How to recognize an image file format using its contents?
If a Image file is of format .png then it will contain ‰PNG, at the beginning of the file. (when read in Text mode)
If a Image file is of format .bmp then it will contain BM, at the beginning of the file. (when read in Text mode)
I know that Image formats contain text (data) of certain size (bytes) in the beginning of the file, which is used as metadata of the Image file?
My Questions are:-
- Is this behavior same in all image file formats (or formats in general)?
Could a image file (of no extension) be recognized just using this data?
Is there information available on how this metadata is broken down? By that I mean, data at which position in the metadata has what meaning?
Is this behavior same in all image file formats (or formats in general)?
For most of them, yes. There are some proprietary formats (e.g. for games) that might have very short or no metadata. Also, metadata might be in another file (e.g. animations together with XML metadata).
Could a image file (of no extension) be recognized just using this data?
Yes. In fact, most image viewers will warn you if an image file has an incorrect extension and ask you if they should fix it.
On Unix systems, there's a file command that identifies files based on their metadata. There is a better tool specific for images called identify (part of ImageMagick) that returns more detailed information on resolution, bitdepth, etc.
Is there information available on how this metadata is broken down? By that I mean, data at which position in the metadata has what meaning?
There are books about (image) file formats and for most formats, this information is available in official specifications (e.g. RFC 2083 for PNG). They list all of the (optional) file contents, describe the compressions and what a viewer/decoder/encoder can/must/should do with the data. A good starting point might be the Wikipedia list of image file formats.
Note that based on the examples you gave I suppose you opened files with a text editor which is not the ideal tool for that task. It's better to use a hex-editor for this. Text editors won't show most bytes (e.g. 255) by default and interprete others (e.g. tab or line feed). They might be good enough to see magic text strings like "BM" and "PNG", but with a hex editor, you can see both these text parts and their numerical representation - e.g. allowing you to extract image width and height. For this, some tool to convert hexademical values to decimal is useful, most calculators can do this.
As an example, let's look at the beginning of a PNG file with a resolution of 6146 x 14293 in both a text editor and a hex editor:
You can see that the file is a PNG image in both of them, that's correct. But the marked part in the hex editor view will show the width and height of the image (matching the PNG chunk specification of the "IHDR" part) - 0x00001802 is 6146 in decimal, 0x000037D5 is 14293. There's no way to do this in the text editor.
Also note that even if you don't know an image format, you might be lucky with just guessing it's uncompressed data (this often works for some game image file formats, most notable Unity's "assets"). E.g. if you rename files to ".raw", the image viewer IrfanView will give you a dialog (see the screenshot below) where you can guess width, height and bit depth of the image and see if the result looks good. This requires some experience in interpreting the outcome though, if width and bitdepth don't match, images will look like noise, warped, or have wrong colors.
This "image geometry guessing" can be improved/automated by trying different widths and computing the correlation coefficent between two lines. The tool raw2tiff can do this. Quote from the site:
There is no magic, it is just a mathematical statistics, so it can be wrong in some cases. But for most ordinary images guessing method will work fine.