2006.10.06 22:50 "Re: [Tiff] Inverting color space values in a TIFF file", by Joris Van Damme

Richard,

I'm working a an extension to tiffcrop that will invert the color space of an image.

'invert' seems like an easy word, but it is in fact very hard to come up with a meaningful definition when applies to imagery.

I'd personally think this is image editing library stuff, not quite a tiff tool job.

For Bilevel and Grayscale images, this makes sense. I use it to convert a negative microfilm image to a positive one.

In bilevel imagery, there can be only one definition, turn black into white and vice versa. So that's feasible and unambiguous.

The way you're doing it now probably involves changing the image data, judging from your description further down this mail. You may want to consider just changing the PhotometricInterpretation, turning PHOTOMETRIC_MINISWHITE into PHOTOMETRIC_MINISBLACK and vice versa.

In greyscale imagery, there is the exact nature of the greyscale space to think of. The way I read the TIFF spec, multibit PHOTOMETRIC_MINISWHITE and PHOTOMETRIC_MINISBLACK should not be interpreted as R=G=B, as in sRGB, but as uniform, which corresponds best to the L channel of Lab. However, I suspect most readers and writers do read and write R=G=B, as in sRGB, therefore reading and writing data that is quite different from L in a non-linear way, therefore reading and writing data that is quite different from human vision and doesn't map uniform to the latter.

So stuff start being more complicated. You need to think about whether 'inversion' should make sense to human vision (L as in Lab), or to the cathode ray tube voltage regulators (R=G=B as in sRGB). Next, you need to think about whether you prefer to go with a strict interpretation of the TIFF spec, or at least my interpretation of the TIFF spec (which may be a different thing altogether), or whether you want to instead aim at compatibility with mainstream readers and writers, or at least my suspicions as to what these do mostly (which may be... etc).

People may argue that thinking too hard on this is not worth it. The difference in results of the different choices you could make is significant, but it's not huge.

> However, is there a meaningful inversion of a palette colour or an

RGBA image and would this be done by modifying the palette rather than the data that points

into the palette/lookup tables? What sort of algorithm should be applied to the

data in the palettes? Eg for 8 bit grayscale, I just subtract the value from 255

and for bilevel data, I just invert the bits with ones complement operator. Doing

the same for palette images produces viewable but bizarre images. Instead, should I simply subtract the values in the palette from 255 for 8 bit per color

images?

It is clear you need to change the palette, not the image data.

As to actual definition of 'inversion', this is even harder in the realm of full-colour. These are some of your options:

You can of course define inversion as inversion of the encoded data, whatever the colour space. That means inversion of an RGB image is quite different from inversion of an image that is stored as Lab, for instance. So it may make numeric sense, but it will surely not make any sense to any user.
If you need to make sense to a visual system of a user, you really need to go with Lab here, the difference is huge in this case. But still, there's two options, and both do another visually meaningful thing. You can define inversion to work on the L channel only. That means dark red becomes light red, light blue becomes dark blue.
The second option when working on Lab is to invert the L value, and negate the a and b values (the numeric operation depends of course on the exact encoding of Lab, that is different for CIE Lab, ICC Lab, and ITU Lab Photometrics, but I'm referring to the actual CIE L*a*b* values as commonly regarded in the range [0..100,-128..128,-128..128]). This is more or less the effect of a negative in analogue photography.
An image editor or image editing library might want to offer both option

2) and 3), as both or meaningful to the human user, and might want to refer to 2) as 'Brightness inversion' and to 3) as 'Negative' to make a clear distinction.

I assume an Alpha Channel value would not be modified.

That seems the most meaningful option. It is also the only options that works when alpha is premultiplied and equal to 0.

Is there any point in allowing such images to be Inverted?

I think the answer is above, because another way to ask the same question is 'Is there a (visually or otherwise) meaningful and useful definition of inversion?', and also 'Is this something that is logically correctly situated in the domain of a TIFF tool, or a file format independent image editing operation that fits more conveniently and logically in a image processing library layer?'.

I think the answer is certainly 'Yes' when applied to binary black and white data. In the greyscale case, I have some doubts. In the full colour case, I've many doubts.

Best regards,

Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
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