A Graphic Equalizer for Images?

May 10, 2009 5:12 PM

Is there such a thing as a graphic equalizer for images? Can I obtain it?

In Photoshop you can high pass filter an image (edges and details) or low pass filter it (blur it). But there isn't continuous control over a range of frequencies.

Does some software exist that I as an average user can get a hold of that does the necessary Fourier analysis (or whatever is necessary) to give me fine control over any frequencies in an image I wish to address?

Sort of a graphic equalizer for photos?

posted by jfrancis to technology (26 answers total)

Well, you could always do highpass and lowpass on different layers, and then merge them together.
posted by delmoi at 5:21 PM on May 10, 2009

I'm not exactly sure what other "whatever's necessary" features you're looking for, but since CS4 (at least), Adobe Photoshop has come in an Extended Version that offers serious image analysis & statistics tools embedded in the app for scientists, engineers, medical professionals, etc. They probably have a lot more in the vein of what you're looking for.
posted by disillusioned at 5:22 PM on May 10, 2009

Try these:

http://arss.sourceforge.net/
http://photosounder.com/
posted by flabdablet at 5:24 PM on May 10, 2009

here is something I threw together in photoshop. The top layer I did Gaussian blur at 3 pixels, highpass at (I think) 10 pixels and on the lower layer I did Gaussian blur at 15 and highpass at 75. Then I set the transparency on the top layer to 27%.
posted by delmoi at 5:29 PM on May 10, 2009

In Photoshop, the Curves control is the equivalent of a graphic equalizer. You can use the pencil tool in the Curves dialog to apply pinpoint changes to specific brightness values, or the Input/Output fields if you know the specific numeric brightness value (and normally, this would be a number between 0 and 255, in an 8-bit-per-channel image), and want to drive it to another value. The High Pass filter is essentially driving everything but the brightest parts of the image to a 50% neutral gray, an effect which could be duplicated with Curves.
posted by dbiedny at 5:50 PM on May 10, 2009

you could always do highpass and lowpass on different layers, and then merge them together.

If by merge you mean add, that would be a notch filter with the center frequencies missing. If you subtracted the highpass and lowpass from the original image, then you would be left with the middle frequencies.

The biggest problem with frequency arithmatic is you really want to work with floating point images. With fixed point you might find yourself doing things like compressing the dynamic range to half of the gamut, and offsetting by adding a middle gray, doing your image operations, and then undoing the compression and offset. Otherwise you can get clipping, wraparound, and other artifacts if you are using what are intended to be graphics arts tools.
posted by StickyCarpet at 5:52 PM on May 10, 2009

dbiedny, those are very expressive tools you are referencing, but what you are saying is not really acurate in terms of, like, science.
posted by StickyCarpet at 5:57 PM on May 10, 2009

ImageJ might have (or have a plugin) that does what you are asking.
posted by ian1977 at 6:01 PM on May 10, 2009 [1 favorite]

Here is a list of ImageJ plugins.
posted by ian1977 at 6:01 PM on May 10, 2009 [1 favorite]

ian1977, looks like ImageJ would also require that images be re-centered with black at the middle gray, effectively permitting negative pixel values, before frequency arithmetic could be accurately performed.
posted by StickyCarpet at 6:11 PM on May 10, 2009

dbiedny, those are very expressive tools you are referencing, but what you are saying is not really acurate in terms of, like, science.

How so? Audio EQ is about the attenuation of specific sonic frequencies, Curves is doing just that with pixel values. Accuracy is exactly what the Curves control is all about, in that sense, it's much more useful for specific density changes than, say, Levels. The angle of the curve going in and out of a specific control point is analogous to the Q value of a EQ freq notch. The OP is asking for an imaging tool that delivers the visual equivalent of a graphic EQ, and I'm stating that Curves is the relevant tool. I'm not sure how my response is not, um, scientific.
posted by dbiedny at 6:12 PM on May 10, 2009

@flabdablet - lol that's quite something. Not quite what I was looking for, but it looks very interesting
posted by jfrancis at 6:20 PM on May 10, 2009

the attenuation of specific sonic frequencies, Curves is doing just that with pixel values.

