signed byte array problem

[Nikos Katsikanis]

I create a byte array

 redPixels = (byte[])red.getPixels(slice);

I want to sum the values and I try this

long totalRed = 0;
for (int j=0; j<redPixels.length; j++)
    totalRed += (byte)redPixels[j];

However because the bytes are signed I get a wrong total.

How to I make the bytes fit into the range 0 - 255 so that me total is
correct. Is there documentation about how ImageJ gets around signed types as
I think this would be helpful.

[Joachim Wesner]

Hi,

yes there is!!!

You need to do a dummy "AND" operation, see the Imagej plugin tutorial at

http://mtd.fh-hagenberg.at/depot/imaging/imagej/tutorial171.pdf

Exerpt:

------------------------------------

Java’s byte data type is signed and has values ranging from -128 to 127,
while we would
expect a 8 bit grayscale image to have values from 0 to 255. If we cast a
byte variable to
another type we have to make sure, that the sign bit is eliminated. This
can be done using
a binary AND operation (&):
int pix = pixels[i] & 0xff;
...
pixels[i] = (byte) pix;

It’s the same with Java’s short data type, which is also signed and has
values ranging
from -32, 768 to 32, 767, while we would expect a 16 bit grayscale image to
have values from
0 to 65, 535. If we cast a short variable to another type we have to make
sure that the sign
bit is eliminated. This can again be done using a binary AND operation:
int pix = pixels[i] & 0xffff;
...
pixels[i] = (short) pix;

------------------------------------


Hope this helps!

JW



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I create a byte array

 redPixels = (byte[])red.getPixels(slice);

I want to sum the values and I try this

long totalRed = 0;
for (int j=0; j<redPixels.length; j++)
    totalRed += (byte)redPixels[j];

However because the bytes are signed I get a wrong total.

How to I make the bytes fit into the range 0 - 255 so that me total is
correct. Is there documentation about how ImageJ gets around signed types
as
I think this would be helpful.

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