Interpolation -- Part II

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS or Photoshop CS2 Used in this Tutorial

For an newer article on this topic, please read Interpolation Revisited

AT 200% interpolation, not much difference could be seen between the various interpolation methods. However, as images are enlarged to greater sizes, the interpolations become more critical. In this article, the images are interpolated at 300% and examined.

Interpolation: Egret at 300%

Figures 1 to 5 show the Egret at 300%.

Figure 1: Bicubic

Figure 2: Bicubic Smoother

Figure 3: Stairstep

Figure 4: Genuine Fractals

Figure 5: S-Spline

At 300% interpolation, differences can now be seen. The S-Spline and Genuine Fractals are sharper and cleaner, with the Genuine Fractals having the most pleasing appearance to my eye. At first, I thought that simply increasing the sharpening of the Bicubic, Bicubic Smoother, and Stairstep interpolations would yield images similar to the Genuine Fractals. This proved not to be the case. Increasing the sharpening on these interpolations started to degrade the images. Clearly, Genuine Fractals has an advantage on this image when compared to the Bicubic, Bicubic Smoother, and Stairstep interpolations. On the other hand, there is a much smaller difference between the Genuine Fractals and S-Spline interpolations.

Interpolation: Mojave Green at 300%

Figures 6 to 10 show the Mojave Green at 300%.

Figure 6: Bicubic

Figure 7: Bicubic Smoother

Figure 8: Stairstep

Figure 9: Genuine Fractals

Figure 10: S-Spline

The differences between the 300% Mojave Green images are not large. The Bicubic, Genuine Fractals, and S-Spline are slightly sharper in the area of the scales than the Bicubic Smoother and Stairstep. I am not sure that there would be much of a difference between the Bicubic, Genuine Fractals, and Spline in a print.

Interpolation: Clock at 300%

Figures 11 to 15 show the clock at 300%.

Figure 11: Bicubic

Figure 12: Bicubic Smoother

Figure 13: Stairstep

Figure 14: Genuine Fractals

Figure 15: S-Spline

On these clock interpolations, I can see no real differences between the Bicubic, Stairstep, Genuine Fractals, and S-Spline interpolations. The only noticeable difference is with the Bicubic Smoother, which has slightly softer detail on the tan part of the clock face.

Interpolation: Doll at 300%

Figures 16 to 20 show the doll at 300%.

Figure 16: Bicubic

Figure 17: Bicubic Smoother

Figure 18: Stairstep

Figure 19: Genuine Fractals

Figure 20: S-Spline

On the 300% doll interpolation, Genuine Fractals is the clear winner. It produced a smoothness of detail in the fine hair that the other interpolations simply can not match. Closely examining the hair in the Bicubic, Bicubic Smoother, and Stairstep images reveals a blotchy look. This is especially evident where the hair crosses the eye of the doll. The S-Spine interpolation does not have the blotchy look, but it has a grainy appearance (to me at least).

300% Interpolation Conclusions

Two conclusions can be drawn from the 300% interpolations:

At 300% interpolation, the best interpolation method for an image may depend on the image itself. With these four images, the clock showed virtually no difference between four of the interpolation methods, the Mojave Green showed only minimal differences, the egret showed sharper and cleaner images with the Genuine Fractals and S-Spline interpolations, and the doll showed a far superior interpolation from Genuine Fractals.

The best interpolation method may depend on the size of the interpolation. At 200% interpolation, the differences between the interpolation methods were relatively small and probably would not be noticeable in a print. At 300% interpolation, differences between the interpolations methods start to become evident.