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Leptonica
1.77.0
Image processing and image analysis suite
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Leptonica
1.77.0
Image processing and image analysis suite
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#include "allheaders.h"Go to the source code of this file.
Functions | |
| l_ok | pixColorContent (PIX *pixs, l_int32 rwhite, l_int32 gwhite, l_int32 bwhite, l_int32 mingray, PIX **ppixr, PIX **ppixg, PIX **ppixb) |
| PIX * | pixColorMagnitude (PIX *pixs, l_int32 rwhite, l_int32 gwhite, l_int32 bwhite, l_int32 type) |
| PIX * | pixMaskOverColorPixels (PIX *pixs, l_int32 threshdiff, l_int32 mindist) |
| PIX * | pixMaskOverColorRange (PIX *pixs, l_int32 rmin, l_int32 rmax, l_int32 gmin, l_int32 gmax, l_int32 bmin, l_int32 bmax) |
| l_ok | pixColorFraction (PIX *pixs, l_int32 darkthresh, l_int32 lightthresh, l_int32 diffthresh, l_int32 factor, l_float32 *ppixfract, l_float32 *pcolorfract) |
| l_ok | pixFindColorRegions (PIX *pixs, PIX *pixm, l_int32 factor, l_int32 lightthresh, l_int32 darkthresh, l_int32 mindiff, l_int32 colordiff, l_float32 edgefract, l_float32 *pcolorfract, PIX **pcolormask1, PIX **pcolormask2, PIXA *pixadb) |
| l_ok | pixNumSignificantGrayColors (PIX *pixs, l_int32 darkthresh, l_int32 lightthresh, l_float32 minfract, l_int32 factor, l_int32 *pncolors) |
| l_ok | pixColorsForQuantization (PIX *pixs, l_int32 thresh, l_int32 *pncolors, l_int32 *piscolor, l_int32 debug) |
| l_ok | pixNumColors (PIX *pixs, l_int32 factor, l_int32 *pncolors) |
| l_ok | pixGetMostPopulatedColors (PIX *pixs, l_int32 sigbits, l_int32 factor, l_int32 ncolors, l_uint32 **parray, PIXCMAP **pcmap) |
| PIX * | pixSimpleColorQuantize (PIX *pixs, l_int32 sigbits, l_int32 factor, l_int32 ncolors) |
| NUMA * | pixGetRGBHistogram (PIX *pixs, l_int32 sigbits, l_int32 factor) |
| l_ok | makeRGBIndexTables (l_uint32 **prtab, l_uint32 **pgtab, l_uint32 **pbtab, l_int32 sigbits) |
| l_ok | getRGBFromIndex (l_uint32 index, l_int32 sigbits, l_int32 *prval, l_int32 *pgval, l_int32 *pbval) |
| l_ok | pixHasHighlightRed (PIX *pixs, l_int32 factor, l_float32 fract, l_float32 fthresh, l_int32 *phasred, l_float32 *pratio, PIX **ppixdb) |
Builds an image of the color content, on a per-pixel basis,
as a measure of the amount of divergence of each color
component (R,G,B) from gray.
l_int32 pixColorContent()
Finds the 'amount' of color in an image, on a per-pixel basis,
as a measure of the difference of the pixel color from gray.
PIX *pixColorMagnitude()
Generates a mask over pixels that have sufficient color and
are not too close to gray pixels.
PIX *pixMaskOverColorPixels()
Generates mask over pixels within a prescribed cube in RGB space
PIX *pixMaskOverColorRange()
Finds the fraction of pixels with "color" that are not close to black
l_int32 pixColorFraction()
Determine if there are significant color regions that are
not background in a page image
l_int32 pixFindColorRegions()
Finds the number of perceptually significant gray intensities
in a grayscale image.
l_int32 pixNumSignificantGrayColors()
Identifies images where color quantization will cause posterization
due to the existence of many colors in low-gradient regions.
l_int32 pixColorsForQuantization()
Finds the number of unique colors in an image
l_int32 pixNumColors()
Find the most "populated" colors in the image (and quantize)
l_int32 pixGetMostPopulatedColors()
PIX *pixSimpleColorQuantize()
Constructs a color histogram based on rgb indices
NUMA *pixGetRGBHistogram()
l_int32 makeRGBIndexTables()
l_int32 getRGBFromIndex()
Identify images that have highlight (red) color
l_int32 pixHasHighlightRed()
Color is tricky. If we consider gray (r = g = b) to have no color content, how should we define the color content in each component of an arbitrary pixel, as well as the overall color magnitude?
