Difference between 8-bit/color RGB and 8-bit colormap

Question

I ran the comand:

file *

and got the output:

spec_wav_17.png:           PNG image data, 5 x 128, 8-bit colormap, non-interlaced
spec_wav_17.pngresize.png: PNG image data, 5 x 128, 8-bit/color RGB, non-interlaced

I have a huge difference in results between these two formats where RGB produces better results, I am trying to figure out what is happening.

I searched the manual for file and found nothing about these formats https://linux.die.net/man/1/file

Answer 1

8-bit colormap means that you have a mapping between an 8 bit integer and a set of colors. With 8 bits you get 256 different entries, each one pointing to a different color, like 1->white, 2->yellow, 3->black... and so on.

8-bit/color RGB means that every pixel is represented by three colors (R, G and B, Red, Green and Blue), and each color is represented by an 8-bit integer. This means that each color can have 256 shades, from black to the color. The three colors together conform the final color. The total palette in this case is 256*256*256 colors in total.

All of this means that the colormap image have very few colors and uses only one bype per pixel, while the RGB image have 16 million colors and uses 3 bytes per pixel (raw images, no compression).