The present paper introduces a method of quantitative
visual colour analyses for the alkaline feldspar, nepheline and sodalite
of ornamental rocks, using digitalised images of polished surface obtained
by means of direct capture method. Initially, each pixel colour of the
selected mineral has been analysed in RGB (red, green, blue) format by
means of graphic softwares. The results show the colour dispersion intolerably
wide for one determined mineral, indicating that direct application of
GIS system based on the RGB is improper for this purpose.
The detailed study of the visual colour property
of the minerals reveals that, in spite of the wide variation in the RGB
values, the R/G, G/B and B/R ratios are relatively constant. On the RGB
triangular diagram, the colour data of the minerals fall in a small area.
In the HSB colour system, H and S parameters
show only a small dispersion, in which, the H is the most stable. In this
system, most of the colour variation is attributed to B (brightness) parameter.
In this sense, the H (hue) is the best and the S (saturation) is the second
best parameter to represent quantitatively the colour. Consequently, the
HS diagram is the most recommended method to evaluate the visual colour
of the minerals constituent of ornamental rocks.
Palavra chave: rocha ornamental, cor de mineral,
quantificação de cores, RGB, HSB