[argyllcms] Re: ImageTarget version 0.1.1 released
- From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
- To: "Alastair M. Robinson" <profiling@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 15 May 2008 19:06:13 +0200
Alastair M. Robinson wrote:
What sort of degree of blurring / downsampling / denoising did you use?
Klaus Karcher wrote:
Before reporting on the results, I have to discuss some strange side
effects I noticed in conjunction with different source profiles (I will
do that in a separate mail).
I just finished my experiments and want to give a brief summary of the
results:
I used the 16bit/chanel data of my oil on canvas scan, transformed it to
MinkaRGB (L* TRC). For comparison, I applied some filters also in
Gamma-1-ProPhoto-RGB, but transformed the results back to MinkaRGB to
avoid side effects with cctiff.
I tested 5 methods to restrict the source gamut (all in Photoshop CS3):
- Gaussian Blur, r = 0.5 ... 10
- Bicubic downsampling to 90 ... 10%
- Median filter, r = 1 ... 10
- "Dust and Scratches" r = 6, threshold = 25
- "Reduce Noise", "Strength" = 10, all other options set to zero
- "Dust and Scratches" had only slight effects on gamut mapping
(slightly increased overall saturation, no objectionable clipping).
Similar results can be obtained with downscaling to approx. 60%.
- "Reduce Noise" had nearly no (at least no positive) influence on gamut
mapping. The VRML file suggests that "Reduce Noise" deforms the gamut
rather than reducing it. viewgam -i confirm this impression:
Intersecting volume = 194686.1 cubic units
'FrauCLMr.gam' volume = 223546.7 cubic units, intersect = 87.09%
'FrauCLMr_RN10.gam' volume = 234535.9 cubic units, intersect = 83.01%
- Gaussian Blur had enormous influence on gamut. Even very small radii
reduce the gamut volume drastically (see attachment "GB_V(r).pdf").
Therefore it's difficult to find the right "dose". Too large radii cause
serious clipping (and even r = 1 can be too large). The scaling of the
slider in ImageTarget's UI should account for the exponential relation
between r an V.
- Median filtering gives reasonable results, but the integer increments
are somewhat coarse and not perceptually uniform.
- Bicubic downsampling gives very good results. It is easy to dose (the
scale is roughly proportional to V^(1/3) and therefore perceptually
nearly uniform). Moreover tiffgamut benefits from the smaller size of
the downscaled images. IMHO it is the best of the tested methods to
control the gamut mapping.
- Bicubic downsampling is strongly TRC dependent e.g. the effect of
Downscaling to 20% with gamma 1 corresponds roughly to downscaling to
60% with L* TRC.
- Gaussian blur is much less TRC dependent.
Klaus
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