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Miscellaneous Image Transformations

AdaptiveThreshold

AdaptiveThreshold(src, dst, maxValue, adaptive_method=CV_ADAPTIVE_THRESH_MEAN_C, thresholdType=CV_THRESH_BINARY, blockSize=3, param1=5) → None

Applies an adaptive threshold to an array.

Parameters:
  • src (CvArr) – Source image
  • dst (CvArr) – Destination image
  • maxValue (float) – Maximum value that is used with CV_THRESH_BINARY and CV_THRESH_BINARY_INV
  • adaptive_method (int) – Adaptive thresholding algorithm to use: CV_ADAPTIVE_THRESH_MEAN_C or CV_ADAPTIVE_THRESH_GAUSSIAN_C (see the discussion)
  • thresholdType (int) –

    Thresholding type; must be one of

    • CV_THRESH_BINARY xxx
    • CV_THRESH_BINARY_INV xxx
  • blockSize (int) – The size of a pixel neighborhood that is used to calculate a threshold value for the pixel: 3, 5, 7, and so on
  • param1 (float) – The method-dependent parameter. For the methods CV_ADAPTIVE_THRESH_MEAN_C and CV_ADAPTIVE_THRESH_GAUSSIAN_C it is a constant subtracted from the mean or weighted mean (see the discussion), though it may be negative

The function transforms a grayscale image to a binary image according to the formulas:

  • CV_THRESH_BINARY

    dst(x,y) = \fork{\texttt{maxValue}}{if $src(x,y) > T(x,y)$}{0}{otherwise}

  • CV_THRESH_BINARY_INV

    dst(x,y) = \fork{0}{if $src(x,y) > T(x,y)$}{\texttt{maxValue}}{otherwise}

where T(x,y) is a threshold calculated individually for each pixel.

For the method CV_ADAPTIVE_THRESH_MEAN_C it is the mean of a \texttt{blockSize} \times \texttt{blockSize} pixel neighborhood, minus param1 .

For the method CV_ADAPTIVE_THRESH_GAUSSIAN_C it is the weighted sum (gaussian) of a \texttt{blockSize} \times \texttt{blockSize} pixel neighborhood, minus param1 .

CvtColor

CvtColor(src, dst, code) → None

Converts an image from one color space to another.

Parameters:
  • src (CvArr) – The source 8-bit (8u), 16-bit (16u) or single-precision floating-point (32f) image
  • dst (CvArr) – The destination image of the same data type as the source. The number of channels may be different
  • code (int) – Color conversion operation that can be specifed using CV_ *src_color_space* 2 *dst_color_space* constants (see below)

The function converts the input image from one color space to another. The function ignores the colorModel and channelSeq fields of the IplImage header, so the source image color space should be specified correctly (including order of the channels in the case of RGB space. For example, BGR means 24-bit format with B_0, G_0, R_0, B_1, G_1, R_1, ... layout whereas RGB means 24-format with R_0, G_0, B_0, R_1, G_1, B_1, ... layout).

The conventional range for R,G,B channel values is:

  • 0 to 255 for 8-bit images
  • 0 to 65535 for 16-bit images and
  • 0 to 1 for floating-point images.

Of course, in the case of linear transformations the range can be specific, but in order to get correct results in the case of non-linear transformations, the input image should be scaled.

The function can do the following transformations:

  • Transformations within RGB space like adding/removing the alpha channel, reversing the channel order, conversion to/from 16-bit RGB color (R5:G6:B5 or R5:G5:B5), as well as conversion to/from grayscale using:

    \text{RGB[A] to Gray:} Y \leftarrow 0.299 \cdot R + 0.587 \cdot G + 0.114 \cdot B

    and

    \text{Gray to RGB[A]:} R \leftarrow Y, G \leftarrow Y, B \leftarrow Y, A \leftarrow 0

    The conversion from a RGB image to gray is done with:

    cvCvtColor(src ,bwsrc, CV_RGB2GRAY)

