I need to check if 2 images have been modified. I have the original image which I input it through 2 different tools, finally resulting the 2nd image.
The tools only alter the format of the image; the inputted image can be of any type (.jpg, .bmp, etc), and the first tool converts the image to .bmp and the second tool converts it to .png.
How can I check if the images display the same thing, with a certain threshold of course. (via cmd if possible, after that I'll write a script in Ruby)
ImageMagick can handle this by compare command.
Usage:
compare im1.png im2.bmp result.jpg
It marks the changed areas with red. After that you can easily check if it's changed or not.
Example: (Note that input images are from slightly different angles, so they are not the same.)
compare orig0.bmp orig1.jpg compare.jpg
orig0.bmp
orig1.jpg
compare.jpg
Related
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.
Using Imagemagick, you can get that information (if available) for formats that Imagemagick can read from its "magick" data in the header file as follows:
convert image -format "%m\n" info:
For example:
convert lena.png -format "%m\n" info:
PNG
convert lena.jpg -format "%m\n" info:
JPEG
convert lena.pnm -format "%m\n" info:
PPM
Even if the suffix is removed, this still works:
convert lena_copy -format "%m\n" info:
PNG
I noticed that PNG files created by Gimp from the same RPG data are identical except for the very beginning. This image shows a diff of otherwise identical PNG files created with Gimp:
What is this data which changes each time and how is it encoded? Are there tools to decode it? Can you learn something from this information, e.g. can you find out when a PNG file was (probably) created by this information?
I was under the impression that PNG files are created deterministically* and don't store meta data which isn't necessary to decode the image. (Obviously, the last part is not true, either, as Gimp writes its own name into the files but doesn't ask the user (which is does if you export something as a JPEG file).)
* I use the word "deterministic" here to refer to things and only such which are the same on each execution/export/whatever given the same input. I'd usually use the word "functional" (i.e. like a mathematical function) but I fear this could be misunderstood by people who don't know what "functional" means in mathematics. Obviously, this is different from the usage of this word in information theory.
See the PNG header definition.
tIME stores the time that the image was last changed, so for me it's the same as the timestamp of the file you create.
bKGD gives the default background color. Possibly the bakcgournd color you are using in Gimp, or the color of the transparent pixels.
tEXT with key Comment and value Created with Gimp is just the default comment. You can change the comment for the image in Image>Properties and you can set a default comment in Edit>Preferences>Default Image
When I export the same PNG twice, I only see a change in tIME. In fact I can't get a bKGD item, even when exporting a PNG with transparent pixels. Are you using any specific options when exporting?
For the purposes of insuring images are not tampered with, I would like to create a unique hash based on the contents of an image file (a PNG specifically). I've googled, and I know it's very possible to create a hash based on a file, but it seems to take into account things other hten the contents of the image?
For instance, to test, I create a very large PNG file with random colors/lines/shapes/etc. Then I saved the file as test1.png. I then created a single pixel black dot in the corner of the image and saved as test2.png.
I ran md5sum on both images, and got different hash values (expected). I then downloaded test2.png, removed the single black pixel, and saved the file as test3.png. test3.png and test1.png contain the EXACT same image.
Now, from what I understand PNG should be a lossless compression, so that shouldn't be the issue (?). I'm a bit in the dark (as you can probably tell) about all of this, so if anybody can give me any ideas, I'd much appreciate it!
You didn't say so, but I guess you are getting different hashes for test1.png and test3.png?
PNG files can contain a fair bit of metadata in addition to the image data; it's possible that some of the metadata is different. It's also possible for the same image data to be compressed in different ways. If you really want to know, compare the files to find out what exactly is different.
If you really want to hash just the contents of the files, you'll most likely have to convert them to a raw RGB format and hash that instead.
I have some raw data in a file that I would like to store in an image file (bmp, jpg, png, or even gif (eegad)). I would like this to be a two way process: I need to be able to reliably convert the image file back later and get a file that is identical to the original file.
