John Williamson wrote:
> Hi,
> 
> Here is an isosurface blob using a exponential density function. Shown
> is the same blob at different thresholds. Seems relatively smooth --
> can anyone suggest a good test that causes non-smooth areas on
> normal blobs?

Wow, they are very smooth indeed. This seems quite promising. What was the 
render time?

It is really quite easy to get non-smooth results with blobs. The attached 
image shows the 2nd-derivative artifact quite clearly, especially in the 
shading. Here's the source

// --- --- ---

global_settings { assumed_gamma 1 }

camera { location <0,12,0> look_at 0 }

light_source { <100,300,-100> 1 }

#declare i = 0;
#while(i < 3)
        
        blob {
                threshold i/6+0.1
                sphere { <-2,0,-1>,3,0.6 }
                sphere { <3,0,-0>,3,0.7 }
                sphere { <1,0,-2>,2,0.9 }

                pigment { rgb 1 }
                finish { phong 0.9 diffuse 0.8 }
                
                scale 0.6
                rotate 90*y
                translate (i*4-4)*x
        }
        
        #declare i = i + 1;
#end


sky_sphere {
        pigment {
                planar
                color_map {
                        [0 rgb <0.1,0.1,0.5> ]
                        [1 rgb 1 ]
                }
        }
}

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Download 'blob_artifact.jpg' (19 KB)

Preview of image 'blob_artifact.jpg'