  • RGB \leftrightarrow CIE XYZ.Rec 709 with D65 white point ( CV_BGR2XYZ, CV_RGB2XYZ, CV_XYZ2BGR, CV_XYZ2RGB ):

    \begin{bmatrix} X \\ Y \\ Z \end{bmatrix} \leftarrow \begin{bmatrix} 0.412453 & 0.357580 & 0.180423 \\ 0.212671 & 0.715160 & 0.072169 \\ 0.019334 & 0.119193 & 0.950227 \end{bmatrix} \cdot \begin{bmatrix} R \\ G \\ B \end{bmatrix}

    \begin{bmatrix} R \\ G \\ B \end{bmatrix} \leftarrow \begin{bmatrix} 3.240479 & -1.53715 & -0.498535 \\ -0.969256 & 1.875991 & 0.041556 \\ 0.055648 & -0.204043 & 1.057311 \end{bmatrix} \cdot \begin{bmatrix} X \\ Y \\ Z \end{bmatrix}

    X , Y and Z cover the whole value range (in the case of floating-point images Z may exceed 1).

  • RGB \leftrightarrow YCrCb JPEG (a.k.a. YCC) ( CV_BGR2YCrCb, CV_RGB2YCrCb, CV_YCrCb2BGR, CV_YCrCb2RGB )

    Y \leftarrow 0.299 \cdot R + 0.587 \cdot G + 0.114 \cdot B

    Cr \leftarrow (R-Y) \cdot 0.713 + delta

    Cb \leftarrow (B-Y) \cdot 0.564 + delta

    R \leftarrow Y + 1.403 \cdot (Cr - delta)

    G \leftarrow Y - 0.344 \cdot (Cr - delta) - 0.714 \cdot (Cb - delta)

    B \leftarrow Y + 1.773 \cdot (Cb - delta)

    where

    delta = \left \{ \begin{array}{l l} 128 & \mbox{for 8-bit images} \\ 32768 & \mbox{for 16-bit images} \\ 0.5 & \mbox{for floating-point images} \end{array} \right .

    Y, Cr and Cb cover the whole value range.

  • RGB \leftrightarrow HSV ( CV_BGR2HSV, CV_RGB2HSV, CV_HSV2BGR, CV_HSV2RGB ) in the case of 8-bit and 16-bit images R, G and B are converted to floating-point format and scaled to fit the 0 to 1 range

    V \leftarrow max(R,G,B)

    S \leftarrow \fork{\frac{V-min(R,G,B)}{V}}{if $V \neq 0$}{0}{otherwise}

    H \leftarrow \forkthree{{60(G - B)}/{S}}{if $V=R$}{{120+60(B - R)}/{S}}{if $V=G$}{{240+60(R - G)}/{S}}{if $V=B$}

    if H<0 then H \leftarrow H+360 On output 0 \leq V \leq 1 , 0 \leq S \leq 1 , 0 \leq H \leq 360 .

    The values are then converted to the destination data type:

    • 8-bit images

      V \leftarrow 255 V, S \leftarrow 255 S, H \leftarrow H/2 \text{(to fit to 0 to 255)}

    • 16-bit images (currently not supported)

      V <- 65535 V, S <- 65535 S, H <- H

    • 32-bit images

      H, S, V are left as is

  • RGB \leftrightarrow HLS ( CV_BGR2HLS, CV_RGB2HLS, CV_HLS2BGR, CV_HLS2RGB ). in the case of 8-bit and 16-bit images R, G and B are converted to floating-point format and scaled to fit the 0 to 1 range.

    V_{max} \leftarrow {max}(R,G,B)

    V_{min} \leftarrow {min}(R,G,B)

    L \leftarrow \frac{V_{max} + V_{min}}{2}

    S \leftarrow \fork{\frac{V_{max} - V_{min}}{V_{max} + V_{min}}}{if $L < 0.5$}{\frac{V_{max} - V_{min}}{2 - (V_{max} + V_{min})}}{if $L \ge 0.5$}

    H \leftarrow \forkthree{{60(G - B)}/{S}}{if $V_{max}=R$}{{120+60(B - R)}/{S}}{if $V_{max}=G$}{{240+60(R - G)}/{S}}{if $V_{max}=B$}

    if H<0 then H \leftarrow H+360 On output 0 \leq L \leq 1 , 0 \leq S \leq 1 , 0 \leq H \leq 360 .

