The importance of Micro-Detail
Every real-world object - whether natural or man-made - has a surface texture. Because neither growth nor manufacturing processes are perfect, real-world surfaces always exhibit tiny and not-so-tiny irregularities. If we are to successfully lend photorealistic credibility to our texturing work, we cannot afford to overlook this crucial aspect of surfacing. Fortunately - as we shall see below - there are some very simple and efficient techniques for simulating this natural micro-detail...
So what's this micro-detail supposed to be? Take a look at the close-up rendering of the Lounge Lizards texture above. Note how the surface reflections appear too clean and somewhat artificial. By adding small micro-bumps to the bump map, subtle distortions are created in the reflection, which greatly enhance the surface's photorealistic credibility. No real-world surface is perfectly smooth, and neither should be our bump maps!
How do we create these Micro-Bumps? The technique is always the same: We take a Perlin Noise with zero Details, zero Roughness and zero Scale, and subtly Blend it over our Height map. The key word here is subtly! Note the Opacity value of 0,5 on the Blend component. Since we're creating subtle detail, less is more. Good Opacity values usually fall somewhere inside the 0,005 to 1 range.
Also be sure to experiment with the Blend modes. In the example above I went with the Overlay mode because I wanted the surface texture to appear abrased in areas that are elevated. However, Screen, Multiply, Lighten, Darken and Normal blend modes also work well usually, and may lead to subtly different results.
Let's take a look at another example: the Decadent Drapery texture. Since we're dealing with a textile surface here, a more complex approach to creating the necessary micro-detail was called for in order to create the illusion of a silky sheen fabric. Note how the reflections and the HDRI lighting are broken up beautifully, courtesy of the surface texture. Without it, this texture would be quite unconvincing.
The textile fabric detail was generated by blending two stretched Perlin Noises, distorted via Offsets and Noise Distortion to follow the folds, and subsequently multiplied over the drapery height map. Again, take note that only a small amount of Opacity is needed on the Blend to create the effect. Some additional subtle micro-bumps further roughen up the fabric and finish off the bump map for this texture.
Note that micro-detail is added only to the Height part of the filter tree in most cases. Since the bump map always indirectly affects the specular/reflective properties of a Surface Filter, we can usually achieve our goal quite economically without resorting to complex maps for the Reflectivity, Reflection Blur and Metallic inputs of the Result component. In our texture example above, the fabric detail was also conveniently re-used for the diffuse map in order to give it a little more interest.
Our last example will be the Rough Plastering texture. Note how the photorealistic credibility of the texture is greatly diminished when micro-detail is absent. Visual chaos is key for this texture, and without the small bumps, dents, brush marks and scratches it would certainly lose its appeal.
One would think that such complex detail is quite difficult to achieve, yet this filter is really quite simple in its construction: The trick to achieve complex detail is to layer several noises using Blends. The micro-bumps have been blended into the noise layers prior to the application of the Noise Distortion components, which effectively 'smear' the small bumps all over the place, creating the brush-stroke-like marks that can be seen in the final result. Again, the effect is subtle, but it goes a long way in lending the texture its photorealistic credibility.
So - in the name of photorealism - be sure to always micro-detail away, all! Try not to overdo the detailing though, or the filter's rendering performance will suffer as a result. Strive for an economy of means!