I am trying to produce some high quality 8bpp bmp from pdf file with ghostscript. For that purpose, I use the bmp256 device.
So far, everything works well and is really fast, but ghostscript use halftoning to dither the image, leading to some uggly patterns when zooming on the picture :
I've managed to reduce their size by playing with the -dDITHERPPI flag, but this is still not satisfying enough. Those are too regular and are too easily spotted, even with little zoom.
Instead of using halftone, I would like to use some error diffusion algorithm, like the Floyd–Steinberg one. I found this algorithm is implemented on other devices, but they are all printer related devices, so I can't really use them.
Plus, I need to be as fast as possible when converting the PDF to 8bpp BMP, and the outputed pictures are very large: so converting it to 24 or 32bpp BMP in the first place to dither it later with another tool is excluded.
I already downloaded the source to try to implement it myself, but the project is really big and complex and I don't know how and where to start.
Is there any way to use some error diffusion algorithm with ghostscript without having to implement it myself ?
If no, is there a prefered way for extending ghostscript ? Any guideline ?
I am reading a book on advanced iOS programming which says "It's generally much faster to work on planar formats than interleaved formats. If at all possible, get your data into planar format as soon as you can , and leave it in the at format through the entire transformation process".
I got to wondering why that is. The only reason I could come up with was that as you are iterating over your pixels, that you can use ++ operator instead of things like w*y+x to calculate your offsets.
I am familiar with planar vs interleaved as well as RGBA vs YUV. I've written a lot of c code to do things like image format conversion, rotations, flips, resizing, etc... The conversions were typically either to or from i420.
Is there something I'm overlooking?
Sweeping generalizations like the one you quote are often of limited practical applicability (i.e. almost, but not quite, entirely generalized from one or two data points ;-)
You say you have written a lot of C code for image processing. Good, take it to the next step: learn how to use a profiler, write microbenchmarks to unit-test the performance of your algorithms, especially when you are exploring the solution space.
What file formats and software could I use to represent vector images over time as an animation, without compromising the advantages of the vector format?
Say I generate data that is best represented as a single point in the plane, moving over time. I would like to make an animation showing the motion of this point. One way to do this is to make a sequence of 2D bitmap images and string these together into an AVI file. But this produces either huge files (orders of magnitude larger than the underlying dataset) or very low quality animations. A stack of raster images is a very inefficient representation of the data.
A much better representation would be a sequence of 2D vector images. Vector images combine very high fidelity with small file size. But is it possible to string such images into an animation? What kind of software could be used to do so, starting from the underlying dataset?
I imagine a tool such as Adobe Flash could be used here, but this seems akin to making scatterplots from scratch in Illustrator: sure, it can be done and will look nice, but this is not how you make scatterplots. You use R, Excel or MATLAB, and then perhaps retouch the plot in a graphics program. I'm looking for a similarly efficient solution, but for making dynamic visualizations rather than plots.
Can somebody help me, to find a library, or a detailed description of algorithm, that could embed a Digital watermark(invisible watermark, just a kind of steganography) to a jpeg/png file. But the quality of algorithm, should be great. It should be possible to extract this mark after rotation and expansion(if possible) of image.
Mark is just a key 32bytes.
I found a good site, but the algorithm are made for the NetPBM format, that is dead...
I know that there is a LSB method, but it is not stable to the expansion. Are there something better?
Changing metadata, is not suitable, because it is visible changes.
This maybe won't really be an answer, as I don't think it would be easy to give a magical, precise answer on this question.Watermarking is complex, and the best way to do it is by yourself : this will make things more hard for an attacker trying to reverse engineer your code. One could even read your question here, guess what library you used, and attack your system more easily.
Making Steganography resist to expansion in JPEG images is also very hard, because the JPEG compression is reapplied after the expansion. There are in fact a bunch of JPEG steganography algorithms. Which one you should use, depends on what exactly do you require :
Data confidentiality ?
Message presence confidentiality ?
Message coherence after JPEG changes ?
Resistance to "Known Cover" attacks (when attackers try to find the message, based on the steganographic system) ?
Resistance to "Known Message" attacks (when attackers try to find the steganographic system used, based on the message) ?
From what I know, usually, algorithm that resist to JPEG changes (picture recompression) are often really easier to attack, whereas algorithms that run the "encode" stage during the JPEG compression (after the DCT (lossy) transform, and before the Huffmann (non-lossy) transform) are more prone to resist.
Also, one key factor about steganography is scale : if you have only 32bytes of data to encode in a, say, 256*256px image, don't use an algo that can encode 512bytes of data in the same size. Either use a scalable algorithm, either use an algorithm at its efficient scale.
