Raw image

From XPUB & Lens-Based wiki

Raw: This is the simplest of all ways to store images, just as "raw" bytes. For example one byte per pixel for grey scale or 3 bytes per pixel for RGB colour. There is no standard header and so even the size of the image needs to be specified for programs that might read the image. [1]

Imagemagick can convert a bit-map file to a raw .dat – a not specific file type

convert image.png image.dat

And, it can also convert the .dat back into a bit-map

Andre Castro (talk) 10:14, 29 January 2018 (CET) if you run:

identify image.dat
img.dat PNG 2102x2799 2102x2799+0+0 8-bit RGB 256c 33.6KB 0.000u 0:00.000

Hence the image still with PNG headers

If you stick to the size, bit-depth and color profile of the original, the conversion will "preserve" the original

convert -depth 8 -size 2102x2799 rgb:image.dat image2.png

If you change those parameters, it is likely that the resulting bitmap will be something else!

convert -depth 8 -size 1000x1000 rgb:image.dat image2.png
convert -depth 24 -size 1000x1000 rgb:image.dat image2.png
convert -depth 8 -size 1000x1000 gray:image.dat image2.png

What we are doing here is, not unlike playing back audio files at bit-rate and sample-rates, different from the ones they were encoded in.

But we can convert from a raw audio to an image file with

convert -depth 8 -size 1000x1000 rgb:Zong3.raw Zong3.jpg

or even an animated gif

convert -depth 8 -size 1000x1000 rgb:Zong3.raw Zong3.jpg

What gets lost in a conversion ??



code experiments with Raw images

With a small amount of code, it's easy to dump out a stream of data as bytes:

# raw.py
import struct, sys
out = open("image.data", "wb")
for x in range(100): 
    out.write(struct.pack('B', 255))
    out.write(struct.pack('B', 0))
    out.write(struct.pack('B', 0))
    out.write(struct.pack('B', 128))

The "wb" option in the open command means that we want to "write" "binary" data (the default behaviour of read would be to read a text file).

When you run the script:

python raw.py

In Python, struct.pack is a way of converting a number (from 0 to 255) into it's corresponding binary representation as a "byte" or 8 bits (where 0 is all bits off 00000000 and 255 is all bits on 11111111). The code above loops 100 times outputting the bits of the sequence 255, 0, 0, 128.

255 0 0 128 255 0 0 128 255 0 0 128 255 0 0 128...

We can then use an image application like the GIMP to interpret this raw data as an image... In this case we tell the GIMP that it should interpret the bytes as being RGBA formatted pixels in the size 10 x 10:

It then interprets the numbers as a stream of pixels in the form:


So for the code above:

255 (full) RED, 0 GREEN, 0 BLUE, 128 (half) ALPHA, ...


The result is an image 10 pixels wide by 10 pixels tall, where every pixel is red with 50% transparency.

Put a header on it

Working with raw data file can be inconvenient however since everytime you want to view the data as an image, you need to explicitly tell an application (such as GIMP) what the size and format of the image is. We can improve the situation by attaching preceding the raw data with a simple "header" to and follow the guidelines (which specify the order of the bytes) of a specific simple image format.

Targa is an early very simple format for images. It comes from an early manufacturer of video display cards, named Targa, who created a minimal format for files to display on their hardware. The format is still popular today in Game development and other communities for whom the simplicity of the format is useful for "wrapping" raw image data with a header so that it's self-contained and directly loadable by different programs without needing to explicitly specify information width and height and bit depth.

import struct, sys

out = open("image.tga", "wb")
width = 320
height = 240
header = struct.pack("<BBBHHBHHHHBB",0,0,2,0,0,8,0,0,width,height,32,1<<5)
for y in xrange(height):
    for x in xrange(width):
        r = 0
        g = 0
        b = 0
        a = 128

        if y < 32:
            r = 255
            a = 255

        if x > 64 and x < 256:
            g = 255

        if y > 120:
            r = 128

        out.write(struct.pack('B', b))
        out.write(struct.pack('B', g))
        out.write(struct.pack('B', r))
        out.write(struct.pack('B', a))

This script outputs a TGA format, which has been opened in the GIMP and exported to PNG


Next: Raw image sequence



  1. Bourke, Paul. n.d. ‘A Beginners Guide to Bitmaps’. Accessed 2 January 2018. http://paulbourke.net/dataformats/bitmaps/.