    The values are then converted to the destination data type:

    • 8-bit images

      V \leftarrow 255 V, S \leftarrow 255 S, H \leftarrow H/2 \text{(to fit to 0 to 255)}

    • 16-bit images (currently not supported)

      V <- 65535 V, S <- 65535 S, H <- H

    • 32-bit images

      H, S, V are left as is

  • RGB \leftrightarrow CIE L*a*b* ( CV_BGR2Lab, CV_RGB2Lab, CV_Lab2BGR, CV_Lab2RGB ) in the case of 8-bit and 16-bit images R, G and B are converted to floating-point format and scaled to fit the 0 to 1 range

    \vecthree{X}{Y}{Z} \leftarrow \vecthreethree{0.412453}{0.357580}{0.180423}{0.212671}{0.715160}{0.072169}{0.019334}{0.119193}{0.950227} \cdot \vecthree{R}{G}{B}

    X \leftarrow X/X_n, \text{where} X_n = 0.950456

    Z \leftarrow Z/Z_n, \text{where} Z_n = 1.088754

    L \leftarrow \fork{116*Y^{1/3}-16}{for $Y>0.008856$}{903.3*Y}{for $Y \le 0.008856$}

    a \leftarrow 500 (f(X)-f(Y)) + delta

    b \leftarrow 200 (f(Y)-f(Z)) + delta

    where

    f(t)= \fork{t^{1/3}}{for $t>0.008856$}{7.787 t+16/116}{for $t<=0.008856$}

    and

    delta = \fork{128}{for 8-bit images}{0}{for floating-point images}

    On output 0 \leq L \leq 100 , -127 \leq a \leq 127 , -127 \leq b \leq 127 The values are then converted to the destination data type:

    • 8-bit images

      L \leftarrow L*255/100, a \leftarrow a + 128, b \leftarrow b + 128

    • 16-bit images

      currently not supported

    • 32-bit images

      L, a, b are left as is

  • RGB \leftrightarrow CIE L*u*v* ( CV_BGR2Luv, CV_RGB2Luv, CV_Luv2BGR, CV_Luv2RGB ) in the case of 8-bit and 16-bit images R, G and B are converted to floating-point format and scaled to fit 0 to 1 range

    \vecthree{X}{Y}{Z} \leftarrow \vecthreethree{0.412453}{0.357580}{0.180423}{0.212671}{0.715160}{0.072169}{0.019334}{0.119193}{0.950227} \cdot \vecthree{R}{G}{B}

    L \leftarrow \fork{116 Y^{1/3}}{for $Y>0.008856$}{903.3 Y}{for $Y<=0.008856$}

    u' \leftarrow 4*X/(X + 15*Y + 3 Z)

    v' \leftarrow 9*Y/(X + 15*Y + 3 Z)

    u \leftarrow 13*L*(u' - u_n) \quad \text{where} \quad u_n=0.19793943

    v \leftarrow 13*L*(v' - v_n) \quad \text{where} \quad v_n=0.46831096

    On output 0 \leq L \leq 100 , -134 \leq u \leq 220 , -140 \leq v \leq 122 .

    The values are then converted to the destination data type:

    • 8-bit images

      L \leftarrow 255/100 L, u \leftarrow 255/354 (u + 134), v \leftarrow 255/256 (v + 140)

    • 16-bit images

      currently not supported

    • 32-bit images

      L, u, v are left as is

    The above formulas for converting RGB to/from various color spaces have been taken from multiple sources on Web, primarily from the Ford98 at the Charles Poynton site.

  • Bayer \rightarrow RGB ( CV_BayerBG2BGR, CV_BayerGB2BGR, CV_BayerRG2BGR, CV_BayerGR2BGR, CV_BayerBG2RGB, CV_BayerGB2RGB, CV_BayerRG2RGB, CV_BayerGR2RGB ) The Bayer pattern is widely used in CCD and CMOS cameras. It allows one to get color pictures from a single plane where R,G and B pixels (sensors of a particular component) are interleaved like this:

    \newcommand{\Rcell}{\color{red}R} \newcommand{\Gcell}{\color{green}G} \newcommand{\Bcell}{\color{blue}B} \definecolor{BackGray}{rgb}{0.8,0.8,0.8} \begin{array}{ c c c c c } \Rcell & \Gcell & \Rcell & \Gcell & \Rcell \\ \Gcell & \colorbox{BackGray}{\Bcell} & \colorbox{BackGray}{\Gcell} & \Bcell & \Gcell \\ \Rcell & \Gcell & \Rcell & \Gcell & \Rcell \\ \Gcell & \Bcell & \Gcell & \Bcell & \Gcell \\ \Rcell & \Gcell & \Rcell & \Gcell & \Rcell \end{array}