Also, the best way to do good steganography is to know its limitations,and to know how steganalyzers work. Try these tools, so you can understand what attackers will do to your picture.^
Now, I cannot tell you what steganographic system will be the best for you, but I can give you some indications :
jSteg - Quite old, I don't think it will resist to JPEG changes
OutGuess - Quite old too, but one of the best algorithms
F5 (and F3/F4) - More recent, good algorithm, scientifical research behind.
stegHide
I think all of these are LSB based : the encoding is done during the JPEG compression, after the DCT and Quantization. The only non LSB-based steganography system I heard of was mentionned in this research paper, however, I did not read it to the end yet, so I cannot tell if this will meet your needs.
However, I'm not sure there exists a real steganography algorithm resisting to JPEG compression, to JPEG resize and rotation, resisting to visual and statisticals attacks. Or I'm not aware of it.
Sorry for the lack of precise answer, I tried to give you what I know on the subject, as it's always better to be more informed. Sorry also for the lack of proper English, I'm French, nobody's perfect :)
Pistache is right in what he told you regarding the watermarking implementation algorithms. I will try to help you by showing one algorithm for the given requirements.
Before explaining you the algorithms first I guess that the distinction between the JPG and PNG formats should be done.
JPEG is a lossy format, i.e, the images are susceptible to compression that could remove the watermark. When you open an image for manipulation purposes and you save it, upon the writing procedure, a compression is made by using DCT filtering that removes some important components of the image.
On the other hand, PNG format is lossless, and that means that images are not susceptible to such kind of compression when stored after manipulation.
As a matter of fact, JPEG is used as a watermarking scheme attack due to its compressing characteristic that could remove the watermark if an attacker performed the compression.
Now that you know the difference between both formats, I can tell you a suitable algorithm resistant to the attacks that you mentioned.
Regarding methods to embed a watermark message for PNG files you can use the histogram embedding method. The histogram embedding method changes values on the histogram by changing the values of the neighbor bins. For example imagine that you have a PNG image in grayscale.
Therefore, you'll have only one channel for embedding and that means that you have one histogram with 256 bins. By selecting the neighbor bins x and x+1, you change the values of x and x+1 by moving the pixels with the bright x to x+1 or the other way around, so that (x/(x+1))>T for embedding a '1' or ((x+1)/x)>T for embedding a '0'.
You can repeat the same procedure for the whole histogram length and therefore you can embed in the best case up to 128bits. However this payload is less than what you asked. Therefore I suggest you to split the image into parts, for example blocks, and if you split one image into 4 components you'd be able to embed in the best case up to
512 bits which means 64 bytes.
This method although is very, susceptible to filtering and compression if applied straight in the space domain. Therefore, I suggest you to compute before the DWT of the image and use its low-frequency sub-band. This will provide you better transparency and robustness increased for the warping, resizing etc attacks and compression or filtering as well.
There are other approaches such as LPM (Log Polar Maps) but they are very complex to implement and I think for your case this approach would be fine.
I can suggest you two papers, the first is:
Watermarking digital image and video data. A state-of-the-art overview
This paper will give you some basic notions of watermarking and explain more in detail the LSB algorithm. And the second paper is:
Real-Time Compressed- Domain Video Watermarking Resistance to Geometric Distortions
This paper will explain the algorithm that I just explained now.
Cheers,
I do not know if you are considering approaches different to steganography. Instead of storing data hidden in the pixel data you could create a new data block in the JPEG file and store encripted data.
Take a look at the JPEG file structure on Wikipedia
You can create an application specific data block, using the marker 0xFF 0xEn. Doing so, any change in the image pixels do not change the information stored in the image. Moreover, many image editing software respect custom data blocks and will keep them even after image manipulation.
I've read some of posts here about png/jpg/gif but still I'm quite confused..
I've got a big header on my website :
width:850px height:380px weight:108kb
And it's jpg. A woman + gradient + some layers on top and behing her..
Do you think 108kb it's too much? I was thinking about cut it to png pieces..Would that be a bad idea? What's your suggestions?;) THX for help;]
It depends on the nature of the image, if it's a photograph, JPEG would give the highest quality/compression ratio, if it was pixel stuff like writing or clipart, or have some transparency, then it's GIF/PNG to choose (GIF,PNG8 offers one level transparency, while PNG24 offers a levelled transparency for each pixel).
When I'm confused, I usually save the picture in all three formats and decide what gives the best quality/size deal within the size limits I need, I also try to lower down JPEG quality to the level where the image is still good quality (because that varies from image to another).
Also, if it was a photograph with some writing, split it into a JPEG photograph with a transparent GIF writing overlay (because writing edges look distorted in JPEG).
So, when you are confused, give the three a try and decide, with time you'll gain experience what format suits what content the most.
Actually, 108kb for an image of that size isn't abnormal. It's even more optimal to have it in 1 image: your browser only needs to perform 1 get-request. If you break it up into multiple images, the user needs to wait longer for the site to load.
PNG probably wouldn't help you since JPG is usually much more efficient at handling gradients. PNG would be better if you had big unicolored spaces in your image.