When using imread, changed the background colour of image?

I had an image of a red cross with a background colour of white, after I used imread, it returned an image with a black background. Why is this? And, does it have anything to with the file format? (.png) This is messing with the entire program.

Answer 1

[Image Analyst]

Extract the color channels and see what the values are

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

Then pick a row and column that's in the background and see what they are

r = redChannel(row, column)
g = greenChannel (row, column)
b = blueChannel (row, column)

Tell us what your values are.

[Samyukta Ramnath]

It says 0, 0, 0 for red, blue and green channels. This is black, right? And after imread when I use imshow, it shows the original red cross but with a black background. Also, I saw this thing on imread, you define a three member vector with each member a value between 0 and 1, and say

>> BG = [0 0 1]; >>I1 = imread('red cross.png','BackgroundColor',BG);

When I put BG as 1 1 1, it shows up with a white background. But this doesn't explain why the colours should be inverted from the original image in the first place.

[Image Analyst]

If it's a uint8 image, the values should be integers going from 0 to 255. If you have a double image (floating point) then all values must be in the range 0-1 if you want to display them with imshow(). [0,0,0] is pure black. [0,0,1] is pure blue. [1,1,1] is pure white. Any transparent pixels in your PNG image will show up in the "BG" color. Maybe your background is not really transparent like you thought.

[Samyukta Ramnath]

This makes sense, because when I try the same thing (setting the background color for another image), it gives an error.