I can think of three ways to define the color content in each component:
(1) Linear. For each component, take the difference from the average
of all three.
(2) Linear. For each component, take the difference from the average
of the other two.
(3) Nonlinear. For each component, take the minimum of the differences
from the other two. How might one choose from among these? Consider two different situations:
(a) r = g = 0, b = 255 {255} /255/
(b) r = 0, g = 127, b = 255 {191} /128/
How much g is in each of these? The three methods above give:
(a) 1: 85 2: 127 3: 0 [85]
(b) 1: 0 2: 0 3: 127 [0]
How much b is in each of these?
(a) 1: 170 2: 255 3: 255 [255]
(b) 1: 127 2: 191 3: 127 [191]
The number I'd "like" to give is in []. (Please don't ask why, it's
just a feeling.So my preferences seem to be somewhere between (1) and (2). (3) is just too "decisive!" Let's pick (2).
We also allow compensation for white imbalance. For each component, we do a linear TRC (gamma = 1.0), where the black point remains at 0 and the white point is given by the input parameter. This is equivalent to doing a global remapping, as with pixGlobalNormRGB(), followed by color content (or magnitude) computation, but without the overhead of first creating the white point normalized image.
Another useful property is the overall color magnitude in the pixel.
For this there are again several choices, such as:
(a) rms deviation from the mean
(b) the average L1 deviation from the mean
(c) the maximum (over components) of one of the color
content measures given above. For now, we will choose two of the methods in (c):
L_MAX_DIFF_FROM_AVERAGE_2
Define the color magnitude as the maximum over components
of the difference between the component value and the
average of the other two. It is easy to show that
this is equivalent to selecting the two component values
that are closest to each other, averaging them, and
using the distance from that average to the third component.
For (a) and (b) above, this value is in {..}.
L_MAX_MIN_DIFF_FROM_2
Define the color magnitude as the maximum over components
of the minimum difference between the component value and the
other two values. It is easy to show that this is equivalent
to selecting the intermediate value of the three differences
between the three components. For (a) and (b) above,
this value is in /../.
Definition in file colorcontent.c.
| l_ok getRGBFromIndex | ( | l_uint32 | index, |
| l_int32 | sigbits, | ||
| l_int32 * | prval, | ||
| l_int32 * | pgval, | ||
| l_int32 * | pbval | ||
| ) |
| [in] | index | rgbindex |
| [in] | sigbits | 2-6, significant bits retained in the quantizer for each component of the input image |
| [out] | prval,pgval,pbval | rgb values |
Notes:
(1) The index is expressed in bits, based on the the
sigbits of the r, g and b components, as
r7 r6 ... g7 g6 ... b7 b6 ...
(2) The computed rgb values are in the center of the quantized cube.
The extra bit that is OR'd accomplishes this.
Definition at line 1674 of file colorcontent.c.
| l_ok makeRGBIndexTables | ( | l_uint32 ** | prtab, |
| l_uint32 ** | pgtab, | ||
| l_uint32 ** | pbtab, | ||
| l_int32 | sigbits | ||
| ) |
| [out] | prtab,pgtab,pbtab | 256-entry index tables |
| [in] | sigbits | 2-6, significant bits retained in the quantizer for each component of the input image |
Notes:
(1) These tables are used to map from rgb sample values to
an rgb index, using
rgbindex = rtab[rval] | gtab[gval] | btab[bval]
where, e.g., if sigbits = 3, the index is a 9 bit integer:
r7 r6 r5 g7 g6 g5 b7 b6 b5
Definition at line 1584 of file colorcontent.c.
| l_ok pixColorContent | ( | PIX * | pixs, |
| l_int32 | rwhite, | ||
| l_int32 | gwhite, | ||
| l_int32 | bwhite, | ||
| l_int32 | mingray, | ||
| PIX ** | ppixr, | ||
| PIX ** | ppixg, | ||
| PIX ** | ppixb | ||
| ) |
| [in] | pixs | 32 bpp rgb or 8 bpp colormapped |
| [in] | rwhite,gwhite,bwhite | color value associated with white point |
| [in] | mingray | min gray value for which color is measured |
| [out] | ppixr | [optional] 8 bpp red 'content' |
| [out] | ppixg | [optional] 8 bpp green 'content' |
| [out] | ppixb | [optional] 8 bpp blue 'content' |
Notes:
(1) This returns the color content in each component, which is
a measure of the deviation from gray, and is defined
as the difference between the component and the average of
the other two components. See the discussion at the
top of this file.
(2) The three numbers (rwhite, gwhite and bwhite) can be thought
of as the values in the image corresponding to white.
They are used to compensate for an unbalanced color white point.
They must either be all 0 or all non-zero. To turn this
off, set them all to 0.
(3) If the maximum component after white point correction,
max(r,g,b), is less than mingray, all color components
for that pixel are set to zero.
Use mingray = 0 to turn off this filtering of dark pixels.
(4) Therefore, use 0 for all four input parameters if the color
magnitude is to be calculated without either white balance
correction or dark filtering.
Definition at line 179 of file colorcontent.c.
References CCBorda::h, pixGetDimensions(), and CCBorda::w.
| l_ok pixColorFraction | ( | PIX * | pixs, |
| l_int32 | darkthresh, | ||
| l_int32 | lightthresh, | ||
| l_int32 | diffthresh, | ||
| l_int32 | factor, | ||
| l_float32 * | ppixfract, | ||
| l_float32 * | pcolorfract | ||
| ) |
| [in] | pixs | 32 bpp rgb |
| [in] | darkthresh | threshold near black; if the lightest component is below this, the pixel is not considered in the statistics; typ. 20 |
| [in] | lightthresh | threshold near white; if the darkest component is above this, the pixel is not considered in the statistics; typ. 244 |
| [in] | diffthresh | thresh for the maximum difference between component value; below this the pixel is not considered to have sufficient color |
| [in] | factor | subsampling factor |
| [out] | ppixfract | fraction of pixels in intermediate brightness range that were considered for color content |
| [out] | pcolorfract | fraction of pixels that meet the criterion for sufficient color; 0.0 on error |
Notes:
(1) This function is asking the question: to what extent does the
image appear to have color? The amount of color a pixel
appears to have depends on both the deviation of the
individual components from their average and on the average
intensity itself. For example, the color will be much more
obvious with a small deviation from white than the same
deviation from black.
(2) Any pixel that meets these three tests is considered a
colorful pixel:
(a) the lightest component must equal or exceed darkthresh
(b) the darkest component must not exceed lightthresh
(c) the max difference between components must equal or
exceed diffthresh.
(3) The dark pixels are removed from consideration because
they don't appear to have color.
(4) The very lightest pixels are removed because if an image
has a lot of "white", the color fraction will be artificially
low, even if all the other pixels are colorful.
(5) If pixfract is very small, there are few pixels that are neither
black nor white. If colorfract is very small, the pixels
that are neither black nor white have very little color
content. The product 'pixfract * colorfract' gives the
fraction of pixels with significant color content.
(6) One use of this function is as a preprocessing step for median
cut quantization (colorquant2.c), which does a very poor job
splitting the color space into rectangular volume elements when
all the pixels are near the diagonal of the color cube. For
octree quantization of an image with only gray values, the
2^(level) octcubes on the diagonal are the only ones
that can be occupied.
Definition at line 678 of file colorcontent.c.
References CCBorda::h, and CCBorda::w.
| [in] | pixs | 32 bpp rgb or 8 bpp colormapped |
| [in] | rwhite,gwhite,bwhite | color value associated with white point |
| [in] | type | chooses the method for calculating the color magnitude: L_MAX_DIFF_FROM_AVERAGE_2, L_MAX_MIN_DIFF_FROM_2, L_MAX_DIFF |
Notes:
(1) For an RGB image, a gray pixel is one where all three components
are equal. We define the amount of color in an RGB pixel as
a function depending on the absolute value of the differences
between the three color components. Consider the two largest
of these differences. The pixel component in common to these
two differences is the color farthest from the other two.
