A (much) simpler program to reproduce the problem:
Open in online IDEgraphics3d 800, 600 camera = createcamera() positionentity(camera, 0, 0, -3) const BLOCK_SIZE# = 1.0 const BLOCK_BEVEL_WIDTH# = 0.2 ; lookup coordinates (x and y are identical) global x#[3] global y#[3] global z#[3] x[0] = -BLOCK_SIZE / 2 x[1] = -BLOCK_SIZE / 2 + BLOCK_BEVEL_WIDTH x[2] = BLOCK_SIZE / 2 - BLOCK_BEVEL_WIDTH x[3] = BLOCK_SIZE / 2 y[0] = -BLOCK_SIZE / 2 y[1] = -BLOCK_SIZE / 2 + BLOCK_BEVEL_WIDTH y[2] = BLOCK_SIZE / 2 - BLOCK_BEVEL_WIDTH y[3] = BLOCK_SIZE / 2 z[0] = BLOCK_SIZE / 2 z[1] = BLOCK_SIZE / 2 - BLOCK_BEVEL_WIDTH z[2] = -BLOCK_SIZE / 2 + BLOCK_BEVEL_WIDTH z[3] = -BLOCK_SIZE / 2 ; create block block = createmesh() surface_id = createsurface(block) vertex_0 = addvertex(surface_id, x[1], y[0], z[3]) vertex_1 = addvertex(surface_id, x[1], y[3], z[3]) vertex_2 = addvertex(surface_id, x[2], y[3], z[3]) addtriangle(surface_id, vertex_0, vertex_1, vertex_2) vertex_3 = addvertex(surface_id, x[2], y[0], z[3]) addtriangle(surface_id, vertex_0, vertex_2, vertex_3) vertex_4 = addvertex(surface_id, x[2], y[2], z[3]) vertex_5 = addvertex(surface_id, x[3], y[2], z[3]) vertex_6 = addvertex(surface_id, x[3], y[1], z[3]) addtriangle(surface_id, vertex_4, vertex_5, vertex_6) vertex_7 = addvertex(surface_id, x[2], y[1], z[3]) addtriangle(surface_id, vertex_4, vertex_6, vertex_7) ; translucency better shows the artifacts entityalpha(block, .5) ambientlight(255, 255, 255) ; move it around with the mouse to see artifacts movemouse(400, 300) while true if keyhit(1) then end rotateentity(block, -mousey()+300, mousex()-400, 0) renderworld flip wend