How to look at the anaglyphs to get a good vision of relief.
The anaglyph technique gives a three dimensional vision by the use of color filters. The pictures are formed by the superposition of the two channels of a stereo pair. Originally, each member of the stereoscopic pair is a color picture which is transformed by software to eliminate some color channels of the RGB image. In one stereo image, only the red channel is kept while the second image retains the green and blue channels. Finally, the two pictures are superposed with transparency added so that both can be seen simultaneously. The horizontal shift between the two stereo pictures is adjusted in order to position the final 3D image in front or behind the screen.
To give a good rendition of relief, the two stereo images embedded in the final photograph must be separated so that each eye sees its own image and only that one. Two filters must be used: a red filter and a blue-green filter. Sometimes, a green filter is used instead of blue green but this method gives only a monochrome image. To correctly see the pictures of this site, you must place the red filter in front of your right eye and the blue-green in front of the left.
To obtain a good impression of relief you must "remove" all things from the video screen except of course the stereo picture. The full screen option of some software is very useful: for instance full screen of "Adobe Photoshop" with its black background and the F11 key of "Internet Explorer 5". As the pictures of this site are most of the time situated in front of the screen, even the small arrow of the mouse pointer can disturb the 3D image construction by our brain. In particular, the 3D objects must never cross the border of the screen, you must adjust the size of images to avoid that situation. The best distance to see a mineral image covering the video screen is about one meter from the screen.
How to choose the filters? Normally, the method of RGB channels separation keeps all the color information in the final 3 dimensional photograph but the filters used for examination are never perfect. The filters used are wide spectral band contrast filters usually utilized in black and white photography. For a perfect view of the relief, a filter must give a good contrasted image of its complementary color picture and no image at all of its own color picture. Of course to fulfill this condition we could use very narrow bands filter with a very good bands separation but with the consequence of a very dark monochrome image. In practice wide bands filters are used. The precise adjustment of the spectral band can improve the color vision and the stereo channels separation. Generally, the choice is a compromise between the two parameters. The graph below is a summary of the filters I have used up to now with the addition of a few spectral curves of Kodak gelatin filters (from: Kodak filters for Scientific and Technical Uses N°. B3).
All filter curves measurement have been recorded with a Mac-Beth spectro-colorimeter. The Kodak filter curves have been taken from the Kodak publication except the Kodak 25 which has been also measured. The agreement between the theoretical spectral response given by Kodak for this filter and the measured curve is reasonably good for the purpose of choosing the filters. The best relief vision on the cathode ray tube of a computer monitor was obtained with the "Media Relief" filters (Paris, France). The Kodak 25 red filter is not dark enough and produces some cross talk between the stereo channels. Probably, the Kodak 29 would be a better choice (to be tested later). The choice of the cyan filter is more delicate: a curve like the "SAR ELEC" cyan filter gives a clear 3D image with good colors but as the spectral curves of the red and cyan filters cross each other, the vision of relief could be disturbed. A cyan filter like the "TV red" (distributed by a Belgian television some years ago) having less absorption in the yellow part of the spectrum gives better colors but again problems with the relief of some pictures. Some interesting Kodak cyan filters have been reproduced in the graph, they will be tested later on different color monitors to give an advice in the choice of filters and to try finding the best compromise between color and relief vision.
The spectra of filters above demonstrate evidently that the color rendition of anaglyphs is far from perfect due to the strong absorption of both filters in the yellow part of the spectrum. Some filters with a decreased absorption in that region (SAR ELEC or TV cyan) improve the yellow color in the final image.
An other limitation of the system is the poor 3D effect observed with some strongly colored objects like ferricyanide crystals (dark red). The problem comes from the fact that one of the stereo image is very clear without good contrast an the other is very contrasted. In that case, it is still possible to get a good relief by rotating the color space and thus transforming the dark red color of ferricyanide to yellow.