The color magnitude in an RGB pixel can be taken as one
of these three definitions:
(a) The average of these two differences. This is the
average distance from the two components that are
nearest to each other to the third component.
(b) The minimum value of these two differences. This is
the intermediate value of the three distances between
component values. Stated otherwise, it is the
maximum over all components of the minimum distance
from that component to the other two components.
(c) The maximum difference between component values.
(2) As an example, suppose that R and G are the closest in
magnitude. Then the color is determined as either:
(a) The average distance of B from these two:
(|B - R| + |B - G|) / 2
(b) The minimum distance of B from these two:
min(|B - R|, |B - G|).
(c) The maximum distance of B from these two:
max(|B - R|, |B - G|)
(3) The three methods for choosing the color magnitude from
the components are selected with these flags:
(a) L_MAX_DIFF_FROM_AVERAGE_2
(b) L_MAX_MIN_DIFF_FROM_2
(c) L_MAX_DIFF
(4) The three numbers (rwhite, gwhite and bwhite) can be thought
of as the values in the image corresponding to white.
They are used to compensate for an unbalanced color white point.
They must either be all 0 or all non-zero. To turn this
off, set them all to 0.
Definition at line 363 of file colorcontent.c.
References CCBorda::h, pixGetDimensions(), and CCBorda::w.
| l_ok pixColorsForQuantization | ( | PIX * | pixs, |
| l_int32 | thresh, | ||
| l_int32 * | pncolors, | ||
| l_int32 * | piscolor, | ||
| l_int32 | debug | ||
| ) |
| [in] | pixs | 8 bpp gray or 32 bpp rgb; with or without colormap |
| [in] | thresh | binary threshold on edge gradient; 0 for default |
| [out] | pncolors | the number of colors found |
| [out] | piscolor | [optional] 1 if significant color is found; 0 otherwise. If pixs is 8 bpp, and does not have a colormap with color entries, this is 0 |
| [in] | debug | 1 to output masked image that is tested for colors; 0 otherwise |
Notes:
(1) This function finds a measure of the number of colors that are
found in low-gradient regions of an image. By its
magnitude relative to some threshold (not specified in
this function), it gives a good indication of whether
quantization will generate posterization. This number
is larger for images with regions of slowly varying
intensity (if 8 bpp) or color (if rgb). Such images, if
quantized, may require dithering to avoid posterization,
and lossless compression is then expected to be poor.
(2) If pixs has a colormap, the number of colors returned is
the number in the colormap.
(3) It is recommended that document images be reduced to a width
of 800 pixels before applying this function. Then it can
be expected that color detection will be fairly accurate
and the number of colors will reflect both the content and
the type of compression to be used. For less than 15 colors,
there is unlikely to be a halftone image, and lossless
quantization should give both a good visual result and
better compression.
(4) When using the default threshold on the gradient (15),
images (both gray and rgb) where ncolors is greater than
about 15 will compress poorly with either lossless
compression or dithered quantization, and they may be
posterized with non-dithered quantization.
(5) For grayscale images, or images without significant color,
this returns the number of significant gray levels in
the low-gradient regions. The actual number of gray levels
can be large due to jpeg compression noise in the background.
(6) Similarly, for color images, the actual number of different
(r,g,b) colors in the low-gradient regions (rather than the
number of occupied level 4 octcubes) can be quite large, e.g.,
due to jpeg compression noise, even for regions that appear
to be of a single color. By quantizing to level 4 octcubes,
most of these superfluous colors are removed from the counting.
(7) The image is tested for color. If there is very little color,
it is thresholded to gray and the number of gray levels in
the low gradient regions is found. If the image has color,
the number of occupied level 4 octcubes is found.
(8) The number of colors in the low-gradient regions increases
monotonically with the threshold thresh on the edge gradient.
(9) Background: grayscale and color quantization is often useful
to achieve highly compressed images with little visible
distortion. However, gray or color washes (regions of
low gradient) can defeat this approach to high compression.
