Color conversions

See cv::cvtColor and cv::ColorConversionCodes

Todo:

document other conversion modes

RGB <-> GRAY

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:

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

and

\[\text{Gray to RGB[A]:} \quad R \leftarrow Y, G \leftarrow Y, B \leftarrow Y, A \leftarrow \max (ChannelRange)\]

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

cvtColor(src, bwsrc, cv::COLOR_RGB2GRAY);

More advanced channel reordering can also be done with cv::mixChannels.

See also

cv::COLOR_BGR2GRAY, cv::COLOR_RGB2GRAY, cv::COLOR_GRAY2BGR, cv::COLOR_GRAY2RGB

RGB <-> CIE XYZ.Rec 709 with D65 white point

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

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

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

See also

cv::COLOR_BGR2XYZ, cv::COLOR_RGB2XYZ, cv::COLOR_XYZ2BGR, cv::COLOR_XYZ2RGB

RGB <-> YCrCb JPEG (or YCC)

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

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

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

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

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

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

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.

See also

cv::COLOR_BGR2YCrCb, cv::COLOR_RGB2YCrCb, cv::COLOR_YCrCb2BGR, cv::COLOR_YCrCb2RGB

RGB <-> YUV with subsampling

Only 8-bit values are supported. The coefficients correspond to BT.601 standard with resulting values Y [16, 235], U and V [16, 240] centered at 128.

Two subsampling schemes are supported: 4:2:0 (Fourcc codes NV12, NV21, YV12, I420 and synonimic) and 4:2:2 (Fourcc codes UYVY, YUY2, YVYU and synonimic).

In both subsampling schemes Y values are written for each pixel so that Y plane is in fact a scaled and biased gray version of a source image.

In 4:2:0 scheme U and V values are averaged over 2x2 squares, i.e. only 1 U and 1 V value is saved per each 4 pixels. U and V values are saved interleaved into a separate plane (NV12, NV21) or into two separate semi-planes (YV12, I420).

In 4:2:2 scheme U and V values are averaged horizontally over each pair of pixels, i.e. only 1 U and 1 V value is saved per each 2 pixels. U and V values are saved interleaved with Y values for both pixels according to its Fourcc code.

Note that different conversions are perfomed with different precision for speed or compatibility purposes. For example, RGB to YUV 4:2:2 is converted using 14-bit fixed-point arithmetics while other conversions use 20 bits.

\[R \leftarrow 1.164 \cdot (Y - 16) + 1.596 \cdot (V - 128)\]

\[G \leftarrow 1.164 \cdot (Y - 16) - 0.813 \cdot (V - 128) - 0.391 \cdot (U - 128)\]

\[B \leftarrow 1.164 \cdot (Y - 16) + 2.018 \cdot (U - 128)\]

\[Y \leftarrow (R \cdot 0.299 + G \cdot 0.587 + B \cdot 0.114) \cdot \frac{236 - 16}{256} + 16 \]

\[U \leftarrow -0.148 \cdot R_{avg} - 0.291 \cdot G_{avg} + 0.439 \cdot B_{avg} + 128 \]

\[V \leftarrow 0.439 \cdot R_{avg} - 0.368 \cdot G_{avg} - 0.071 \cdot B_{avg} + 128 \]

See also

cv::COLOR_YUV2RGB_NV12, cv::COLOR_YUV2RGBA_YUY2, cv::COLOR_BGR2YUV_YV12 and similar ones

RGB <-> HSV

In case of 8-bit and 16-bit images, R, G, and B are converted to the 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 \forkfour{{60(G - B)}/{(V-min(R,G,B))}}{if \(V=R\)} {{120+60(B - R)}/{(V-min(R,G,B))}}{if \(V=G\)} {{240+60(R - G)}/{(V-min(R,G,B))}}{if \(V=B\)} {0}{if \(R=G=B\)}\]

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) \(V \leftarrow 65535 V, S \leftarrow 65535 S, H \leftarrow H\)
• 32-bit images: H, S, and V are left as is

