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5
Spearhead Display
Figure 6.
Color Corrector Adjustments with Spearhead Display.
Figure 7.
Spearhead Display & Image before (right side) and after (left side) Color
Correction.
Spearhead for Color Correction
Figure 6 illustrates how to use the Spearhead display to quickly make color correction adjustments. As shown, the setup or
black level is easily set by adjusting the image dot locations for alignment to the lower corner of the Spearhead triangle. The
RGB White or Gain affects the image dot locations near the upper side of the triangle increasing or decreasing the color Value
or intensity. The RGB black-level controls affect the image dot locations near the lower side of the Spearhead triangle increasing
or decreasing color Saturation. A chroma level change stretches or compresses the image dot locations along the horizontal
changing both Saturation and Value. Lastly, the gray-scale balance of the RGB gamma controls affects the alignment of the
monochrome components of the image to the left side of the Spearhead. Almost anyone can tell if a video image is not properly
gray-scale balanced so this is an important adjustment. For example, adjusting the red gamma control can stretch or compress
the dark red components of the image. This will cause the gray parts of the image to pull away from the vertical or left side of the
Spearhead and cause the image to have a reddish tint. Thereafter, the green and blue gamma controls can be optimally adjusted
to push back the bowing to the left side thereby establishing gray-scale balance very quickly and returning the monochrome
parts of the image to the displayed white point.
Color Corrector Adjustment with Spearhead
Figure 7 shows a practical example of using the Spearhead for color correction adjustment. The upper right tile shows the
Spearhead display of the image just below it in Figure 7. Note that the image looks a little dark and has RGB gamut errors.
For example, looking at the Spearhead display we can see the blacks near the bottom of the triangle are below the 100%
Saturation line and exceed the valid RGB gamut space. Also note that there is a dot below the 0% point of the left side
(Lightness axis) of the Spearhead triangle causing a luma gamut error. Both of these could have occurred due to the removal
of setup at an earlier conversion stage. It is also clear that the image has been reduced to about 75% color Value not fully
utilizing the full RGB gamut space.
Spearhead Display and Image after Color Correction
The left side of Figure 7 shows the image and Spearhead display after color correction. The color correction adjustments were
made by looking only at the Spearhead display. The black level gamut error was corrected by increasing the RGB black-level
adjustments whilst tweaking the three black-level controls to place the dot and the very bottom of the triangle without exceeding
the boundaries. The interaction of the black level controls quickly converges to the location shown on the left Spearhead display.
The Y gain and Chroma gain were adjusted to fill the valid gamut area by looking at the sections of the Spearhead triangle
affected by each adjustment as illustrated in Figure 7. The result is a more vivid image with more detail in the blacks and the
removal of both the Luma and RGB gamut errors without any changes in Hue. A Vector display could also be used to verify that
color correction adjustments are not changing the Hue of higher Saturation and Value parts of the image. Finally, it is interesting to
note that the bright red nose of the ape is visible as a higher intensity series of dots near the tip of the Spearhead and the nose
is at full 100% Value and Saturation.