How can one determine if an image is expected to compress
well using gray or color quantization? We use the fact that
* gray washes, when quantized with less than 50 intensities,
have posterization (visible boundaries between regions
of uniform 'color') and poor lossless compression
* color washes, when quantized with level 4 octcubes,
typically result in both posterization and the occupancy
of many level 4 octcubes.
Images can have colors either intrinsically or as jpeg
compression artifacts. This function reduces but does not
completely eliminate measurement of jpeg quantization noise
in the white background of grayscale or color images.
Definition at line 1145 of file colorcontent.c.
References CCBorda::h, and CCBorda::w.
| l_ok pixFindColorRegions | ( | PIX * | pixs, |
| PIX * | pixm, | ||
| l_int32 | factor, | ||
| l_int32 | lightthresh, | ||
| l_int32 | darkthresh, | ||
| l_int32 | mindiff, | ||
| l_int32 | colordiff, | ||
| l_float32 | edgefract, | ||
| l_float32 * | pcolorfract, | ||
| PIX ** | pcolormask1, | ||
| PIX ** | pcolormask2, | ||
| PIXA * | pixadb | ||
| ) |
| [in] | pixs | 32 bpp rgb |
| [in] | pixm | [optional] 1 bpp mask image |
| [in] | factor | subsample factor; integer >= 1 |
| [in] | lightthresh | threshold for component average in lightest of 10 buckets; typ. 210; -1 for default |
| [in] | darkthresh | threshold to eliminate dark pixels (e.g., text) from consideration; typ. 70; -1 for default. |
| [in] | mindiff | minimum difference (b - r) and (g - r), used to find blue or green pixels; typ. 10; -1 for default |
| [in] | colordiff | minimum difference in (max - min) component to qualify as a color pixel; typ. 90; -1 for default |
| [in] | edgefract | fraction of image half-width and half-height for which color pixels are ignored; typ. 0.05. |
| [out] | pcolorfract | fraction of 'color' pixels found |
| [out] | pcolormask1 | [optional] mask over background color, if any |
| [out] | pcolormask2 | [optional] filtered mask over background color |
| [out] | pixadb | [optional] debug intermediate results |
Notes:
(1) This function tries to determine if there is a significant
color or darker region on a scanned page image, where part
of the image is background that is either white or reddish.
This also allows extraction of regions of colored pixels that
have a smaller red component than blue or green components.
(2) If pixm exists, pixels under its fg are combined with
dark pixels to make a mask of pixels not to be considered
as color candidates.
(3) There are four thresholds.
* lightthresh: compute the average value of each rgb pixel,
and make 10 buckets by value. If the lightest bucket gray
value is below lightthresh, the image is not considered
to have a light bg, and this returns 0.0 for colorfract.
* darkthresh: ignore pixels darker than this (typ. fg text).
We make a 1 bpp mask of these pixels, and then dilate it to
remove all vestiges of fg from their vicinity.
* mindiff: consider pixels with either (b - r) or (g - r)
being at least this value, as having color.
* colordiff: consider pixels where the (max - min) difference
of the pixel components exceeds this value, as having color.
(4) All components of color pixels that are touching the image
border are removed. Additionally, all pixels within some
normalized distance edgefract from the image border can
be removed. This insures that dark pixels near the edge
of the image are not included.
(5) This returns in pcolorfract the fraction of pixels that have
color and are not in the set consisting of an OR between
pixm and the dilated dark pixel mask.
(6) No masks are returned unless light color pixels are found.
If colorfract > 0.0 and pcolormask1 is defined, this returns
a 1 bpp mask with fg pixels over the color background.
This mask may have some holes in it.
(7) If colorfract > 0.0 and pcolormask2 is defined, this returns
a version of colormask1 where small holes have been filled.
(8) To generate a boxa of rectangular regions from the overlap
of components in the filtered mask:
boxa1 = pixConnCompBB(colormask2, 8);
boxa2 = boxaCombineOverlaps(boxa1, NULL);
This is done here in debug mode.
Definition at line 805 of file colorcontent.c.
