Manipulating the K-channel in CMYK.

In a previous post I showed that you can manipulate just the L-channel in Lab and leave the colors unchanged.  Can we do the same thing in CMYK?  Yes we can.

This is the original of a photograph that I took in Dresden, Germany. 

The usual unsalvageable deep shadows of early morning along with burnt highlights where the sun is striking. Let's convert this to CMYK and take out the K channel and look at it.

Here's the K channel (Black) which I extracted from the previous photograph.  This is where brightness info is encoded in the foregoing picture. (N.B. You can also sharpen in this channel as you can in the L channel of Lab.) Now I'm going to transform this K channel in a series of HDR tone-mapping operations and put it back into the original photo.

Here's the K channel tone-mapped with Dynamic Photo HDR's Auto-Adaptive algorithm. Now let's put it back into the original photograph.

 

Here it is. Most of the bad shadows are gone and there are decent retrieved highlights. There are halos, though, because this is a local operator. You can see them better if you click on this photo and look under the cars and around the planters at the front of the picture-space.  Still, we could definitely start from here and salvage the rest in Photoshop.
Let's try the 'eye-catching' variant next.

This is the K channel again. It's been tone-mapped with the 'eye-catching' tone mapper from Dynamic Photo HDR.

When it replaces the K-channel of the CMYK original it produces this (above). It's nearly identical to the previous except that the halos are under control here.

 

Here's the K channel tone mapped with the Halo tonemapper. The halos are pretty obvious and this seems the worst of the four variants.

  

Original with the halo tone-mapped K channel. The halos are quite obvious, particularly if you click through.

 

Dynamic Photo HDR's 'human eye' algorithm is supposed to 'mimic the rods and the cones of the human eye'. No. I don't know what that means either.  This is the K-channel (above) as transformed by that algorithm.

Above see the finished product.  Notice the amazingly unchanged color. HDR is an idea about brightness. It is NOT an idea about color. That becomes obvious when we isolate HDR tonemapping just to the brightness layer of the original (either L channel or K channel).  I've said that the color layer doesn't change.  That's true as long as you don't manipulate the C,M, or Y channels.  If you made this CMYK from an RGB, say, and then perform these manipulations and convert it back to RGB then the RGB representation of the colors WILL change.  But as long as you're transforming Lab to Lab or CMYK to CMYK then they will NOT change.  You may like the garish colors of HDR transformation but this tutorial will allow you to hold the colors still if that's what you want.

I've compared Lab versions of the original to Lab versions where the L channel has been modified with HDR. The L channel varies significantly but the a and b channels (color) stay within 1/2 of 1% of each other (and even that is a rounding artifact). So I say that L or K will vary, but not a or b (or CMY) and this is what we expected. You can easily replicate this for yourselves.

 

I took the final 'human eye' version and produced this from it with some brightness map work and a touch of contrast. No sharpening. Compare it to the original.  From a photographic disaster zone it has become a reasonable picture.

By the way, their 'orange-juicer' dome is famous.

(signed)  Robert Consoli