I wanted to create a very basic test file. I created a 2400x3000 pixel file in the sRGB color space at 8 bits (I actually started out with ProPhoto but after seeing the results I was getting I dumbed it down to sRGB). I then created 14 divisions, each with a gradient in it (the divisions were running vertically on the page). I created two sets of gradients for each color those being X to black (where X is the color) and X to white. The values I used are as follows (numbers are in the format of Red, Green, Blue)
So with two sets of that (one going to black, the other going to white) I had 12 gradient strips. I also created a white to black gradient and finally one strip that was grey (128, 128, 128).
I made my own testimage and got somewhat different results with the gamut warning as shown below for perceptual rendering with the Canon Matte Profile which I downloaded from the Canon USA site. The map is not useful because it does not indicate if the color error is large or small:
The standard way to express a color error is with ΔE (which measures both luminance and chrominance in the 3 dimensional relatively perceptually uniform L*a*b space. Some workers prefer ΔC which ignores luminance and takes only color into account). There are several varieties of these measurements, but I am using *CMC. One loads the image in to Gamutvision and a soft proof is given on the upper right. ΔE or ΔC magnitudes are shown by a pseudo color display with the key on the right. Note that the ΔE on the top has a gamut problem because of with low luminance in the shadows. This does not show on the ΔC plot or on the Photoshop Gamut display, but it should show up with softproofing with black point compensation turned on.
Correlation between the Gamutvison result and PS is not that good.
The whole test is not that useful because the sRGB color space contains colors that can't be printed and there are many colors that the printer can handle, but are out of the sRGB gamut.
The 3D plot below demonstrates the sRGB (the wireframe) contains high luminance greens and a few yellows that can't be printed, and that some yellows and oranges at high luminance are in gamut for sRGB and out of the printer gamut. By contrast, at lower luminance there are greens, blues, and cyans that are out of sRGB but well within the printer gamut. There is no rendering in these views, which only show the two gamuts. The wire frame is sRGB and the solid color represents the printer space.
Here is what happens with perceptual rendering. The wire frame is the ouput (printer) space and the vectors show how the rendering affects low luminance values (the matte paper does not have deep blacks) and high luminance saturated primaries. There are a great many out of gamut colors, which are clipped by the relative colorimetric rendering. Perceptual rendering can not handle such large out of gamut conditions, and the results are similar.
Research at Kodak and elsewhere has shown a gamut of colors which represent real work surface colors which actually appear in photographs. Excluded are highly saturated self-luminous or florescent objects, neon lights, and computer generated display. Here is a saturation plot demonstrating the real world surface colors along with the gamuts of ProPhotoRGB, aRGB, and sRGB drawn up by Prof. Dr Gernot Hoffman in Germany. The dotted line shows the gamut of the surface colors, and the gamuts of three common spaces are also shown. ProPhotoRGB covers the whole surface gamut, but the smaller spaces do not.
These are 2 dimensional plots showing mid luminances that are most important. At high and low luminances, even sRGB has some colors which can not be printed, but these are not real world colors and are not important. This is shown in this Kodak paper on Color.org. Download and view at your convenience.
Note how the Canon printer colors include important greens at mid luminance that are well outside the sRGB gamut shown by most monitors and not fully in gamut for aRGB. This eplains why ProPhotoRGB is the preferred working color space for digigal photography with modern inkjet printers. Glossy papers have an even better gamut. Softproofing might be problematic with these colors. Herr Schewe can indicate how he handles them.