References CCBorda::h, and CCBorda::w.
| l_ok pixGetMostPopulatedColors | ( | PIX * | pixs, |
| l_int32 | sigbits, | ||
| l_int32 | factor, | ||
| l_int32 | ncolors, | ||
| l_uint32 ** | parray, | ||
| PIXCMAP ** | pcmap | ||
| ) |
| [in] | pixs | 32 bpp rgb |
| [in] | sigbits | 2-6, significant bits retained in the quantizer for each component of the input image |
| [in] | factor | subsampling factor; use 1 for no subsampling |
| [in] | ncolors | the number of most populated colors to select |
| [out] | parray | [optional] array of colors, each as 0xrrggbb00 |
| [out] | pcmap | [optional] colormap of the colors |
Notes:
(1) This finds the ncolors most populated cubes in rgb colorspace,
where the cube size depends on sigbits as
cube side = (256 >> sigbits)
(2) The rgb color components are found at the center of the cube.
(3) The output array of colors can be displayed using
pixDisplayColorArray(array, ncolors, ...);
Definition at line 1395 of file colorcontent.c.
References CCBorda::n.
| [in] | pixs | 32 bpp rgb |
| [in] | sigbits | 2-6, significant bits retained in the quantizer for each component of the input image |
| [in] | factor | subsampling factor; use 1 for no subsampling |
Notes:
(1) This uses a simple, fast method of indexing into an rgb image.
(2) The output is a 1D histogram of count vs. rgb-index, which
uses red sigbits as the most significant and blue as the least.
(3) This function produces the same result as pixMedianCutHisto().
Definition at line 1516 of file colorcontent.c.
References CCBorda::h, and CCBorda::w.
| l_ok pixHasHighlightRed | ( | PIX * | pixs, |
| l_int32 | factor, | ||
| l_float32 | fract, | ||
| l_float32 | fthresh, | ||
| l_int32 * | phasred, | ||
| l_float32 * | pratio, | ||
| PIX ** | ppixdb | ||
| ) |
| [in] | pixs | 32 bpp rgb |
| [in] | factor | subsampling; an integer >= 1; use 1 for all pixels |
| [in] | fract | threshold fraction of all image pixels |
| [in] | fthresh | threshold on a function of the components; typ. ~2.5 |
| [out] | phasred | 1 if red pixels are above threshold |
| [out] | pratio | [optional] normalized fraction of threshold red pixels that is actually observed |
| [out] | ppixdb | [optional] seed pixel mask |
Notes:
(1) Pixels are identified as red if they satisfy two conditions:
(a) The components satisfy (R-B)/B > fthresh (red or dark fg)
(b) The red component satisfied R > 128 (red or light bg)
Masks are generated for (a) and (b), and the intersection
gives the pixels that are red but not either light bg or
dark fg.
(2) A typical value for fract = 0.0001, which gives sensitivity
to an image where a small fraction of the pixels are printed
in red.
(3) A typical value for fthresh = 2.5. Higher values give less
sensitivity to red, and fewer false positives.
Definition at line 1757 of file colorcontent.c.
References CCBorda::h, and CCBorda::w.
| [in] | pixs | 32 bpp rgb or 8 bpp colormapped |
| [in] | threshdiff | threshold for minimum of the max difference between components |
| [in] | mindist | minimum allowed distance from nearest non-color pixel |
Notes:
(1) The generated mask identifies each pixel as either color or
non-color. For a pixel to be color, it must satisfy two
constraints:
(a) The max difference between the r,g and b components must
equal or exceed a threshold threshdiff.
(b) It must be at least mindist (in an 8-connected way)
from the nearest non-color pixel.
(2) The distance constraint (b) is only applied if mindist > 1.
For example, if mindist == 2, the color pixels identified
by (a) are eroded by a 3x3 Sel. In general, the Sel size
for erosion is 2 * (mindist - 1) + 1.
Why have this constraint? In scanned images that are
essentially gray, color artifacts are typically introduced
in transition regions near sharp edges that go from dark
to light, so this allows these transition regions to be removed.
Definition at line 502 of file colorcontent.c.
