Color Space Converter

Convert color information between color spaces

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Libraries:
Vision HDL Toolbox / Conversions

Description

The Color Space Converter block converts between R'G'B' and Y'CbCr color spaces, and also converts R'G'B' to intensity.

Note

The Color Space Converter block operates on gamma-corrected color spaces, usually indicated with prime notation ('). However, for simplicity, the block and mask labels do not include the prime notation.

Examples

Convert RGB Image to YCbCr 4:2:2 Color Space

Convert a pixel stream from R'G'B' color space to Y'CbCr 4:2:2 color space.

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Ports

This block uses a streaming pixel interface with a pixelcontrol bus for frame control signals. This interface enables the block to operate independently of image size and format. All Vision HDL Toolbox™ blocks use the same streaming interface. The block accepts and returns a scalar pixel value and a bus that contains five control signals. The control signals indicate the validity of each pixel and its location in the frame. To convert a frame (pixel matrix) into a serial pixel stream and control signals, use the Frame To Pixels block. For a full description of the interface, see Streaming Pixel Interface.

This block also supports multipixel streams, where the pixel input is a matrix of M-by-3 values. M is the number of pixels and each pixel has 3 R'G'B' or Y'CbCr components. These values correspond to the Number of pixels and Number of components parameters of the Frame To Pixels block.

Input

pixel — Input pixel stream

vector | matrix

For scalar pixel streams, specify pixel as a vector of 1-by-3 values. For multipixel streams, specify pixel as a matrix of Number of pixels-by-3 pixel intensity values. Number of pixels can be two, four, or eight.

The pixel stream must be in Y'CbCr or R'G'B' color space. Integer and fixed-point data types must be between 8 and 16 bits.

The software supports double and single data types for simulation, but not for HDL code generation.

Data Types: single | double | uint8 | uint16 | fixed point

ctrl — Control signals associated with pixel stream

pixelcontrol bus

The pixelcontrol bus contains five signals. The signals describe the validity of the pixel and its location in the frame. For more information, see Pixel Control Bus.

For multipixel streaming, each vector of pixel values has one set of control signals. Because the vector has only one valid signal, the pixels in the vector must be either all valid or all invalid. The hStart and vStart signals apply to the pixel with the lowest index in the vector. The hEnd and vEnd signals apply to the pixel with the highest index in the vector.

Data Types: bus

Output

pixel — Output pixel stream in new colorspace

scalar | vector | matrix

Output pixel stream in intensity, Y'CbCr, or R'G'B' color space, returned as a single pixel stream or multipixel stream. The data type and Number of pixels of the output stream is the same as the input pixel stream. If the output is intensity values, each pixel has one component. If the output is Y'CbCr or R'G'B', each pixel has three components.

The software supports double and single data types for simulation, but not for HDL code generation.

Data Types: single | double | uint8 | uint16 | fixed point

ctrl — Control signals associated with pixel stream

pixelcontrol bus

The pixelcontrol bus contains five signals. The signals describe the validity of the pixel and its location in the frame. For more information, see Pixel Control Bus.

For multipixel streaming, each vector of pixel values has one set of control signals. Because the vector has only one valid signal, the pixels in the vector must be either all valid or all invalid. The hStart and vStart signals apply to the pixel with the lowest index in the vector. The hEnd and vEnd signals apply to the pixel with the highest index in the vector.

Data Types: bus

Parameters

Conversion — Type of color space conversion

RGB to YCbCr (default) | YCbCr to RGB | RGB to intensity

The block accepts input pixels as vectors of three values that represent a single pixel. If you choose RGB to intensity, each output pixel is a scalar. Otherwise, each output pixel is a vector of three values.

Use conversion specified by — Conversion equation

Rec. 601 (SDTV) (default) | Rec. 709 (HDTV)

Conversion equation used between R'G'B' and Y'CbCr color spaces.

Dependencies

This parameter applies only when you set Conversion to RGB to YCbCr or YCbCr to RGB.

Scanning standard — HDTV scanning standard

1250/50/2:1 (default) | 1125/60/2:1

Scanning standard used to convert between R'G'B' and Y'CbCr color spaces in HDTV format.

Dependencies

This parameter applies when you set Use conversion specified by to Rec. 709 (HDTV).

Algorithms

When you use multipixel streaming, the block replicates the conversion algorithm for each of the M input pixels, in parallel. This increase in hardware resources is a trade off for increasing throughput compared to single-pixel streaming.

Conversion Between R'G'B' and Y'CbCr Color Spaces

The following equations define R'G'B' to Y'CbCr conversion and Y'CbCr to R'G'B' conversion:

$$\left[\begin{array}{c}{Y}^{\prime }\\ Cb\\ Cr\end{array}\right]=\left[\begin{array}{c}16\\ 128\\ 128\end{array}\right]+{\rm A}\times \left[\begin{array}{c}{R}^{\prime }\\ G^{\prime }\\ B^{\prime }\end{array}\right]$$

$$\left[\begin{array}{c}{R}^{\prime }\\ G^{\prime }\\ B^{\prime }\end{array}\right]={\rm B}\times \left(\left[\begin{array}{c}{Y}^{\prime }\\ Cb\\ Cr\end{array}\right]-\left[\begin{array}{c}16\\ 128\\ 128\end{array}\right]\right)$$

The values in matrices A and B are based on your choices for the Use conversion specified by and Scanning standard parameters.