    The output RGB components of a pixel are interpolated from 1, 2 or 4 neighbors of the pixel having the same color. There are several modifications of the above pattern that can be achieved by shifting the pattern one pixel left and/or one pixel up. The two letters C_1 and C_2 in the conversion constants CV_Bayer C_1 C_2 2BGR and CV_Bayer C_1 C_2 2RGB indicate the particular pattern type - these are components from the second row, second and third columns, respectively. For example, the above pattern has very popular “BG” type.

DistTransform

DistTransform(src, dst, distance_type=CV_DIST_L2, mask_size=3, mask=None, labels=NULL) → None

Calculates the distance to the closest zero pixel for all non-zero pixels of the source image.

Parameters:
  • src (CvArr) – 8-bit, single-channel (binary) source image
  • dst (CvArr) – Output image with calculated distances (32-bit floating-point, single-channel)
  • distance_type (int) – Type of distance; can be CV_DIST_L1, CV_DIST_L2, CV_DIST_C or CV_DIST_USER
  • mask_size (int) – Size of the distance transform mask; can be 3 or 5. in the case of CV_DIST_L1 or CV_DIST_C the parameter is forced to 3, because a 3\times 3 mask gives the same result as a 5\times 5 yet it is faster
  • mask (sequence of int) – User-defined mask in the case of a user-defined distance, it consists of 2 numbers (horizontal/vertical shift cost, diagonal shift cost) in the case ofa 3\times 3 mask and 3 numbers (horizontal/vertical shift cost, diagonal shift cost, knight’s move cost) in the case of a 5\times 5 mask
  • labels (CvArr) – The optional output 2d array of integer type labels, the same size as src and dst

The function calculates the approximated distance from every binary image pixel to the nearest zero pixel. For zero pixels the function sets the zero distance, for others it finds the shortest path consisting of basic shifts: horizontal, vertical, diagonal or knight’s move (the latest is available for a 5\times 5 mask). The overall distance is calculated as a sum of these basic distances. Because the distance function should be symmetric, all of the horizontal and vertical shifts must have the same cost (that is denoted as a ), all the diagonal shifts must have the same cost (denoted b ), and all knight’s moves must have the same cost (denoted c ). For CV_DIST_C and CV_DIST_L1 types the distance is calculated precisely, whereas for CV_DIST_L2 (Euclidian distance) the distance can be calculated only with some relative error (a 5\times 5 mask gives more accurate results), OpenCV uses the values suggested in [Borgefors86] :

CV_DIST_C (3\times 3) a = 1, b = 1
CV_DIST_L1 (3\times 3) a = 1, b = 2
CV_DIST_L2 (3\times 3) a=0.955, b=1.3693
CV_DIST_L2 (5\times 5) a=1, b=1.4, c=2.1969

And below are samples of the distance field (black (0) pixel is in the middle of white square) in the case of a user-defined distance:

User-defined 3 \times 3 mask (a=1, b=1.5)

4.5 4 3.5 3 3.5 4 4.5
4 3 2.5 2 2.5 3 4
3.5 2.5 1.5 1 1.5 2.5 3.5
3 2 1   1 2 3
3.5 2.5 1.5 1 1.5 2.5 3.5
4 3 2.5 2 2.5 3 4
4.5 4 3.5 3 3.5 4 4.5

User-defined 5 \times 5 mask (a=1, b=1.5, c=2)

4.5 3.5 3 3 3 3.5 4.5
3.5 3 2 2 2 3 3.5
3 2 1.5 1 1.5 2 3
3 2 1   1 2 3
3 2 1.5 1 1.5 2 3
3.5 3 2 2 2 3 3.5
4 3.5 3 3 3 3.5 4

Typically, for a fast, coarse distance estimation CV_DIST_L2 , a 3\times 3 mask is used, and for a more accurate distance estimation CV_DIST_L2 , a 5\times 5 mask is used.