References CCBorda::h, pixGetDimensions(), and CCBorda::w.
| PIX* pixMaskOverColorRange | ( | PIX * | pixs, |
| l_int32 | rmin, | ||
| l_int32 | rmax, | ||
| l_int32 | gmin, | ||
| l_int32 | gmax, | ||
| l_int32 | bmin, | ||
| l_int32 | bmax | ||
| ) |
| [in] | pixs | 32 bpp rgb or 8 bpp colormapped |
| [in] | rmin,rmax | min and max allowed values for red component |
| [in] | gmin,gmax | |
| [in] | bmin,bmax |
Definition at line 569 of file colorcontent.c.
References CCBorda::h, pixGetDimensions(), and CCBorda::w.
| l_ok pixNumColors | ( | PIX * | pixs, |
| l_int32 | factor, | ||
| l_int32 * | pncolors | ||
| ) |
| [in] | pixs | 2, 4, 8, 32 bpp |
| [in] | factor | subsampling factor; integer |
| [out] | pncolors | the number of colors found, or 0 if there are more than 256 |
Notes:
(1) This returns the actual number of colors found in the image,
even if there is a colormap. If factor == 1 and the
number of colors differs from the number of entries
in the colormap, a warning is issued.
(2) Use factor == 1 to find the actual number of colors.
Use factor > 1 to quickly find the approximate number of colors.
(3) For d = 2, 4 or 8 bpp grayscale, this returns the number
of colors found in the image in 'ncolors'.
(4) For d = 32 bpp (rgb), if the number of colors is
greater than 256, this returns 0 in 'ncolors'.
Definition at line 1287 of file colorcontent.c.
References CCBorda::h, pixGetData(), pixGetDimensions(), and CCBorda::w.
| l_ok pixNumSignificantGrayColors | ( | PIX * | pixs, |
| l_int32 | darkthresh, | ||
| l_int32 | lightthresh, | ||
| l_float32 | minfract, | ||
| l_int32 | factor, | ||
| l_int32 * | pncolors | ||
| ) |
| [in] | pixs | 8 bpp gray |
| [in] | darkthresh | dark threshold for minimum intensity to be considered; typ. 20 |
| [in] | lightthresh | threshold near white, for maximum intensity to be considered; typ. 236 |
| [in] | minfract | minimum fraction of all pixels to include a level as significant; typ. 0.0001; should be < 0.001 |
| [in] | factor | subsample factor; integer >= 1 |
| [out] | pncolors | number of significant colors; 0 on error |
Notes:
(1) This function is asking the question: how many perceptually
significant gray color levels is in this pix?
A color level must meet 3 criteria to be significant:
~ it can't be too close to black
~ it can't be too close to white
~ it must have at least some minimum fractional population
(2) Use -1 for default values for darkthresh, lightthresh and minfract.
(3) Choose default of darkthresh = 20, because variations in very
dark pixels are not visually significant.
(4) Choose default of lightthresh = 236, because document images
that have been jpeg'd typically have near-white pixels in the
8x8 jpeg blocks, and these should not be counted. It is desirable
to obtain a clean image by quantizing this noise away.
Definition at line 1022 of file colorcontent.c.
References CCBorda::h, and CCBorda::w.
| [in] | pixs | 32 bpp rgb |
| [in] | sigbits | 2-4, significant bits retained in the quantizer for each component of the input image |
| [in] | factor | subsampling factor; use 1 for no subsampling |
| [in] | ncolors | the number of most populated colors to select |
Notes:
(1) If you want to do color quantization for real, use octcube
or modified median cut. This function shows that it is
easy to make a simple quantizer based solely on the population
in cells of a given size in rgb color space.
(2) The ncolors most populated cells at the sigbits level form
the colormap for quantizing, and this uses octcube indexing
under the covers to assign each pixel to the nearest color.
(3) sigbits is restricted to 2, 3 and 4. At the low end, the
color discrimination is very crude; at the upper end, a set of
similar colors can dominate the result. Interesting results
are generally found for sigbits = 3 and ncolors ~ 20.
(4) See also pixColorSegment() for a method of quantizing the
colors to generate regions of similar color.
Definition at line 1470 of file colorcontent.c.
References CCBorda::h, and CCBorda::w.
1.8.14