Color rendering

The book did not consider specifically the problem of color template matching. However, as briefly pointed out in Chapter TM:2, even when monochromatic images are considered, the details of the full imaging process should be considered. It is possible to generate realistic images of the same scene under different illuminations:

  cd theRenderingWorkshop/povray/visual-lab
  ./cameras.sh persp gretag           small incandescent 1
  ./cameras.sh persp gretag           small daylight     1

1   incandescent <- ia.readAnimage(file.path(tiffdir,
2 ...                                  "persp_gretag_small_incandescent.tif"))
3   daylight     <- ia.readAnimage(file.path(tiffdir,
4 ...                                   "persp_gretag_small_daylight.tif"))
5   #
6   tm.dev("figures/differentLights", width=6, height=3)
7   par(mfrow = c(1,2))
8   ia.show(incandescent, main="incandescent")
9   ia.show(daylight,     main="daylight")
10   dev.off()

Figure B.2:

The same scene lit by different illuminants appears different.

If we generate the corresponding luminance images by averaging the color channels

1   iL <- ia.scale(ia.add(incandescent[[1]],
2 ...                    ia.add(incandescent[[2]],
3 ...                           incandescent[[3]])))
4   dL <- ia.scale(ia.add(daylight[[1]],
5 ...                    ia.add(daylight[[2]],
6 ...                           daylight[[3]])))
7   #
8   tm.dev("figures/differentLightsMono", width=6, height=6)
9   par(mfrow = c(2,2))
10   ia.show(iL, main="incandescent")
11   ia.show(dL,     main="daylight")
12   hist(iL, main="incandescent",xlab="")
13   hist(dL, ylab="",main="daylight",xlab="")
14   dev.off()

we obtain different images (see Figure B.3): monochromatic template matching is affected by the illumination conditions of the scene.

Figure B.3:

Monochromatic images of the same scene under different illumination are different.

B.2 Color rendering