3.2.1 Acquisition and definition of alpha
The alpha mask channel is a channel image that is consistent with the
basic appearance of the mountain stone by defining the particle density,
hardness and fusion degree in the particle stacking (or overlapping)
model according to the needs of digital landscape painting freehand
effect. The mask channel image participates in the rendering process of
texture and model composite mapping. In order to achieve a better fit
between the texturing texture and the shape of the mountain stone, that
is, the ink and texturing texture can blend and complement each other,
or some places only display light ink without texturing lines, and some
places only display texturing lines without light ink. In Maya software,
we redefine and set threshold values for the black, white, and gray
levels in the alpha mask channel, and set the weight value of the
texturing texture display degree between the block gray value
0<RGB<255. Among them, the RGB gray value starts
from 255 to 0, and the display degree of texturing texture rendering
gradually weakens. When the texturing texture display degree of the
darkest block (that is, pure black with RGB of 0, 0, 0) is 0%, The
texturing texture of the whitest block (that is, pure white with RGB 255
255 255) is 100%. Figure 5-A and Figure 5-B show the RGB gray value of
each block of the mask image of the alpha channel and the texture
display corresponding to the gray value. The final alpha mask channel
image generated by custom rendering is shown in Figure 6.