I am not looking for a how-to on steganography; the image file will probably be one pixel wide and millions of pixels high and look like garbage. That is fine.
I looked into the Imagemagick utility convert, but am intimidated by the large number of options and terse man page. I am guessing I could just use this to convert from a 'raw' black channel to png, but would have to specify a bunch of other stuff. Any hints? I would prefer to work within Imagemagick or using Linux utilities.
If you are wondering, there's nothing black hat or cloak and dagger about my request. I simply want to automatically backup some important data to a photo-sharing site.
I'd plow into ImageMagick if that's what you'd prefer anyway.
Specific image formats support storing text data to different degrees, and ImageMagick supports all of the formats you mentioned. I'd choose the one that lets you store what you need.
What is the preferred way to convert various images, bitmap and vector, for use in a LaTeX and PDFLaTeX document?
There are many ways to do this, some make use of standard inclusions in the various LaTeX packages, others give better results.
You can include a PDF image directly into a LaTeX document if you want to produce your final output using pdflatex, but not if you want to produce a dvi file.
pdflatex can use PDF, PNG, and JPEG
latex/dvips can use PS, EPS
See more details:
Including images in LaTeX files
Watch what you name graphics files in LaTeX
I convert bitmaps into PNG, and vector graphics (e.g. SVG) into PDF. pdflatex understand both PNG and PDF.
If you have an image "as PDF", and you don't want to include it as pdf, you may want to extract the complete image data first with pdfimages. Other conversions may render the image only with reduced resolution.
My current preferred way is using bmeps and epstopdf included in MikTeX. For the generation of pdf and eps versions of a png.
In a file called convertimage.bat,
bmeps -p3 -c -e8f -tpng %1.png > %1.eps
epstopdf %1.eps
Use by including in the path and writing convertimage.bat filenameminusextension
Include in the documents using,
\begin{figure}[h]
\begin{center}
\includegraphics[scale=0.25]{path/to/fileminuxextension}
\caption{My caption here}
\label{somelabelforreference}
\end{center}
\end{figure}
I only use Encapsulated PostScript (.eps) figures (converting bitmaps with NetPBM first), since I always use dvips + ps2pdf anyway, and then I do \includegraphics{file}.
As John D. Cook says, your available image formats depend on whether you are using latex or pdflatex.
I find ImageMagick a useful tool for converting images between formats. Handles bitmap images, plus ps/pdf/eps (with ghostscript) and a zillion others. Available through apt, macports, etc.
I use a mac so I use GraphicConverter to load images and export as PDFs.
When I draw diagrams, I use Omnigraffle which lets me export as PDFs.
On windows I used to use Visio which supported EPSs which I also had no problems embedding.
The basic issues are that a) you want to handle raster and vector images differently and b) this introduces potential pitfalls.
The "right" thing to do depends a bit on your final output.
If your final output is going to be a .pdf file, and you don't need pstricks or anything else that these days you're probably better off just using pdflatex to directly produce the file.
In this case:
store all vector figures as .pdf
store all raster figures as .png (or jpeg if they were originally jpeg)
use graphicx package and \includegraphics{filename-without-suffix}
If you don't do the above, your raster figures will be converted to jpegs and may gain compression artifacts. png is the best bet if you can choose output.
If you are headed for .dvi file you're going to want .eps for everything. (You can gzip these files as long as you generate a bounding box file).
If you're careful you can do both. I store all vector figures as (compressed) .eps because there are a few things .pdf can't do that .eps can. I store all raster figures as .png. Using make, I can have temporary copies of these canonical versions generated on the fly for .dvi or .pdf output as needed.
Someone above pointed out the filename issue. You want to avoid "." in the file names, and avoid suffixes always in your latex file itself.
I always include images in PNG format.
If you compile your code with pdflatex, then you also can use the \includegraphics to include images in pdf (you have to include the package graphix