visionhdl.ChromaResampler
Downsample or upsample chrominance component
Description
visionhdl.ChromaResampler downsamples or upsamples a
pixel stream.
Downsampling reduces bandwidth and storage requirements in a video system by combining pixel chrominance components over multiple pixels. You can specify a filter to prevent aliasing, by selecting the default filter or by entering coefficients.
Upsampling restores a signal to its original rate. You can use interpolation or replication to calculate the extra sample.
This object uses a streaming pixel interface with a structure
for frame control signals. This interface enables the object to operate independently of image
size and format, and to connect with other Vision HDL Toolbox™ objects. The object accepts and returns a scalar pixel value and control signals
as a structure containing five signals. The control signals indicate the validity of each pixel
and its location in the frame. To convert a pixel matrix into a pixel stream and control
signals, use the visionhdl.FrameToPixels object. For a full
description of the interface, see Streaming Pixel Interface.
The object accepts luma and the chrominance components. The object does not modify the luma component and applies delay to align with the resampled chrominance outputs. The rate of the output luma component is the same as the input.
Note
Starting in R2016b, instead of using the step
method to perform the operation defined by the System object™, you can call the object with arguments, as if it were a function. For
example, y = step(obj,x) and y = obj(x)
perform equivalent operations.
Construction
CR = visionhdl.ChromaResamplerCR, that downsamples from 4:4:4 to 4:2:2 and applies
the default antialiasing filter.
CR = visionhdl.ChromaResampler(Name,Value)CR, with additional options specified by one or more
Name,Value pair arguments. Name is a property name and
Value is the corresponding value. Name must
appear inside single quotes (''). You can specify several name-value
pair arguments in any order as Name1,Value1,...,NameN,ValueN.
Properties not specified retain their default values.
Properties
|
Resampling format.
|
|
Lowpass filter to accompany a downsample operation.
This property applies when you set |
|
Coefficients for the antialiasing filter. Enter the coefficients as a vector. This property applies when you set
Default: |
|
Interpolation method for an upsample operation.
This property applies when you set |
|
Rounding mode used for fixed-point operations. The object uses fixed-point arithmetic for internal calculations
when the input is any integer or fixed-point data type. This option
does not apply when the input data type is Default: |
|
Overflow action used for fixed-point operations. The object uses fixed-point arithmetic for internal calculations
when the input is any integer or fixed-point data type. This option
does not apply when the input data type is Default: |
|
Data type for the antialiasing filter coefficients. Specify a custom data type as a character vector. This parameter applies
when you set Default: |
Methods
| step | Compute next pixel in upsampled or downsampled pixel stream |
| Common to All System Objects | |
|---|---|
release | Allow System object property value changes |
Examples
Downsample a Y'CbCr Image
Resample a 4:4:4 Y'CbCr image to 4:2:2. The example also shows how to convert a R'G'B' input image to Y'CbCr color space.
Prepare a test image by selecting a portion of an image file.
frmActivePixels = 64;
frmActiveLines = 48;
frmOrig = imread('fabric.png');
frmInput = frmOrig(1:frmActiveLines,1:frmActivePixels,:);Create a serializer and specify the size of inactive pixel regions. The number of padding pixels on each line must be greater than the latency of each pixel-processing object.
frm2pix = visionhdl.FrameToPixels(... 'NumComponents',3,... 'VideoFormat','custom',... 'ActivePixelsPerLine',frmActivePixels,... 'ActiveVideoLines',frmActiveLines,... 'TotalPixelsPerLine',frmActivePixels+40,... 'TotalVideoLines',frmActiveLines+10,... 'StartingActiveLine',6,... 'FrontPorch',5);
Create a color space converter and resampler, using the default property values. The default conversion is 'RGB to YCbCr'. The default resampling mode is '4:4:4 to 4:2:2'. The default anti-aliasing filter is a 29-tap lowpass filter. This gives the object a latency of 30 cycles.
convert2ycbcr = visionhdl.ColorSpaceConverter(); downsampler = visionhdl.ChromaResampler();
Serialize the test image using the serializer object. pixIn is a numPixelsPerFrame -by-3 matrix. ctrlIn is a vector of control signal structures. Preallocate vectors for the output signals.
Note: This syntax runs only in R2016b or later. If you are using an earlier release, replace each call of an object with the equivalent step syntax. For example, replace myObject(x) with step(myObject,x).
[pixIn,ctrlIn] = frm2pix(frmInput); [~,~,numPixelsPerFrame] = getparamfromfrm2pix(frm2pix); pix444 = zeros(numPixelsPerFrame,3,'uint8'); ctrl444 = repmat(pixelcontrolstruct,numPixelsPerFrame,1); pix422 = zeros(numPixelsPerFrame,3,'uint8'); ctrl422 = repmat(pixelcontrolstruct,numPixelsPerFrame,1);
For each pixel in the stream, convert to YCbCr, then downsample.
for p = 1:numPixelsPerFrame [pix444(p,:),ctrl444(p)] = convert2ycbcr(pixIn(p,:),ctrlIn(p)); [pix422(p,:),ctrl422(p)] = downsampler(pix444(p,:),ctrl444(p)); end
Create deserializers with a format matching that of the serializer. Convert the 4:4:4 and 4:2:2 pixel streams back to image frames.
pix2frm444 = visionhdl.PixelsToFrame(... 'NumComponents',3,... 'VideoFormat','custom',... 'ActivePixelsPerLine',frmActivePixels,... 'ActiveVideoLines',frmActiveLines); pix2frm422 = visionhdl.PixelsToFrame(... 'NumComponents',3,... 'VideoFormat','custom',... 'ActivePixelsPerLine',frmActivePixels,... 'ActiveVideoLines',frmActiveLines); [frm444,frmValid] = pix2frm444(pix444,ctrl444); [frm422,frmValid] = pix2frm422(pix422,ctrl422);
There are the same number of pixels in the 4:2:2 and 4:4:4 pixel-streams and frames. To examine the resampled data, regroup the pixel data for the first 8 pixels of the first line. The first row is the Y elements of the pixels, the second row is the Cb elements, and the third row is the Cr elements. Notice that, in the 4:2:2 data, the Cb and Cr elements change only every second sample.
YCbCr444 = [frm444(1,1:8,1); frm444(1,1:8,2); frm444(1,1:8,3)]
YCbCr444 = 3x8 uint8 matrix
132 134 129 124 125 122 118 119
116 118 119 122 122 121 123 123
135 131 125 121 119 116 118 118
YCbCr422 = [frm422(1,1:8,1); frm422(1,1:8,2); frm422(1,1:8,3)]
YCbCr422 = 3x8 uint8 matrix
132 134 129 124 125 122 118 119
116 116 120 120 122 122 123 123
135 135 126 126 119 119 118 118
figure imshow(frm422,'InitialMagnification',300) title '4:2:2'
figure imshow(frm444,'InitialMagnification',300) title '4:4:4'
Algorithms
This object implements the algorithms described on the Chroma Resampler block reference page.
See Also
Chroma Resampler | visionhdl.FrameToPixels | vision.ChromaResampler (Computer Vision Toolbox)