When the output parameter labels is not NULL , for every non-zero pixel the function also finds the nearest connected component consisting of zero pixels. The connected components themselves are found as contours in the beginning of the function.

In this mode the processing time is still O(N), where N is the number of pixels. Thus, the function provides a very fast way to compute approximate Voronoi diagram for the binary image.

CvConnectedComp

class CvConnectedComp

Connected component, represented as a tuple (area, value, rect), where area is the area of the component as a float, value is the average color as a CvScalar , and rect is the ROI of the component, as a CvRect .

FloodFill

FloodFill(image, seed_point, new_val, lo_diff=(0, 0, 0, 0), up_diff=(0, 0, 0, 0), flags=4, mask=NULL) → comp

Fills a connected component with the given color.

Parameters:
  • image (CvArr) – Input 1- or 3-channel, 8-bit or floating-point image. It is modified by the function unless the CV_FLOODFILL_MASK_ONLY flag is set (see below)
  • seed_point (CvPoint) – The starting point
  • new_val (CvScalar) – New value of the repainted domain pixels
  • lo_diff (CvScalar) – Maximal lower brightness/color difference between the currently observed pixel and one of its neighbors belonging to the component, or a seed pixel being added to the component. In the case of 8-bit color images it is a packed value
  • up_diff (CvScalar) – Maximal upper brightness/color difference between the currently observed pixel and one of its neighbors belonging to the component, or a seed pixel being added to the component. In the case of 8-bit color images it is a packed value
  • comp (CvConnectedComp) – Returned connected component for the repainted domain. Note that the function does not fill comp->contour field. The boundary of the filled component can be retrieved from the output mask image using FindContours
  • flags (int) –

    The operation flags. Lower bits contain connectivity value, 4 (by default) or 8, used within the function. Connectivity determines which neighbors of a pixel are considered. Upper bits can be 0 or a combination of the following flags:

    • CV_FLOODFILL_FIXED_RANGE if set, the difference between the current pixel and seed pixel is considered, otherwise the difference between neighbor pixels is considered (the range is floating)
    • CV_FLOODFILL_MASK_ONLY if set, the function does not fill the image ( new_val is ignored), but fills the mask (that must be non-NULL in this case)
  • mask (CvArr) – Operation mask, should be a single-channel 8-bit image, 2 pixels wider and 2 pixels taller than image . If not NULL, the function uses and updates the mask, so the user takes responsibility of initializing the mask content. Floodfilling can’t go across non-zero pixels in the mask, for example, an edge detector output can be used as a mask to stop filling at edges. It is possible to use the same mask in multiple calls to the function to make sure the filled area do not overlap. Note : because the mask is larger than the filled image, a pixel in mask that corresponds to (x,y) pixel in image will have coordinates (x+1,y+1)

The function fills a connected component starting from the seed point with the specified color. The connectivity is determined by the closeness of pixel values. The pixel at (x,y) is considered to belong to the repainted domain if:

  • grayscale image, floating range

    src(x',y')- \texttt{lo\_diff} <= src(x,y) <= src(x',y')+ \texttt{up\_diff}

  • grayscale image, fixed range

    src(seed.x,seed.y)- \texttt{lo\_diff} <=src(x,y)<=src(seed.x,seed.y)+ \texttt{up\_diff}

  • color image, floating range

    src(x',y')_r- \texttt{lo\_diff} _r<=src(x,y)_r<=src(x',y')_r+ \texttt{up\_diff} _r

    src(x',y')_g- \texttt{lo\_diff} _g<=src(x,y)_g<=src(x',y')_g+ \texttt{up\_diff} _g

    src(x',y')_b- \texttt{lo\_diff} _b<=src(x,y)_b<=src(x',y')_b+ \texttt{up\_diff} _b

  • color image, fixed range

    src(seed.x,seed.y)_r- \texttt{lo\_diff} _r<=src(x,y)_r<=src(seed.x,seed.y)_r+ \texttt{up\_diff} _r

    src(seed.x,seed.y)_g- \texttt{lo\_diff} _g<=src(x,y)_g<=src(seed.x,seed.y)_g+ \texttt{up\_diff} _g

    src(seed.x,seed.y)_b- \texttt{lo\_diff} _b<=src(x,y)_b<=src(seed.x,seed.y)_b+ \texttt{up\_diff} _b

where src(x',y') is the value of one of pixel neighbors. That is, to be added to the connected component, a pixel’s color/brightness should be close enough to the:

  • color/brightness of one of its neighbors that are already referred to the connected component in the case of floating range
  • color/brightness of the seed point in the case of fixed range.