See also

cv::COLOR_BGR2HSV, cv::COLOR_RGB2HSV, cv::COLOR_HSV2BGR, cv::COLOR_HSV2RGB

RGB <-> HLS

In case of 8-bit and 16-bit images, R, G, and B are converted to the 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 \forkfour {{60(G - B)}/{(V_{max}-V_{min})}}{if \(V_{max}=R\) } {{120+60(B - R)}/{(V_{max}-V_{min})}}{if \(V_{max}=G\) } {{240+60(R - G)}/{(V_{max}-V_{min})}}{if \(V_{max}=B\) } {0}{if \(R=G=B\) }\]

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: \(V \leftarrow 255 \cdot V, S \leftarrow 255 \cdot S, H \leftarrow H/2 \; \text{(to fit to 0 to 255)}\)
• 16-bit images: (currently not supported) \(V \leftarrow 65535 \cdot V, S \leftarrow 65535 \cdot S, H \leftarrow H\)
• 32-bit images: H, S, V are left as is

See also

cv::COLOR_BGR2HLS, cv::COLOR_RGB2HLS, cv::COLOR_HLS2BGR, cv::COLOR_HLS2RGB

RGB <-> CIE L*a*b*

In case of 8-bit and 16-bit images, R, G, and B are converted to the 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 \(t\leq 0.008856\)}\]

and

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

This outputs \(0 \leq L \leq 100\), \(-127 \leq a \leq 127\), \(-127 \leq b \leq 127\) . 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, and b are left as is

See also

cv::COLOR_BGR2Lab, cv::COLOR_RGB2Lab, cv::COLOR_Lab2BGR, cv::COLOR_Lab2RGB

RGB <-> CIE L*u*v*

In case of 8-bit and 16-bit images, R, G, and B are converted to the 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} - 16}{for \(Y>0.008856\)}{903.3 Y}{for \(Y\leq 0.008856\)}\]

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

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

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

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

This outputs \(0 \leq L \leq 100\), \(-134 \leq u \leq 220\), \(-140 \leq v \leq 122\) .

The values are then converted to the destination data type:

• 8-bit images: \(L \leftarrow 255/100 L, \; u \leftarrow 255/354 (u + 134), \; v \leftarrow 255/262 (v + 140)\)
• 16-bit images: (currently not supported)
• 32-bit images: L, u, and v are left as is

Note that when converting integer Luv images to RGB the intermediate X, Y and Z values are truncated to \( [0, 2] \) range to fit white point limitations. It may lead to incorrect representation of colors with odd XYZ values.

The above formulae for converting RGB to/from various color spaces have been taken from multiple sources on the web, primarily from the Charles Poynton site http://www.poynton.com/ColorFAQ.html

See also

cv::COLOR_BGR2Luv, cv::COLOR_RGB2Luv, cv::COLOR_Luv2BGR, cv::COLOR_Luv2RGB

Bayer -> RGB

The Bayer pattern is widely used in CCD and CMOS cameras. It enables you to get color pictures from a single plane where R, G, and B pixels (sensors of a particular component) are interleaved as follows:

Bayer patterns (BGGR, GBRG, GRGB, RGGB)

The output RGB components of a pixel are interpolated from 1, 2, or 4 neighbors of the pixel having the same color.

Note

See the following for information about correspondences between OpenCV Bayer pattern naming and classical Bayer pattern naming.

Bayer pattern

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 a very popular "BG" type.

See also

cv::COLOR_BayerRGGB2BGR, cv::COLOR_BayerGRBG2BGR, cv::COLOR_BayerBGGR2BGR, cv::COLOR_BayerGBRG2BGR, cv::COLOR_BayerRGGB2RGB, cv::COLOR_BayerGRBG2RGB, cv::COLOR_BayerBGGR2RGB, cv::COLOR_BayerGBRG2RGB cv::COLOR_BayerBG2BGR, cv::COLOR_BayerGB2BGR, cv::COLOR_BayerRG2BGR, cv::COLOR_BayerGR2BGR, cv::COLOR_BayerBG2RGB, cv::COLOR_BayerGB2RGB, cv::COLOR_BayerRG2RGB, cv::COLOR_BayerGR2RGB