Alphas and stamps

An alpha is a greyscale image used to shape a sculpting brush. Where the image is bright the brush acts strongly; where it is dark it does little or nothing. Blender's manual puts the principle simply: for sculpting, the brush texture is "used to determine the strength of the brush" (Blender Manual: Texture & Texture Mask). Pressed once, an alpha works like a stamp: a rubber stamp of a crack, a bolt, a pore pattern or a patch of rock surface. Dragged along a stroke, it becomes a textured brush.

ZBrush calls these images alphas; Blender calls them brush textures. The idea is the same.

#How alphas are applied

The same image can be laid onto the surface in several ways. Blender's mapping modes are a clear, tool-neutral list (Blender Manual: Texture & Texture Mask):

Blender also has a separate Texture Mask, which controls brush strength with a second image (Blender Manual: Texture & Texture Mask). In ZBrush, the Alphas palette lets you flip, rotate and invert an alpha, adjust blur, intensity and contrast, and Make 3D turns an alpha into a mesh (ZBrush for iPad Help: Alpha). Maxon notes that alpha settings work the same on the iPad and desktop versions (ZBrush for iPad Help: Alpha).

#What alphas are used for

• Surface breakup on stone. Sergii Zlobin adds cracks and damage to his stone wall sculpt with crack and damage alphas, balancing the detail against the surrounding areas (80 Level: Creating a Stone Wall Material in ZBrush…).
• Tiling rock materials. Jared Sobotta's tiling rock materials for The Last of Us Part I started as sculpts that combined hand sculpting and alphas (ArtStation: Rock Breakdown). See tileable sculpts.
• Hard-surface detail. Bolts, vents, panel seams and stitching are common stamp subjects, because one clean alpha replaces dozens of identical hand-made details. See greebles.
• Repeated organic detail. Pores, scales, bark and pitting, often with Random mapping so no two stamps line up.

#Making your own

Any greyscale image can be an alpha: a height map baked from a sculpt, a photo turned to greyscale, or a pattern painted by hand. Baking your own from a small sculpt gives the cleanest result, because the values describe real height rather than photographed light and shadow. The same thinking applies at a larger scale: Sobotta sculpted two atlases of small rocks, one smooth and one sharp, and scattered them in Substance 3D Designer to build the small-stone layer of his river materials (ArtStation: River Rock Breakdown).

#Common mistakes

• Visible stamp edges. An alpha whose border is not pure black leaves a square outline. Check the edges of every custom alpha.
• Same scale everywhere. Vary size and strength; real damage is clustered, not evenly spread.
• Stamping on a low level. An alpha can only show detail the mesh has polygons for. Work at a dense subdivision level.
• Stretching on steep faces. Use a normal-following mapping such as Blender's Area Plane (Blender Manual: Texture & Texture Mask).

#Related terms

digital sculpting · tileable sculpts · sculpting damage and wear · height and displacement maps · primary, secondary and tertiary forms · multiresolution and subdivision levels

#Further reading and watching