Matrix	Use conversion specified by = Rec. 601 (SDTV)	Use conversion specified by = Rec. 709 (HDTV)
		Scanning standard = 1125/60/2:1	Scanning standard = 1250/50/2:1
A	$$\left[\begin{array}{ccc}0.25678824& 0.50412941& 0.09790588\\ -0.1482229& -0.29099279& 0.43921569\\ 0.43921569& -0.36778831& -0.07142737\end{array}\right]$$	$$\left[\begin{array}{l}\text{ 0}\text{.18258588 0}\text{.61423059 0}\text{.06200706}\\ \text{ -0}\text{.10064373 -0}\text{.33857195 0}\text{.43921569}\\ \text{ 0}\text{.43921569 -0}\text{.39894216 -0}\text{.04027352}\end{array}\right]$$	$$\left[\begin{array}{ccc}0.25678824& 0.50412941& 0.09790588\\ -0.1482229& -0.29099279& 0.43921569\\ 0.43921569& -0.36778831& -0.07142737\end{array}\right]$$
B	$$\left[\begin{array}{ccc}1.1643836& 0& 1.5960268\\ 1.1643836& -0.39176229& -0.81296765\\ 1.1643836& 2.0172321& 0\end{array}\right]$$	$$\left[\begin{array}{ccc}\text{1}\text{.16438356}& \text{0}& \text{1}\text{.79274107}\\ \text{1}\text{.16438356}& \text{-0}\text{.21324861}& \text{-0}\text{.53290933}\\ \text{1}\text{.16438356}& \text{2}\text{.11240179}& \text{0}\end{array}\right]$$	$$\left[\begin{array}{ccc}1.1643836& 0& 1.5960268\\ 1.1643836& -0.39176229& -0.81296765\\ 1.1643836& 2.0172321& 0\end{array}\right]$$

Conversion from R'G'B' to Intensity

The following equation defines conversion from the R'G'B' color space to intensity:

$$\text{intensity}=\left[\begin{array}{ccc}0.299& 0.587& 0.114\end{array}\right]\left[\begin{array}{c}{R}^{\prime }\\ G^{\prime }\\ B^{\prime }\end{array}\right]$$

Data Types

For fixed-point and integer input, the block converts matrix A to fixdt(1,17,16), and matrix B to fixdt(1,17,14).

For double or single input, the block applies the conversion matrices in double type, and scales the Y'CbCr offset vector ([16,128,128]) by 1/255. The block saturates double or single R'G'B' and intensity outputs to the range [0,1].

The Y'CbCr standard includes headroom and footroom. For 8-bit data, luminance values in the range 16–235 and chrominance values in the range 16–240 are valid. The Color Space Converter block pins out-of-range input to these limits before calculating the conversion. The block scales the offset vector and the allowed headroom and footroom depending on the word length of the input signals. For example, when you convert a Y'CbCr input of type fixdt(0,10,0) to R'G'B', the block multiplies the offset vector by 2^{(10 – 8)} = 4. As a result, the valid luminance range becomes 64–940 and the valid chrominance range becomes 64–960.

Latency

When you use this block with R'G'B' input, the block has a latency of 9 cycles. When you use this block with Y'CbCr input, the block has a latency of 10 cycles. The extra cycle is required to check for and correct headroom and footroom violations.

Note

When you use edge padding, use a horizontal blanking interval of at least twice the kernel width. This interval lets the block finish processing one line before it starts processing the next one, including adding padding pixels before and after the active pixels in the line.

The horizontal blanking interval is equal to TotalPixelsPerLine – ActivePixelsPerLine or, equivalently, FrontPorch + BackPorch. Standard streaming video formats use a horizontal blanking interval of about 25% of the frame width. This interval is much larger than the filters applied to each frame.

The horizontal blanking interval must also meet these minimums:

• If the kernel size is less than 4, and you use edge padding, the total porch must be at least 8 pixels.

• When you disable edge padding, the horizontal blanking interval must be at least 12 cycles and is independent of the kernel size.

• The BackPorch must be at least 6 pixels. This parameter is the number of inactive pixels before the first valid pixel in a frame.

For more information, see Configure Blanking Intervals.

Performance

This table shows the resource use after synthesis of the block for the Xilinx^® Zynq^®-7000 SoC ZC706 Evaluation Kit with single-pixel uint8 input and the default parameter settings. The design achieves a clock frequency of 438 MHz.

Resource	Usage
Slice LUTs	205
Slice Registers	300
DSP48	9
Block RAM	0

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

This block supports C/C++ code generation for Simulink^® accelerator and rapid accelerator modes and for DPI component generation.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Version History

Introduced in R2015a

R2022a: Two pixels-per-clock streaming

The block now supports multipixel streams that have 2 pixels per clock cycle.

R2021b: Multipixel streaming

The Color Space Converter block now supports multipixel streams. The HDL implementation replicates the algorithm for each pixel in parallel.

The block supports input matrices of NumPixels-by-3 values, and output matrices of NumPixels-by-NumComponents values, where NumComponents is 3 or 1. The ctrl ports remain scalar, and the control signals in the pixelcontrol bus apply to all pixels in the matrix.