Inpaint

Inpaint(src, mask, dst, inpaintRadius, flags) → None

Inpaints the selected region in the image.

Parameters:
  • src (CvArr) – The input 8-bit 1-channel or 3-channel image.
  • mask (CvArr) – The inpainting mask, 8-bit 1-channel image. Non-zero pixels indicate the area that needs to be inpainted.
  • dst (CvArr) – The output image of the same format and the same size as input.
  • inpaintRadius (float) – The radius of circlular neighborhood of each point inpainted that is considered by the algorithm.
  • flags (int) –

    The inpainting method, one of the following:

    • CV_INPAINT_NS Navier-Stokes based method.
    • CV_INPAINT_TELEA The method by Alexandru Telea [Telea04]

The function reconstructs the selected image area from the pixel near the area boundary. The function may be used to remove dust and scratches from a scanned photo, or to remove undesirable objects from still images or video.

Integral

Integral(image, sum, sqsum=NULL, tiltedSum=NULL) → None

Calculates the integral of an image.

Parameters:
  • image (CvArr) – The source image, W\times H , 8-bit or floating-point (32f or 64f)
  • sum (CvArr) – The integral image, (W+1)\times (H+1) , 32-bit integer or double precision floating-point (64f)
  • sqsum (CvArr) – The integral image for squared pixel values, (W+1)\times (H+1) , double precision floating-point (64f)
  • tiltedSum (CvArr) – The integral for the image rotated by 45 degrees, (W+1)\times (H+1) , the same data type as sum

The function calculates one or more integral images for the source image as following:

\texttt{sum} (X,Y) = \sum _{x<X,y<Y} \texttt{image} (x,y)

\texttt{sqsum} (X,Y) = \sum _{x<X,y<Y} \texttt{image} (x,y)^2

\texttt{tiltedSum} (X,Y) = \sum _{y<Y,abs(x-X+1) \leq Y-y-1} \texttt{image} (x,y)

Using these integral images, one may calculate sum, mean and standard deviation over a specific up-right or rotated rectangular region of the image in a constant time, for example:

\sum _{x_1<=x<x_2, \, y_1<=y<y_2} = \texttt{sum} (x_2,y_2)- \texttt{sum} (x_1,y_2)- \texttt{sum} (x_2,y_1)+ \texttt{sum} (x_1,x_1)

It makes possible to do a fast blurring or fast block correlation with variable window size, for example. In the case of multi-channel images, sums for each channel are accumulated independently.

PyrMeanShiftFiltering

PyrMeanShiftFiltering(src, dst, sp, sr, max_level=1, termcrit=(CV_TERMCRIT_ITER+CV_TERMCRIT_EPS, 5, 1)) → None

Does meanshift image segmentation

Parameters:
  • src (CvArr) – The source 8-bit, 3-channel image.
  • dst (CvArr) – The destination image of the same format and the same size as the source.
  • sp (float) – The spatial window radius.
  • sr (float) – The color window radius.
  • max_level (int) – Maximum level of the pyramid for the segmentation.
  • termcrit (CvTermCriteria) – Termination criteria: when to stop meanshift iterations.

The function implements the filtering stage of meanshift segmentation, that is, the output of the function is the filtered “posterized” image with color gradients and fine-grain texture flattened. At every pixel (X,Y) of the input image (or down-sized input image, see below) the function executes meanshift iterations, that is, the pixel (X,Y) neighborhood in the joint space-color hyperspace is considered:

(x,y): X- \texttt{sp} \le x \le X+ \texttt{sp} , Y- \texttt{sp} \le y \le Y+ \texttt{sp} , ||(R,G,B)-(r,g,b)|| \le \texttt{sr}

where (R,G,B) and (r,g,b) are the vectors of color components at (X,Y) and (x,y) , respectively (though, the algorithm does not depend on the color space used, so any 3-component color space can be used instead). Over the neighborhood the average spatial value (X',Y') and average color vector (R',G',B') are found and they act as the neighborhood center on the next iteration:

