why 32bits RGB image, not 24bits RGB

[Ye Chen]

Hello,

I found in ImageJ, if you open a RGB image, if click show info, you see
that it is 32bits per pixel. As I understand, RGB image should be 24 bits
RGB instead of 32 bits, since each channel is 8 bits. In ImageJ, it
calculate the size of a RGB image based on 32 bits per pixel. For example,
one RGB (1024x1024) image I used shows size 4MB. But if I save it to a
disk, the file size is 3MB which is calculated based on 24 bits per pixel.

Why in ImageJ, it shows 32bis per pixel for RGB image? Is that a mistake or
ImageJ has special data structure for RGB image?

Thanks