Changing Blackground of Image to gray?

Hi Guys, I have an jpg / bmp image (3 D) with a black background. Can anybody tell me how can I modify this image to convert black background to gray. I only want to modify the background without changing the picture in between.

Answer 1

[Walter Roberson]

if isreal(YourImage)
  greycol = 0.5;
else
  greycol = intmax(class(YourImage)) / 2;
end

[rows, cols, panes] = size(YourImage);
blacks = find( ~sum(YourImage,3) );
YourImage( [blacks, blacks + rows*cols, blacks + 2*rows*cols]) = greycol;

[Sachin]

Hi Walter, Could you please explain the logic as well, since I have to do some modification to use the same logic when there is white background? Thanks a ton. It works just fine.

SD

[Walter Roberson]

Black is represented by 0. When the sum() of the third dimension (R, G, B plane) is non-0, then the implication is that at least one of the R, G, or B individually must have been non-zero. The logical negation of that (~) is the case where the R, G, and B were all 0 -- that is, that the pixel was black.

The rest of the code has to do with extending the 2D result (the locations where the pixels were black) up into the 3rd dimension so as to set the R, G, and B pixels there to the greycolor (1/2 or 127 as appropriate depending on the data type.)

Instead of sum(YourImage,3) you could use any(YourImage,3)

Detecting white is not as nice because white can be represented by 1 (double precision) or 255 (uint8) or 65535 (uint16). None the less, it isn't too bad:

if isread(YourImage)

whitecol = 1;

else

whitecol = intmax(class(YourImage));

end

greycol = whitecol / 2;

[rows, cols, panes] = size(YourImage);

whites = find( all(YourImage == whitecol, 3) );

YourImage( [whites, whites + rows*cols, whites + 2*rows*cols]) = greycol;