From: michael rees (zedand00@sound.net)
Date: Wed Feb 17 1999 - 19:59:36 EET

razdan wrote:
> I remember somebody posted information about papers on texturing surfaces
> for RP. I am interested in finding out any information on the subject.
> Thanks in advance
Dear Anshuman,

I published an article in Prototypine Technology INternational, January
March 1998, called "Casting a Shadow". The technology that I discuss is
called shapped displacement shading with examples from FORM Z.

The article, without images, is attached below

michael rees SCULPTOR 		http://www.sound.net/~zedand00/
1212 w 8th St. Bldg B #2, 	816 753 3020 voice    zedand00@sound.net
KC, Mo 64101			816 753 1542 fax

Shaped Displacement Shading

Michael Rees

Shaped displacement shading or image based displacements are terms used by some software manufactures to define a process whereby a grayscale bitmapped image is used to texture or model a CAD (computer aided design) design. For my purposes in this article, I will refer to this as SDS (shaped displacement shading). It is distinct from texture mapping, where that same bitmap image can be applied as a visual texture to an object for rendering purposes. Using shaped displacement shading alters the geometry of the CAD model so that a rapid prototyped model derives its texture or, in some cases, form from the bitmapped image.

SDS is elegantly simple. It interprets the grayscale of a bitmapped image and assigns height value to each tone of gray between black and white. Typically, what is white in an image can be assigned as the highest points and what is black, the lowest points. Every gray tone in-between has a corresponding value. For example, a fingerprint, with the proper parameters defined, becomes a plausible fingerprint in the RP model.

I first became aware of this technology by mistake, a valuable misunderstanding. In the early days of my research into rapid prototyping, I was taken by the stair-stepped texture of the models. They created an attractive moiré pattern across the surface and were pleasing to the touch. As my first images were rendered from the CAD program via texture mapping, I made the assumption that those textures were incorporated into the geometry of the model. Receiving a RP part was a mixture of excitement and disappointment. I called the bureau and asked, "where's the texture?" The bureau laughed and patiently explained the misunderstanding. They also didn't know how to texture the model.

SDS also has parameters to accommodate the designer's intent in various ways (fig.6). In Form Z, a solids surface hybrid modeler, these parameters include: mapping types (flat, cylindrical, spherical mapping); tiling options (user determines the number of tiles he/she wants in an object from 1 to infinity); the proportions of the bitmapped image; the top and bottom height of the extrusion based on the bitmapped image; smoothing attributes; adaptive meshing; and triangulation (employed to maintain the planarity of polygonal facets). A SDS operation is similar in most respects to texture mapping (in which an image is used to render the object). These options allow for applications that range between a simple texturing of a design, to operations that are more akin to creating an object. Both functions will be examined.

The function of SDS has several applications towards the design of a product. A simple example would be a tape player for use in water environments. A smooth even surface with acceptable radii would easily slip from the consumers' hand. If a texture is applied in specific places, or perhaps over the entire model, that smooth surface with its radii, becomes just slightly coarser, allowing the consumer a better grip on the object. There are many more such applications, ranging from functional to aesthetic, that justify the application of displacement on a model.

Bitmapped images can create this effect as a texture or as a form. In the following example, the same image is used to create an even texture over the entire surface or to create a "landscape". Keep in mind that the original object can be of any geometry though there may be some difficulties in certain designs.

An image for use in a SDS application must first be prepared (scanned, copied from another image, imported through a digital camera, etc.,) in an image editing program. The smaller the image the better for each pixel will translate into many polygons. The scanned image of the palm print is roughly 280 pixels square. This translates into a model with over 12,000 facets (figs. 1 and 2). For the palm print to look "natural" on the object, it must have a sufficient level of detail. (Note: This is not so much a factor of the number of pixels in the image but rather of the maximum segment length as defined in the displacement dialogue window. fig 8)

In figure 1 the palm print is applied to the top surface of a rectangular cube. The goal is to create a texture in the model. In the left hand corner of the figure is the original image. For this model, the mapping type is flat, the height of displacement set to .25", the tiling set to 1 in vertical and horizontal directions, and no smoothing applied. With adaptive meshing checked, the maximum segment length is set to .03125". This gives the displacement significant detail which is very close to the photograph of the palm print and to the print itself.

In figure 2, the print is displaced up to 3". This is the only difference from figure 1. The look is of a harsh landscape. It is a texture but SDS is functioning more as a modeler that forms an object. Image 3 also demonstrates this modeling function. The only difference from figure 2 is the use of smoothing, which is set to 50 (of a possible 100). This gives an even terrain look that would rival the rolling hills of the most pleasant landscape. But again, the image is used to model, rather than texture, the model.

Figures 4,5,7, and 9, exhibit some of the problems that using SDS may encounter. In Figure 4, the image used is still the palm print. Applied over a saddle shaped surface, the palm print is no longer recognizable as such. It has become much more abstract. In Figure 5 the image is changed to a polka dot pattern. Because this particular pattern does not employ a mapping type that would allow it to be adjusted to each facet of the surface, the dots are applied perpendicular to the reference plane (in this case the xy plane). (Note: Form z can handle several types of mapping styles for texture mapping, but can only use flat, cylindrical, or spherical types for displacement.) Using the knurled image produces a more even texture on the model but one can still notice distortions. Figure 9 demonstrates the same knurled image applied to the saddle shape and to a square cube. The former has distortions, while the later is faithful to the image. (Notice that the knurled image is a color image. The program reads it as gray scale regardless.)

Figures 6 and 8 are screen shots from Form Z's displacement dialogue window. It shows all of the adjustments that can be controlled when displacing an object. In figure 6, in the preview window, the blue lines show the tiles, as they will be mapped to the object. In the figure 8 the preview shows the object in a wireframe view according to its current determination.

Shaped displacement shading can produce some pretty spectacular results, but make no mistake about it; it must be finessed to accomplish the desired effect. Various techniques applied in the image-editing program to the image will also affect the model. Trial and error, and good judgement about the cost effectiveness of this process will prove the most useful tools for its implementation.

All of the exercises in this article are executed in Form Z from Auto Des Sys, 2011 Riverside Drive, Columbus, Ohio, USA, 43221. 614 488 8838 voice. 614 488 0848 fax. <http://www.formZ.com>.

For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/

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