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.