A Typical Problem in SIHDR - noise.

I want to devote this post to the problem of noise in SIHDR.  It's more of a problem in single- than it is in multiple-image HDR because, of course, there's less information to work with.  Here's a typical example.  It's the Art Academy in Dresden and, taken in the early morning, it's underexposed.  There's almost no information under the intrados of the arch.  Here's the original:

Under the arch it was easy to find (RGB) 1,0,0 points and lots of other points like 5,3,4, etc. I just know that this is going to be noisy after an HDR transform.  But before we try HDR let's look at the noise by turning the Levels slider in Photoshop all the way up.  Let's really look at that shadow.

 

Here there was a huge gap at the top end so I moved the white point down to 189. The middle tone slider was moved to 2.22.  Yes.  It's pretty bad.  Not only chromatic noise but luminance noise.  What to do? 

 

This (above) is an HDR transform made with Dynamic Photo HDR and it's the one I'm going to use. The color noise is not as gross as if we'd just used 'Levels' but the luminance noise is still a real problem.

In Photoshop I converted the image to grayscale. The color noise is now gone but, having done that, I'm committed to using this as the L channel in the final image. Let's do something about the luminance noise.

 

   

To cure the luminance noise I went into quick mask mode and painted the area under the arch. Having selected just that area I removed the noise with Noise Ninja. In Noise Ninja the contrast setting under luminance had the greatest effect. There's now slight blurring here but, in the context of the whole picture it won't matter much.

  

 

   

Here's the composite (above) after using the HDR-transformed image as the new L-channel.

  

  

   

This has been sharpened using Unsharp Mask. I used a high radius, low amount setting (HIRALOAM). Radius 49.8, Amount 15. This put the contrast right where I wanted it.  So the moral of the story is that you may have to use aggressive noise-riddance techniques if you use single image HDR.

One last point.  There is still a little luminance noise directly under the arch (not on the glass).  Sometimes you can get rid of that minor noise with the blur operator in Photoshop.  That's only for very minor noise on smooth surfaces.  Otherwise you're better off with the control afforded by Noise Ninja or the like.