[HDRI] Scaling artifacts in Photoshop CS2 w/ Radiance files?

[Gregory J. Ward]

Hi Mark,

The Radiance RGBE format is not able to record negative pixel values,  
so such primary values should be clamped to zero.  Apparently,  
Photoshop is not doing this.  Photosphere (and the Radiance tools)  
check for and truncate negative primaries, writing them out as 0's.   
OpenEXR doesn't have this issue, as it can read and write negative  
primary values -- in fact, you are better off using OpenEXR for your  
intermediates also as it is a higher-precision format.  It doesn't  
really make sense to use a lower-precision format like RGBE for  
intermediate results then write out your final as OpenEXR, as you've  
lost information along the way.  It would make more sense the other  
way around.

-Greg

> From: "Mark Banas (lists)" <listmail at mab3d.com>
> Date: June 15, 2006 2:10:26 PM BDT
>
> I've seen this a lot before: resizing a 32-bit file in Photoshop  
> CS2 with their bicubic interpolator produces artifacts that look  
> like "excessive sharpening halos" around high contrast details.  
> Because of this, I simply stopped scaling images in Photoshop and  
> instead use Photosphere (no problems).
>
> However, I was recently perusing the Adobe user-to-user forums, and  
> it was determined that the "sharpening artifacts" are only present  
> if you subsequently save the file in Radiance format. It was then  
> proposed by an Adobe employee that "the radiance file format code  
> may have some problems dealing with extreme values." (keep in mind  
> these are user-to-user forums, not developer comments).
>
> Here is the thread in question:
> http://www.adobeforums.com/cgi-bin/webx/.3bbe9c64
>
> Now I know that Photoshop's Bicubic does some sharpening (even  
> without using "Bicubic Sharper"), and that sharpening of any kind  
> on an HDR file results in extreme values, but does the Radiance  
> file format itself have a problem with extreme values? Or is this a  
> combo of PS Bicubic and their Radiance I/O? I pretty much store all  
> of my "in progress" files in Radiance format and then create "final  
> HDR files" from them in EXR, so this has effected me and my workflow.
>
> -Mark
>
> BTW, I've also found a huge flaw in Photoshop CS2's "Merge to HDR"  
> when small, bright details are moving between shots (think ghosted  
> leaves in a mild breeze) - so I don't even use PS CS2 for  
> *creating* HDRs anymore, but the PS CS2 "clone tool" and the plugin  
> Flexify 2 are pretty much indispensable for retouching HDRs, so I'm  
> using Photoshop in any case.