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Rounding Error Help

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xirja
Idididoll Forcabbage

Posts: 1698
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I'm getting a most unusual pixelation using a simple modulo (floor subtract) operation
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Posted: February 5, 2014 4:01 pm   Details E-Mail
xirja
Idididoll Forcabbage

Posts: 1698
Filters: 8
Oddly it seems to be related only to the export of an image.

Try the following with the attached filter:


1. Load any image into 'Source A'

2. Make sure the switch is set to 'Encrypt'

3. Save the output. Open the filter to see the unconnected reverse operation yield the original image.

4. Load the output into 'Source A'

5. Set the switch is set to 'Decrypt'


A real puzzle here, as there are also different results depending on the exported bit depth.

RoundingErrorTest.ffxml
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Posted: February 5, 2014 4:09 pm   Details E-Mail
xirja
Idididoll Forcabbage

Posts: 1698
Filters: 8
Image

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http://web.archive.org/web/2021062908...rjadesign/
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Posted: February 5, 2014 4:19 pm   Details E-Mail
xirja
Idididoll Forcabbage

Posts: 1698
Filters: 8
Forgot to mention that its best to use a high contrast / B&W image to see the problem clearly, like the one above.
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Posted: February 5, 2014 10:44 pm   Details E-Mail
xirja
Idididoll Forcabbage

Posts: 1698
Filters: 8
Perhaps this post should be in 'Bugs and Problems (Mac)', but I haven't been able to test this on another platform yet. I'm guessing however that it is either a general FF export problem or an OS issue.

Any ideas official or otherwise on what this problem is? So bizarre that the result internally is good; its only the file encoding/decoding on save/open that generates the error.

I'm afraid I'll have to... go ask apple, when I'm two feet tall. smile:D

Here's hoping this isn't some sort of http://en.wikipedia.org/wiki/Printer_steganography
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Posted: February 9, 2014 8:33 am   Details E-Mail
Vladimir Golovin
Administrator
Posts: 3446
Filters: 55
xirja, I didn't test this in full detail, but I'd like to point out that FF uses dithering when reducing bit depth from double-precision floats to 8 or 16 bit. Could you please try to turn dithering off and repeat your test?
Posted: February 10, 2014 7:57 am   Details E-Mail
Vladimir Golovin
Administrator
Posts: 3446
Filters: 55
Here's the relevant help quote on Filter Forge dithering:

Quote
Use output dithering

When turned on, the renderer applies dithering to the output image. Dithering is a method of simulating shades that are absent in the color palette by mixing pixels of the nearest colors available in the palette. Dithering helps reduce unwanted artifacts known as color banding that occur in color gradients during the conversion from the internal Filter Forge color depth (double-precision floating point) to the target color depth (usually 8 or 16 bit per channel). Filter Forge uses noise-based dithering (as opposed to pattern-based dithering).
Posted: February 10, 2014 7:58 am   Details E-Mail
Vladimir Golovin
Administrator
Posts: 3446
Filters: 55
Also, you could try to use .OpenEXR or .PFM as intermediate file formats: they both are floating-point (though, sadly, not double-precision, if I remember correctly).
Posted: February 10, 2014 8:05 am   Details E-Mail
xirja
Idididoll Forcabbage

Posts: 1698
Filters: 8
Even keeping the 'output dithering' on and using the 'float' formats of a tiff or exr or pfm does the trick, thank you! smile:D
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Posted: February 10, 2014 9:35 am   Details E-Mail
Vladimir Golovin
Administrator
Posts: 3446
Filters: 55
Glad to hear that this solved the problem!

(Which reminds me to consider support for OpenEXR 2.0)
Posted: February 10, 2014 10:24 am   Details E-Mail

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