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- 大饼先生 -

- Viab Cheng -







2006-06-15 14:56:08|  分类: 其他东西 |  标签: |举报 |字号 订阅

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Realism and Shading
Steven Stahlberg


译:大饼 2002.5

The devil's in the details
I'll make a sweeping statement here - good cg pays attention to the details, bad cg doesn't.
This doesn't mean such an image necessarily and always has to be realistic, like a photograph is realistic - but in practise, it turns out that's usually the direction it should be taken.
First let me define what I mean by 'realistic detail' here.
The universe around us, the one outside our computers, has everywhere we look a certain hard-to-describe chaotic, natural ‘richness’, the opposite to ‘mathematical perfection’; the dirt and dust on our man-made things, the accidental factors that flaw them, the dents, cracks, spots, mold and rot, the branching of trees, the varying shapes of mountains... The poor 3d artist has to jump through all sorts of hoops to achieve this kind of richness. This is because what we're dealing with in cg is just numbers - mathematical perfection. What we enjoy resting our eyes on most in the world around us - the precarious balance between chaos and patterns, randomness and sudden discontinuity, contrasts, is all alien to the computer, in fact the direct opposite to what it does best.
True, the computer is good at fractal detail, and patterns, but there is a uniformity and 'coldness', a sharpness and smoothness, that becomes boring and ugly without the other factors mentioned. (I'll get back to that later.) So a single layer of strong Fractal Noise just isn’t enough - we want more, qualitatively speaking. More of what, exactly?
Every successful work of art has (among other things) some or all of these aspects I mentioned before:
1. Detail (complexity)
2. Variation of detail
3. Discontinuities (and edges)
4. Patterns
5. Contrasts
6. Randomness, Chaos (applied to all the above)
How to apply these points to your images? Here are 6 things to remember, to have strong kung fu in cg.
1. 细节(复杂性)
2. 细节的变化
3. 不连续(和边缘)
4. 图案
5. 对照
6. 随机的混乱(应用于以上几点)
1. Shading
Add shading to the objects in the scene, this raises the complexity/variation level. This may seem obvious, but I start at the beginning to better make my point. In Maya it's just a matter of placing at least one light, Don't use high incandescence or ambience too much, since these will tend to blow out the shadows.
Note that the ambient light is too strong in the left one below (a common mistake), and corrected at right. We want the darkest shadows to be dark, almost black but not quite, except maybe for small parts of the image. (Unless we're going for a high-key scheme, I'll discuss that in the lighting segment.)

2. Shadows
Now let's add shadows - this also seems simple enough, but this step is very often skipped, and it's amazing how much some simple shadows can add to an image. We add them to all objects that are not transparent, except if the shadow directly interferes with what we are trying to achieve of course.

3. Textures complexity
All textures must be above a certain level of complexity, or they must be very muted and subtle. NEVER use a simplistic texture, like a procedural (fractal noise, ramps etc), on it's own with very high contrasts, or strong colors. Watch out for strong colors - a surface that only reflects light and emits none of its own, should never have a 100% saturated color on it. Also, on composition - don't cover too much of your image area with a very strong color.

4. Modeling or texturing detail
Thin planes not very far away must have some thickness, bevels should always be added if visible, subtle details - there are never enough, so add more (fillets, flanges, ridges, dents, seams, bolts, panels, defects, dirt, etc), and the polygon resolution must not be too low in rounded objects (if you can see the facets, it's too low). Points 3 and 4 is where 50 to 90% of all the effort and time should be placed (depending on the image) - a good general rule is: The More Detail And Variation The Better. Now I don't mean you should go putting 10,000 exact copies of rivets on a wall - there's no variation in that. To stick with the riveted wall as an example - you should add as many rivets as will look cool (never add detail that will not be seen!), but make sure some of them are different from the others. Also make sure they are not evenly spaced and in perfectly straight lines (and maybe some are missing), that the wall is slightly bumpy and buckled, maybe scratched and dented, that some dirt has collected around the rivets and maybe some rust has run down the wall from some of them, the wall should have a different texture at the top than at the bottom, etc etc...

5. Realistic lighting FX
6. Post effects
Motion blur, if you have fast movement (fast as measured in screen pixels per frame, not absolute speeds). Depth of Field if necessary/applicable. And lastly: Gaussian blur and film grain, in that order (do that always, a computer will render things too sharp and smooth every time).

Additional notes:
Be careful with Glows and Lensflares - beginners very often use too much of these.
The concept of multiple pass rendering for later compositing is a very useful one, and can be used in many different ways to speed up production, and add flexibility and choices towards the end of a project. It can be used to separate light sources and their effects on the scene, it can be used to create your own motion blur, it can be used to separate glow effects, particles, foreground/background, super-high-resolution parts from normal or low res, raytraced effects from raycast, faked fog and faked Depth-of-Field effects, etc.
For printing, you can the final image in Photoshop 200 - 400% without obvious loss of quality, especially if you add grain. A big problem with cg in most cases is that it's too sharp anyway, especially at high resolutions, so lower the rendering resolution and save time as well.

About textures
The best texture is almost always a retouched photo. Why? Because we're directly using a piece of 'Reality', with all that entails in regards to chaotic patterns, detail and variation.
The best tools to use for photo retouching are the clone/stamp tool, and filters applied in masks with varying degrees of feathering on the mask borders.
The worst texture is usually a simple straight forward procedural with no layers.
Sometimes worse are the ones hand painted from scratch in a computer. The reason for this is partly that painting anything is very difficult, and especially using a computer. Mainly there's the problem with the pen tool. A real airbrush, for instance, should never be used freehand, the results are almost always terrible, it should always be masked one way or another. Same thing applies to the computer.The reason the result looks so bad (if we don’t watch out) is the uniformity and smoothness of the lines, sometimes with the little globules at the start and end of every stroke.
Avoid repeating textures at all costs - an example: you have to shade the floor of a large hall with a granite texture. It's a nice texture, but the problem is it's not big enough, it has to be repeated 100 X 150 times at least. But when you render a view from average human eye-level looking across the hall, you see this horrible weird zig-zag pattern extending across the floor. Mirroring and Staggering doesn't seem to help, in fact Mirroring made it worse by making the zig-zags bigger. How to fix it?
One way is to use a 'smear-map' with much less repeats than 100 layered on the main texture - a subtle one will be enough. Another way is to layer textures on each other, with different repeats. Say you have a checkered pattern that repeats 12 times across a floor.
Another way is to open the image in Photoshop, paste it as many times as you like in a larger window, and then make sure it's non-repeating by painting and retouching, then reapplying it in your shader with less repeats. But be aware that this creates larger image files, eating disk space, RAM, and slowing shader-updating.

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