(X,Y)~(X',Y'), (R,G,B)~(R',G',B'). After the iterations over, the color components of the initial pixel (that is, the pixel from where the iterations started) are set to the final value (average color at the last iteration):

I(X,Y) <- (R*,G*,B*) Then \texttt{max\_level}>0 , the gaussian pyramid of \texttt{max\_level}+1 levels is built, and the above procedure is run on the smallest layer. After that, the results are propagated to the larger layer and the iterations are run again only on those pixels where the layer colors differ much ( >\texttt{sr} ) from the lower-resolution layer, that is, the boundaries of the color regions are clarified. Note, that the results will be actually different from the ones obtained by running the meanshift procedure on the whole original image (i.e. when \texttt{max\_level}==0 ).

PyrSegmentation

PyrSegmentation(src, dst, storage, level, threshold1, threshold2) → comp

Implements image segmentation by pyramids.

Parameters:
  • src (IplImage) – The source image
  • dst (IplImage) – The destination image
  • storage (CvMemStorage) – Storage; stores the resulting sequence of connected components
  • comp (CvSeq) – Pointer to the output sequence of the segmented components
  • level (int) – Maximum level of the pyramid for the segmentation
  • threshold1 (float) – Error threshold for establishing the links
  • threshold2 (float) – Error threshold for the segments clustering

The function implements image segmentation by pyramids. The pyramid builds up to the level level . The links between any pixel a on level i and its candidate father pixel b on the adjacent level are established if p(c(a),c(b))<threshold1 . After the connected components are defined, they are joined into several clusters. Any two segments A and B belong to the same cluster, if p(c(A),c(B))<threshold2 . If the input image has only one channel, then p(c^1,c^2)=|c^1-c^2| . If the input image has three channels (red, green and blue), then

p(c^1,c^2) = 0.30 (c^1_r - c^2_r) + 0.59 (c^1_g - c^2_g) + 0.11 (c^1_b - c^2_b).

There may be more than one connected component per a cluster. The images src and dst should be 8-bit single-channel or 3-channel images or equal size.

Threshold

Threshold(src, dst, threshold, maxValue, thresholdType) → None

Applies a fixed-level threshold to array elements.

Parameters:
  • src (CvArr) – Source array (single-channel, 8-bit or 32-bit floating point)
  • dst (CvArr) – Destination array; must be either the same type as src or 8-bit
  • threshold (float) – Threshold value
  • maxValue (float) – Maximum value to use with CV_THRESH_BINARY and CV_THRESH_BINARY_INV thresholding types
  • thresholdType (int) – Thresholding type (see the discussion)

The function applies fixed-level thresholding to a single-channel array. The function is typically used to get a bi-level (binary) image out of a grayscale image ( CmpS could be also used for this purpose) or for removing a noise, i.e. filtering out pixels with too small or too large values. There are several types of thresholding that the function supports that are determined by thresholdType :

  • CV_THRESH_BINARY

    \texttt{dst} (x,y) = \fork{\texttt{maxValue}}{if $\texttt{src}(x,y) > \texttt{threshold}$}{0}{otherwise}

  • CV_THRESH_BINARY_INV

    \texttt{dst} (x,y) = \fork{0}{if $\texttt{src}(x,y) > \texttt{threshold}$}{\texttt{maxValue}}{otherwise}

  • CV_THRESH_TRUNC

    \texttt{dst} (x,y) = \fork{\texttt{threshold}}{if $\texttt{src}(x,y) > \texttt{threshold}$}{\texttt{src}(x,y)}{otherwise}

  • CV_THRESH_TOZERO

    \texttt{dst} (x,y) = \fork{\texttt{src}(x,y)}{if $\texttt{src}(x,y) > \texttt{threshold}$}{0}{otherwise}

  • CV_THRESH_TOZERO_INV

    \texttt{dst} (x,y) = \fork{0}{if $\texttt{src}(x,y) > \texttt{threshold}$}{\texttt{src}(x,y)}{otherwise}

Also, the special value CV_THRESH_OTSU may be combined with one of the above values. In this case the function determines the optimal threshold value using Otsu’s algorithm and uses it instead of the specified thresh . The function returns the computed threshold value. Currently, Otsu’s method is implemented only for 8-bit images.

_images/threshold.png

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