qtgetblk

Block values in quadtree decomposition

Syntax

[vals,r,c] = qtgetblk(I,S,dim)

[vals,idx] = qtgetblk(I,S,dim)

Description

[vals,r,c] = qtgetblk(I,S,dim) returns blocks of size dim-by-dim from image I with quadtree decomposition S. The function returns the block values in vals and the row and column coordinates of the upper left corner of the blocks in r and c.

[vals,idx] = qtgetblk(I,S,dim) returns the block values in vals and the linear indices of the upper left corners of the blocks in idx.

Examples

Get Blocks from Quadtree Decomposition

Create a sample matrix representing a small image.

I = [1    1    1    1    2    3    6    6
     1    1    2    1    4    5    6    8
     1    1    1    1   10   15    7    7
     1    1    1    1   20   25    7    7
    20   22   20   22    1    2    3    4
    20   22   22   20    5    6    7    8
    20   22   20   20    9   10   11   12
    22   22   20   20   13   14   15   16];

Perform a quadtree decomposition of the image, specifying a threshold of 5. qtdecomp splits a block if the maximum value of the block elements minus the minimum value of the block elements is greater than the threshold.

S = qtdecomp(I,5)

S = 
   (1,1)        4
   (5,1)        4
   (1,5)        2
   (3,5)        1
   (4,5)        1
   (5,5)        2
   (7,5)        2
   (3,6)        1
   (4,6)        1
   (1,7)        2
   (3,7)        2
   (5,7)        2
   (7,7)        2

Get the blocks of size 4-by-4 from the quadtree decomposition. qtgetblk finds two blocks of this size.

[vals,r,c] = qtgetblk(I,S,4);

Select the second returned block. Display the values and the (row,column) coordinates of the upper left corner of the block.

blknum = 2;
blockValues = vals(:,:,blknum)

blockValues = 4×4

    20    22    20    22
    20    22    22    20
    20    22    20    20
    22    22    20    20

blockCoordinates = ['(',num2str(r(blknum)),',',num2str(c(blknum)),')']

blockCoordinates = 
'(5,1)'

Input Arguments

I — Grayscale image

numeric matrix

Grayscale image, specified as a numeric matrix.

Data Types: single | double | int16 | uint8 | uint16 | logical

S — Quadtree structure

sparse matrix

Quadtree structure, specified as a sparse matrix. If S(m,n) is nonzero, then the coordinate (m,n) is the upper left corner of a block in the decomposition, and the size of the block is given by S(m,n). You can get a quadtree structure by using the qtdecomp function.

Data Types: double

dim — Block size

positive integer

Block size, specified as a positive integer.

Output Arguments

vals — Block values

dim-by-dim-by-k array | []

Block values, returned as a dim-by-dim-by-k array, where k is the number of dim-by-dim blocks in the quadtree decomposition. If the quadtree decomposition contains no blocks of the specified size, then vals is returned as an empty matrix.

The ordering of the blocks in vals matches the column-wise order of the blocks in I. For example, if vals is 4-by-4-by-2, then vals(:,:,1) contains the values from the first 4-by-4 block in I, and vals(:,:,2) contains the values from the second 4-by-4 block.

r — Row coordinates of upper left corners of blocks

k-element column vector | []

Row coordinates of the upper left corners of blocks, returned as a k-element column vector of positive integers, where k is the number of dim-by-dim blocks in the quadtree decomposition. If the quadtree decomposition contains no blocks of the specified size, then r is returned as an empty matrix.

c — Column coordinates of upper left corners of blocks

k-element column vector | []

Column coordinates of the upper left corners of blocks, returned as a k-element column vector of positive integers, where k is the number of dim-by-dim blocks in the quadtree decomposition. If the quadtree decomposition contains no blocks of the specified size, then c is returned as an empty matrix.

idx — Linear indices of upper left corners of blocks

k-element column vector | []

Linear indices of upper left corners of blocks, returned as a k-element column vector of positive integers, where k is the number of dim-by-dim blocks in the quadtree decomposition. If the quadtree decomposition contains no blocks of the specified size, then idx is returned as an empty matrix.