Incorrect. I guess the easiest way to disprove your thesis is to point out that the Curves tool performs its operations on lone pixel values, mapping each value in directly to a lone pixel value out, even if it does so for every pixel in the image sequentially. It is impossible to select spatial frequencies without incorporating neighboring pixels in the computation.
posted by StickyCarpet at 6:21 PM on May 10, 2009

Sigh... I've attempted to alert the OP to a useful tool, one they already have access to, for achieving a specific type of tonal correction on an image, based on a metaphor that the OP understands. I look forward to your suggestion to the OP regarding a tool or technique that will help them with their problem. Or not.
posted by dbiedny at 6:30 PM on May 10, 2009

dbiedny - thank you for the suggestion. It's close, but not exactly what I'm looking for. I'm looking for something that works on the various spatial frequencies that make up the shapes in the image.
posted by jfrancis at 6:31 PM on May 10, 2009

My first thought was Curves too, until I read the question more carefully.
posted by grouse at 6:35 PM on May 10, 2009

ian1977 - the ImageJ plugins look pretty interesting. I'll heve to check into them more.
posted by jfrancis at 6:36 PM on May 10, 2009

dbiedny: I will agree witgh you on the whole sigh thing. The only way to use photoshop to do the equivalent of graphic frequency EQ is the rather convoluted recentering of the image with black at 128 instead of 0, and subtract various degrees of blur from the unblurred image, and then rescale the image as the final step. I wish the world was a different place too, sometimes.
posted by StickyCarpet at 6:36 PM on May 10, 2009

Actually, if you want to stick to a canned photoshop tool, unsharp masking is the closest thing to what you are asking for.
posted by StickyCarpet at 6:44 PM on May 10, 2009

dbiedny, those are very expressive tools you are referencing, but what you are saying is not really acurate in terms of, like, science.

Hah! I was so sure someone was going to confuse levels and curves as being what jfrancis was talking about (in fact, at first I thought he might have meant that, but then I realized he actually using the terminology correctly). I was thinking of preemptively saying something like "how long until someone mentions curves and levels?" but after I finished my experiment in Photoshop it seemed like most people understood what was going on...

Sigh... I've attempted to alert the OP to a useful tool, one they already have access to, for achieving a specific type of tonal correction on an image, based on a metaphor that the OP understands.

Right, but the jfracis wasn't using a metaphor. Images have frequencies, just like sounds do. Except they have frequencies in two dimensions (x and y).
posted by delmoi at 9:38 PM on May 10, 2009

In Mathematica:

image = ImageResize[ExampleData[{"AerialImage", "Pentagon"}], 128]

levels = Table[1/2 + 1/2 Cos[x/40 y/40], {x, 1, 128}, {y, 1, 128}];

Image[Re@InverseFourier[levels*Fourier[ImageData[image]]]]

Add Manipulate[...] to get a graphic-equalizer like user interface.
posted by hAndrew at 11:51 PM on May 10, 2009 [1 favorite]

hAndrew - Mathematica - very interesting. I'll have to look into this. I checked a while ago about pricing - have to remind myself
posted by jfrancis at 4:33 PM on May 12, 2009

What about MATLAB? This looks interesting...

One of the sleeper features making its debut in Photoshop CS3 Extended is its ability to interface with MATLAB, the number-crunching toolkit from Mathworks.

http://blogs.adobe.com/jnack/2007/06/photoshopmatlab.html
posted by jfrancis at 11:49 AM on June 13, 2009

List of Matlab-like software

Scilab

Octave


Rlab (no longer active) and Rlabplus
posted by jfrancis at 12:50 PM on June 13, 2009

Also interesting - a sort of 3 level Graphic Equalizer - and more

http://tech-slop.serveit.org/wiki/index.php?title=Lum_Frequencies
posted by jfrancis at 12:36 PM on July 12, 2009

Syd's Graphic Equalizer [Photoshop action] from this Model Mayhem thread
posted by jfrancis at 2:20 AM on August 6, 2009