Re: Converting 256 level image to a 16 level image

On Jun 9, 10:29 am, ImageAnalyst <imageanal...@xxxxxxxxxxxxxx> wrote:
On Jun 8, 7:40 pm, gun...@xxxxxxxxx wrote:





On Jun 7, 4:21 am, "Roger Stafford"

<ellieandrogerxy...@xxxxxxxxxxxxxxxxxxxxxx> wrote:
"Roger Stafford" <ellieandrogerxy...@xxxxxxxxxxxxxxxxxxxxxx> wrote in
message <g2bdai$pu...@xxxxxxxxxxxxxxxxxx>...>   Good point, Walter.  If it is to be some kind of mapping, one can define a
15-element monotonically increasing mapping vector g with g(i) being the
smallest value in the range 0:255 which is to map into i and if the value is
less than g(1), it maps into 0.  Let x be an array of values, each in the range
from 0 to 255.  Then do this binary search table lookup:

 y = repmat(8,size(x));
 for k = [4,2,1,1/2]
  y = y + (2*(x>=reshape(g(y),size(x)))-1)*k;
 end
 y = y - 1/2;

Then y would be the mapping of array x into the range 0:15 in accordance
with the mapping defined by g.

-----------
  To avoid two of the reshapes, I should probably have written the binary
search above as follows.  First, make sure g is defined as a column vector.

 x2 = x(:);
 y = repmat(8,size(x2));
 for k = [4,2,1,1/2]
  y = y + (2*(x2>=g(y))-1)*k;
 end
 y = reshape(y-1/2,size(x));

Roger Stafford

Hi Roger and walter

Thanks for the replies. As I am new to this field I am having problems
in showing the image once I scale to a different level.  For example
if I run the following code,

A = imread('C:\TestData\lena.tif');
D = floor(A/16);
figure; imshow(D);

A has values upto 255 and D upto 16, but the image shown is a blank
(black) image. Is there anything else I have to do? Your help is
greatly appreciated

thanks,

Dev- Hide quoted text -

- Show quoted text -

-------------------------------------------------------------------
Dev:
It by default displays the range 0-255 since you probably have a uint8
image.  16 gray levels is so dark that it's virtually black.  Use []
as the second argument in imshow() to get it to display your 16 gray
levels with the full range of the display.
imshow(D, []);
By the way, it's a good idea to use descriptive variable names in case
someone else ever has to support your code.  For example
originalImage = imread('C:\TestData\lena.tif');
reducedGrayLevelImage = floor(originalImage / 16);
figure;
imshow(reducedGrayLevelImage , []);

Regards,
ImageAnalyst
P.S. If you see equals3D instead of the equals sign, it's because
you're using a newsreader that doesn't understand the "quoted
printable" standard for messages (like the MATLAB web-based newsread.)- Hide quoted text -

- Show quoted text -

Thanks, it works.
.

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