converting bmp image to double vector ?

I need to convert .bmp image to double vector, I will use these vectors for training neural network. I did the following :

img = imread('test.bmp');
number=im2double(img);   
number = number';       
data=number(:);

but the resulted vector (data) has 1938 binary values (0 and 1). when I tried the same code for jpg image It gave me vector with 256 double values. what is the difference between jpg and bmp in this case ? note the bmp image is actually image for number which is black and background is white. but the jpg image has white number on black background

[Geoff Hayes]

Fatma - what can you say about the test.bmp? What do you observe when you run the unique command

 img = imread('test.bmp');
 unique(img)

What can you say about the numbers that are returned from the above command? Now repeat this for the jpeg. What do you notice?

[Fatma Zamil]

test.bmp unique values are 0 and 1 but for the jpeg unique values are numbers from 0 to 255 I'm training a neural networks, I saw example on internet, when they converted their training images they had vector for each image with double 256 value but in my case (i have bmp images) I got binary vector with 1938 value so I got confused !!

[Image Analyst]

Fatma, please attach both the BMP and the JPG images (like people usually do when they ask for advice on image processing).

[Fatma Zamil]

I attached the images, thank u for ur advise

[Fatma Zamil]

do any one know how I can convert different bmp images to vectors with same number of elements ! when I tried converting 2 bmp images as I did above in my code, the resulted vectors are different in size !

Answer 1

[Image Analyst]

JPEG is a lossy compression, so it basically will blur the image and introduce gray levels and colors that are not in the original non-lossy image (like BMP, TIFF, or PNG). That is why the JPG image has more gray levels than the BMP version.

[Image Analyst]

"how I can convert different bmp images to vectors with same number of elements"

vector = bmpImage(:);

The number of elements and number of colors or gray levels in vector will be the same as in the variable bmpImage.

Also, the number of rows and columns in a variable read in from a JPG image file will equal the number of rows and columns in a variable read in from a BMP format file (same image, just different file formats). However the number of color channels may be different - sometimes the JPG image reads in as a color image (3 channels or planes) even though it appears to you to be a gray scale or binary image. And the number of gray scales or colors may be different for a JPG image than a BMP, TIF, or PNG image because of lossy JPG artifacts - the block artifacts that I'm sure you've seen in heavily compressed images.

Answer 2

[Guillaume]

There is a field in a the bitmap file that indicates how many colours are encoded in the file, it can be 2, 16, 256, 2^16, 2^24 or 2^32. Your bitmap obviously only has 2 colours while the ones you saw in examples had more.

JPEG images always have at least 256 colours (they could all be the same). Furthermore, if you took a 2 colours bitmap and encoded it as a jpeg you'd get more than two colours due to the way jpeg encodes colours.

[Image Analyst]

First paragraph - not necessarily. It could still be an 8 bit grayscale image or even an RGB image even though it has pixel values of only 0 and 1.