Category: NPR

Siggraph paper summary: Digital Facial Engraving by Victor Ostromoukhov 1999

Another interesting technique for the post-processing of photographs or other images, this paper proposes a system for creating the appearance of traditional engraving, like you see on money or in the Wall Street Journal.

User places a series of 2D bezier patches to guide the direction of the lines, and creates masks to separate features. A ‘universal copperplate’–essentially a greyscale heightmap of the furrow shape–is warped to follow the shape of each patch. A set of merging rules allow multiple line directions and features to overlap and interact. Finally, the image source brightness value is used to drive a threshold operation on the warped and merged plate. Somewhat more complex than a basic clamp, it takes into account characteristics of the output medium and human vision to improve the result and keep tones in balance.

One innovation this allows is color engraving: creating CMYK separations with engraved lines rather than halftones or stochastic stippling. Also, engravings need not be limited to the traditional lines and mezzotint; other shapes and patterns could easily be used.

As presented, this technique is limited to still images. What would it take to make this operate as a shader or post-process for animation? Could the UV information provided by the patches could be driven by surface UVs? Or some kind of surface normal information? Line size would need to be driven by output resolution, of course. I’ve seen CG art that (so far as I could tell) mapped an engraving pattern to the geometry and then clamped the result based on light direction/intensity. Used carefully, it can look good but the lines are locked to the geometry, like zebra stripes, while the lines in a true engraving seem to me to be somewhere between the object and the picture. Swimming through lines locked to the picture frame would not be acceptable, of course, but when the object recedes, the line size needs to stay consistent in screen space. Seems like a better solution would be to render surface direction and light intensity, and use those to draw and clamp the lines after the fact.

a painterly chameleon

One of the more interesting styles created in this paper

Painterly Rendering with Curved Brush Strokes of Multiple Sizes by Aaron Hertzmann 1998
A novel approach to algorithmically painting photographs, video, or other source images. Builds up the painted frame from large strokes to smaller ones, drawing strokes along the normals of image gradients where there is sufficient contrast, the idea being to place more small strokes in areas of high detail, and fewer larger strokes in low-detail areas. Some of the example images are pretty neat, but the algorithm does not address temporal coherence, so it’s not suitable for animation. Perhaps using oflow to move longer-lived brush strokes would take it further.

aQtree: What Is It?

via David’s Really Interesting Pages comes news of, well, I guess David’s big project: aQtree. This is some kind of non-photorealistic rendering tool or, as it is described on the site, “a dedicated non-photorealistic environment.” What is meant by “environment” I’m not sure, but I look forward to finding out. The sample movies on the site are lovely; go check ‘em out.

Short film: FARD

“FARD” by David Alapont and Luis Briceno is a wonderful example of non-photorealistic rendering done well and in the service of the story.

Clever story, the design is great, and the NPR styling of the characters and the world is really well handled. I never had that uncomfortable feeling of “procedural lines algorithmically drawn over 3D solid” that is the bane of poorly-made NPR. The geometry and lines of the character faces have been optimized and carefully crafted to look their best in each different view, which I think is essential. Were they indeed 3D? Looking at that beefy credit list, I wonder how much was drawn. Either way, it’s nice work, and the integration of the photographic elements is handled perfectly. I’ll definitely investigate the production of this short.

Polygon-Based NPR

via Dave’s Really Interesting Pages, a method for NPR that doesn’t rely on cel shading techniques but instead rapid polygon displacement to create a spatially coherent brush-stroke look. Boy howdy, all that motion blur sure would be fun in mental ray. :\

Also, a short bit of MEL script by Duncan Brinsmead to achieve this effect in Maya.

UVs and NPR

Researching the best free UV solutions, and spent a little bit of time with Maya 2009’s Unfold tool. I can’t tell yet if it’s as powerful as the purpose-built tools like Headus, but it’s a start and it’s built in! The search for a fast fast fast UV solution continues.

Meanwhile, here’s an entertaining bit of non-photorealistic rendering (mixed with IBL, interestingly) that’s pretty entertaining for a student project. I don’t think the style is 100% successful, especially the lack of consistent contact shadows on the ground, but I like how adventurous it is all the same.

Elk Hair Caddis from peter smith on Vimeo.

Goal: Non-Photorealistic Rendering

I think non-photorealistic rendering is cool. In some ways, more exciting than photorealistic rendering. It has a huge potential for artistic expression and visual innovation, it could be a good niche to inhabit, and I want to get better at it. Embargoed project X was a fun start. Goal: become a demonstrated expert in non-photorealistic rendering with a substantial portfolio of NPR work.

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