp, deskewed, English text, 150 - 300 ppi) &conf (<return> confidence that text is rightside-up) mincount (min number of up + down; use 0 for default) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) Faster (DWA) version of pixUpDownDetect(). (2) This is a special case (but typical and slightly faster) of pixUpDownDetectGeneralDwa(), where the pixels identified through the HMT (hit-miss transform) are not clipped by a truncated word mask pixm. See pixUpDownDetectGeneral() for usage and other details. (3) The returned confidence is the normalized difference between the number of detected up and down ascenders, assuming that the text is either rightside-up or upside-down and not rotated at a 90 degree angle.
Definition at line 627 of file flipdetect.c.
References pixUpDownDetectGeneralDwa().
Referenced by pixOrientDetectDwa().
| l_int32 pixUpDownDetectGeneralDwa | ( | PIX * | pixs, |
| l_float32 * | pconf, | ||
| l_int32 | mincount, | ||
| l_int32 | npixels, | ||
| l_int32 | debug | ||
| ) |
Input: pixs (1 bpp, deskewed, English text) &conf (<return> confidence that text is rightside-up) mincount (min number of up + down; use 0 for default) npixels (number of pixels removed from each side of word box) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) See the notes in pixUpDownDetectGeneral() for usage.
Definition at line 650 of file flipdetect.c.
References ADDED_BORDER, boxaDestroy(), boxaGetBox(), boxaGetCount(), boxDestroy(), boxGetGeometry(), DEFAULT_MIN_UP_DOWN_CONF, DEFAULT_MIN_UP_DOWN_COUNT, ERROR_INT, IFF_PNG, L_CLONE, L_MAX, NULL, PIX_SET, pixAddBorderGeneral(), pixAnd(), pixConnComp(), pixCountPixels(), pixCreateTemplate(), pixDestroy(), pixFlipFHMTGen(), pixMorphSequenceDwa(), pixOr(), pixRasterop(), pixReduceRankBinaryCascade(), pixWrite(), and PROCNAME.
Referenced by main(), and pixUpDownDetectDwa().
Input: pixs (1 bpp, deskewed, English text) &conf (<return> confidence that text is not LR mirror reversed) mincount (min number of left + right; use 0 for default) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) For this test, it is necessary that the text is horizontally oriented, with ascenders going up. (2) conf is the normalized difference between the number of right and left facing characters with ascenders. Left-facing are {d}; right-facing are {b, h, k}. At least that was the expectation. In practice, we can really just say that it is the normalized difference in hits using two specific hit-miss filters, textsel1 and textsel2, after the image has been suitably pre-filtered so that these filters are effective. See (4) for what's really happening. (3) A large positive conf value indicates normal text, whereas a large negative conf value means the page is mirror reversed. (4) The implementation is a bit tricky. The general idea is to fill the x-height part of characters, but not the space between them, before doing the HMT. This is done by finding pixels added using two different operations -- a horizontal close and a vertical dilation -- and adding the intersection of these sets to the original. It turns out that the original intuition about the signal was largely in error: much of the signal for right-facing characters comes from the lower part of common x-height characters, like the e and c, that remain open after these operations. So it's important that the operations to close the x-height parts of the characters are purposely weakened sufficiently to allow these characters to remain open. The wonders of morphology!
Definition at line 803 of file flipdetect.c.
References DEFAULT_MIN_MIRROR_FLIP_CONF, DEFAULT_MIN_MIRROR_FLIP_COUNT, ERROR_INT, L_MAX, NULL, pixAnd(), pixCountPixels(), pixDebugFlipDetect(), pixDestroy(), pixHMT(), pixMorphCompSequence(), pixOr(), pixReduceRankBinaryCascade(), pixXor(), PROCNAME, selCreateFromString(), selDestroy(), textsel1, and textsel2.
Referenced by main().
Input: pixs (1 bpp, deskewed, English text) &conf (<return> confidence that text is not LR mirror reversed) mincount (min number of left + right; use 0 for default) debug (1 for debug output; 0 otherwise) Return: 0 if OK, 1 on error
Notes: (1) We assume the text is horizontally oriented, with ascenders going up. (2) See notes in pixMirrorDetect().
Definition at line 893 of file flipdetect.c.
References ADDED_BORDER, DEFAULT_MIN_MIRROR_FLIP_CONF, DEFAULT_MIN_MIRROR_FLIP_COUNT, ERROR_INT, L_MAX, NULL, pixAddBorderGeneral(), pixAnd(), pixCountPixels(), pixDestroy(), pixFlipFHMTGen(), pixMorphSequenceDwa(), pixOr(), pixReduceRankBinaryCascade(), pixXor(), and PROCNAME.
Referenced by main().
const char* textsel1 = "xxxxxx" [static] |
Definition at line 160 of file flipdetect.c.
Referenced by pixMirrorDetect(), and pixUpDownDetectGeneral().
const char* textsel2 = "xxxxxx" [static] |
Definition at line 166 of file flipdetect.c.
Referenced by pixMirrorDetect(), and pixUpDownDetectGeneral().
const char* textsel3 = "x oo " [static] |
Definition at line 172 of file flipdetect.c.
Referenced by pixUpDownDetectGeneral().
const char* textsel4 = " oo x" [static] |
Definition at line 178 of file flipdetect.c.
Referenced by pixUpDownDetectGeneral().
const l_int32 DEFAULT_MIN_UP_DOWN_COUNT = 70 [static] |
Definition at line 185 of file flipdetect.c.
Referenced by pixOrientDetect(), pixOrientDetectDwa(), pixUpDownDetectGeneral(), and pixUpDownDetectGeneralDwa().
const l_float32 DEFAULT_MIN_UP_DOWN_CONF = 7.0 [static] |
Definition at line 186 of file flipdetect.c.
Referenced by makeOrientDecision(), pixUpDownDetectGeneral(), and pixUpDownDetectGeneralDwa().
const l_float32 DEFAULT_MIN_UP_DOWN_RATIO = 2.5 [static] |
Definition at line 187 of file flipdetect.c.
Referenced by makeOrientDecision().
const l_int32 DEFAULT_MIN_MIRROR_FLIP_COUNT = 100 [static] |
Definition at line 190 of file flipdetect.c.
Referenced by pixMirrorDetect(), and pixMirrorDetectDwa().
const l_float32 DEFAULT_MIN_MIRROR_FLIP_CONF = 5.0 [static] |
Definition at line 191 of file flipdetect.c.
Referenced by pixMirrorDetect(), and pixMirrorDetectDwa().
1.7.3