how to downsample YUV colour image but downsample process only in U and V components ??

how to downsample YUV colour image but downsample process only in U and V components ?? and after that, i'm reaaly confused how to partition images into nonoverlapped blocks and overlapped blocks ?

Answer 1

[Image Analyst]

subSampleRate = 4;
smallImage = fullImage(1:subSampleRate:end, 1:subSampleRate:end, :);

fullImage is either the U or the V image.

[Internazionale]

i dont understand completely. this is my program, i am already transform the color grom RGB to YUV. next, i have to downsmple only U and V and partition that image to overlapped blocks and non overlapped blocks..

RGB = imread('lena.jpeg');

R = RGB(:,:,1); G = RGB(:,:,2); B = RGB(:,:,3);

Y = 0.299 * R + 0.587 * G + 0.114 * B; U = -0.14713 * R - 0.28886 * G + 0.436 * B; V = 0.615 * R - 0.51499 * G - 0.10001 * B;

YUV = cat(3,Y,U,V);

figure, imshow(Y) figure,imshow(U) figure,imshow(V)

A=downsample(U,2) B=downsample(V,2)

figure, imshow(A) figure, imshow(B)

[Image Analyst]

Looks like that should work, though you need to add [] to imshow to display a floating point array.

imshow(A, []);

Was there a problem with it other than that?

Answer 2

[Internazionale]

thank you for your help, and how about my next question ? about partition images into nonoverlapped blocks ?

[Image Analyst]

See my demo, split_image_into_blocks.m:

% Demo to divide a color image up into blocks.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
workspace;  % Make sure the workspace panel is showing.
fontSize = 20;

% Read in a standard MATLAB color demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'peppers.png';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
  % Didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, 'file')
    % Still didn't find it.  Alert user.
    errorMessage = sprintf('Error: %s does not exist.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
% Read the image from disk.
rgbImage = imread(fullFileName);

% Test code if you want to try it with a gray scale image.
% Uncomment line below if you want to see how it works with a gray scale image.
% rgbImage = rgb2gray(rgbImage);

% Display image full screen.
imshow(rgbImage);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
drawnow;
% Get the dimensions of the image.  numberOfColorBands should be = 3.
[rows columns numberOfColorBands] = size(rgbImage)

%==========================================================================
% The first way to divide an image up into blocks is by using mat2cell().
blockSizeR = 150; % Rows in block.
blockSizeC = 100; % Columns in block.

% Figure out the size of each block in rows. 
% Most will be blockSizeR but there may be a remainder amount of less than that.
wholeBlockRows = floor(rows / blockSizeR);
blockVectorR = [blockSizeR * ones(1, wholeBlockRows), rem(rows, blockSizeR)];
% Figure out the size of each block in columns. 
wholeBlockCols = floor(columns / blockSizeC);
blockVectorC = [blockSizeC * ones(1, wholeBlockCols), rem(columns, blockSizeC)];

% Create the cell array, ca.  
% Each cell (except for the remainder cells at the end of the image)
% in the array contains a blockSizeR by blockSizeC by 3 color array.
% This line is where the image is actually divided up into blocks.
if numberOfColorBands > 1
  % It's a color image.
  ca = mat2cell(rgbImage, blockVectorR, blockVectorC, numberOfColorBands);
else
  ca = mat2cell(rgbImage, blockVectorR, blockVectorC);
end

% Now display all the blocks.
plotIndex = 1;
numPlotsR = size(ca, 1);
numPlotsC = size(ca, 2);
for r = 1 : numPlotsR
  for c = 1 : numPlotsC
    fprintf('plotindex = %d,   c=%d, r=%d\n', plotIndex, c, r);
    % Specify the location for display of the image.
    subplot(numPlotsR, numPlotsC, plotIndex);
    % Extract the numerical array out of the cell
    % just for tutorial purposes.
    rgbBlock = ca{r,c};
    imshow(rgbBlock); % Could call imshow(ca{r,c}) if you wanted to.
    [rowsB columnsB numberOfColorBandsB] = size(rgbBlock);
    % Make the caption the block number.
    caption = sprintf('Block #%d of %d\n%d rows by %d columns', ...
      plotIndex, numPlotsR*numPlotsC, rowsB, columnsB);
    title(caption);
    drawnow;
    % Increment the subplot to the next location.
    plotIndex = plotIndex + 1;
  end
end

% Display the original image in the upper left.
subplot(4, 6, 1);
imshow(rgbImage);
title('Original Image');

%==============================================================================
% Another way to split the image up into blocks is to use indexing.
% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'cameraman.tif';
fullFileName = fullfile(folder, baseFileName);
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
  % Didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, 'file')
    % Still didn't find it.  Alert user.
    errorMessage = sprintf('Error: %s does not exist.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
grayImage = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorBands should be = 1.
[rows columns numberOfColorBands] = size(grayImage);
% Display the original gray scale image.
figure;
subplot(2, 2, 1);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

% Divide the image up into 4 blocks.
% Let's assume we know the block size and that all blocks will be the same size.
blockSizeR = 128; % Rows in block.
blockSizeC = 128; % Columns in block.
% Figure out the size of each block. 
wholeBlockRows = floor(rows / blockSizeR);
wholeBlockCols = floor(columns / blockSizeC);
% Preallocate a 3D image
image3d = zeros(wholeBlockRows, wholeBlockCols, 3);
% Now scan though, getting each block and putting it as a slice of a 3D array.
sliceNumber = 1;
for row = 1 : blockSizeR : rows
  for col = 1 : blockSizeR : columns
    % Let's be a little explicit here in our variables
    % to make it easier to see what's going on.
    row1 = row;
    row2 = row1 + blockSizeR - 1;
    col1 = col;
    col2 = col1 + blockSizeC - 1;
    % Extract out the block into a single subimage.
    oneBlock = grayImage(row1:row2, col1:col2);
    % Specify the location for display of the image.
    subplot(2, 2, sliceNumber);
    imshow(oneBlock);
    % Make the caption the block number.
    caption = sprintf('Block #%d of 4', sliceNumber);
    title(caption);
    drawnow;
    % Assign this slice to the image we just extracted.
    image3D(:, :, sliceNumber) = oneBlock;
    sliceNumber = sliceNumber + 1;
  end
end
% Now image3D is a 3D image where each slice, 
% or plane, is one quadrant of the original 2D image.

msgbox('Done with demo!  Check out the two figures.');