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Validation and Limits of the Method Cocorico !

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To understand the functioning of the method of extraction, it is necessary to remember laws of graphic compositions of colours. These are shown in the figure opposite.
With the function "Curve" used in Adobe Photoshop, we vary the tones of the colours in the image. But the trick is to vary the differential shades of very close colours, so that the development of the relative contrasts, so obtained, highlight certain aspects of the image, which otherwise would not be perceptible for the human eye.
Theoretically, this process, used alone, may not generate artificial images artefacts. Its inconvenience is that from "natural" colours, we obtain "artificial" colours. But in astronomy, it is not really inconvenient, the colours of the astronomical photos being very generally "artificial".
We verified, on numerous examples, that there is a never spontaneous generation of artificial images. The pixels, whatever their alignements, never generate by themselves artefacts. The examples below illustrate this. The images are always preserved. When modifications appear, it is only during the final compression of the images for certain formats, JPG in particular. But it becomes annoying only with high compression ratios. Up to 50 % the distortions remain generally acceptable.


The example, opposite, shows the variations of obtained shades. To the left the original image, to the right the final image. We see that the variations of shades can lead to their inversion to the complementary colour:

Red Green for example.

This method takes all its meaning and all its power only with "colour" images. In monochrome, they often still give good results. The obtained effect then becomes equivalent to an accentuation by the method of adjustment of "Light - Contrast"



Here is an orchid, before and after processing. The orchid remains an orchid. What changes they are the colours. And especially the contrasts between these colours. Thus little visible details become obvious. Observe the bottom, imperceptible nuances, in the original image, appeared. It is not an artefact. These shades exist indeed.


And now our galaxy, the Milky Way in Infrared light, presented elsewhere. Here, side by side are, the image after treatment (at the top) and the radio image of the VLA, (at the bottom). There is no better validation of a process, than the comparison with some other already validated methods. Note that here the image of the VLA was turned for the sake of clarity.



Limits of the Method

The experience shows that this method of extraction is very successful on diffuse images, galaxies, nebulas, jets, star clusters, galaxy clusters, clouds of gas and dusts, etc; the case of M87 is a basic example. But it does not enable one to see what is not already present in the initial photography. It does not allow extraction of the information from the image of a compact object such as a planet. It is not a statistical or mathematical method. It remains a technique of photography. With all the "knowledge" of a profession. And this is what is most missing today, men having practical experience. We take the opportunity here to honour Guy Subra, photographer and typographer, talented man who taught us his art and technique and regrettably disappeared too soon.


Last Update: 05.11.2013

The Artefacts: