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).

#How to make one

These steps make a crack alpha from your own sculpt and use it in Blender. The same file works as a ZBrush alpha.

1. Sculpt the detail on a flat plane. Make a 20 × 20 cm plane at a dense subdivision level and sculpt one crack into it, keeping the border of the plane untouched.
2. Bake a height map. Bake the sculpt onto a flat 20 × 20 cm low-poly plane as a height map, at 1024 or 2048 px in 16-bit (see baking). Set the bake's height range so the deepest part of the crack is near black or white and the untouched surface sits at one even value.
3. Clean the image. Make sure the untouched border is pure black for an additive alpha (or invert it later). Any grey at the border leaves a square outline when you stamp.
4. Load it as a brush texture. In Blender, add the image as the brush's Texture and choose a mapping: Area Plane "keeps the texture from stretching on steep angles", Random picks a new texture position per step (Blender Manual: Texture & Texture Mask). [In ZBrush, import it in the Alphas palette, where you can flip, rotate, invert and adjust blur, intensity and contrast (ZBrush for iPad Help: Alpha).]
5. Test it. Stamp it once on a curved surface, then drag it as a stroke. Adjust strength and size until one stamp reads as one crack.
6. Make variants. Repeat for two or three more cracks so you can alternate them and avoid visible repeats.

#Video tutorials

#Use in recent games

Breakdowns rarely list individual alphas, so documented examples are few.

• The Last of Us Part I (2022), Naughty Dog: rocks. Sobotta's tiling rock materials started as two sculpts made with "a combo of hand sculpting and alphas", according to the description of his breakdown (ArtStation: Rock Breakdown).
• The Last of Us Part I (2022), Naughty Dog: river rocks. Small rocks were sculpted once into two atlases, smooth and sharp, and scattered with seven Atlas Scatter nodes from largest to smallest (ArtStation: River Rock Breakdown). The atlas works like a sheet of stamps for a material graph. See texture atlases.

#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