MATLAB 7.9

Integer Arithmetic

This gives examples of performing arithmetic on signal and image integer data.

Contents

Load Integer Signal Data

Load measurement datasets comprising signals from four instruments using 8 and 16-bit A-to-D's resulting in data saved as int8, int16 and uint16. Time is stored as uint16.

load integersignal

% Look at variables
whos Signal1 Signal2 Signal3 Signal4 Time1

  Name            Size            Bytes  Class     Attributes

  Signal1      7550x1              7550  int8
  Signal2      7550x1              7550  int8
  Signal3      7550x1             15100  int16
  Signal4      7550x1             15100  uint16
  Time1        7550x1             15100  uint16

Plot Data

First we will plot two of the signals to see the signal ranges.

plot(Time1, Signal1, Time1, Signal2);
grid;
legend('Signal1','Signal2');

Here we see the values are in the range -128 and 127, which is as we would expect for int8. It is likely that these values would need to be scaled to calculate the actual physical value that the signal represents e.g. Volts.

Process Data

We can perform standard arithmetic on integers such as +, -, *, and /. Let's say we wished to find the sum of Signal1 and Signal2.

SumSig = Signal1 + Signal2; % Here we sum the integer signals.

We can turn on warnings to tell us if the arithmetic saturates. In the case of int8, this will occur if the result of an operation is outside the range -128 to 127. We will turn these warnings on with the intwarning command and re-execute.

intwarning('on')
SumSig = Signal1 + Signal2; % Here we sum the integer signals again.

Warning: Out of range value or NaN computed in integer arithmetic.

However, performing this checking greatly slows down execution. It is best to turn it on during algorithm development and back off for final execution. Now let's plot the sum signal and see where it saturates.

intwarning('off')

cla;
plot(Time1, SumSig);
hold on;
Saturated = (SumSig == intmin('int8')) | (SumSig == intmax('int8')); % Find 
where it has saturated
plot(Time1(Saturated),SumSig(Saturated),'rd');grid;
hold off;

The markers show where the signal has saturated.

Load Integer Image Data

Next we will look at arithmetic on some image data.

street1=imread('street1.jpg'); % Load image data
street2=imread('street2.jpg');
whos street1 street2

  Name           Size                Bytes  Class    Attributes

  street1      480x640x3            921600  uint8
  street2      480x640x3            921600  uint8

Here we see the images are 24-bit color, stored as three planes of uint8 data.

Display Images

Display first image.

cla;
image(street1); % Display image
axis equal; axis off

Display second image

image(street2); % Display image
axis equal; axis off

Scale an Image

We can scale the image by a double precision constant but keep the image stored as integers. For example,

duller = 0.5 * street2; % Scale image with a double constant but create an i
nteger
whos duller

  Name          Size                Bytes  Class    Attributes

  duller      480x640x3            921600  uint8

subplot(1,2,1);
image(street2);
axis off equal tight
title('Original');  % Display image

subplot(1,2,2);
image(duller);
axis off equal tight
title('Duller');    % Display image

Add the Images

We can add the two street images together and plot the ghostly result.

combined = street1 + duller; % Add |uint8| images
subplot(1,1,1)
cla;
image(combined); % Display image
title('Combined');
axis equal; axis off