To understand the problem ImgTarget is intended to solve, we must first look at how a traditional colour-managed workflow operates.

The range of colours (gamut) a typical printer can produce is somewhat smaller than that of an RGB "working space" such as sRGB, AdobeRGB, or especially the various wide-gamut RGB spaces such as ProPhotoRGB.  Given an ICC colour profile for a working space and for a printer, a traditional colour management system will build a transform between the two colourspaces, compressing the colours to fit the printer's limited range.

A hypothetical transform between an RGB working space and a printer's colourspace is illustrated here:

Illustration of traditional gamut-mapping

Figure 1.  Certain colours are unavailable on the printer, so colours must be remapped.


In practice this approach works well enough, but there is still room for improvement.  Because a traditional colour management system has to cater for the general case, the strongest colours in the source colourspace are mapped to the printer's strongest colours, but these colours very rarely occur in real-world images.  Especially if using a wide-gamut working space, a real world image generally has plenty of "headroom" - colours which are available, but not used.  The result is that these "headroom" colours are mapped to the printer's strongest colours, which are then essentially wasted, as illustrated here:

Traditional mapping - images typically do not fill the source colourspace
Figure 2.  A typical image doesn't fill the source colour-space.  A traditional mapping
can't take this into account, so the image isn't rendered using the full gamut of the printer.
ArgyllCMS's tiffgamut utility is capable of analyzing an image to determine precisely which colours are required to reproduce it - the gamut.  The colllink utility is capable of building a transformation tailored specifically for that gamut, avoiding this problem:
A mapping is generated specifically for this image, minimising the printable colours which are unused.
Figure 3.  A mapping is generated specifically for the image,
minimising the number of printable colours which are left unused.
ImgTarget doesn't itself perform any of this magic - it's just a front end to Argyll's utilities, designed to make this advanced colour management capability more accessible.