If you've read the tutorial, Adding Rivets: Patches or Decals?. I made a case for using splinage for rivets because they look better and really don't add to the size of model. However after putting almost 900 rivets in the mail car, I realize they do bog down the modeler. So I decided to compromise and use bump decals for the rivets attaching the sheet metal side panels.
However if I'm going to use a decal for a rivet, I want to minimize the lack of realistic shadows. So I modeled a flush style rivet that doesn't protrude much from the surrounding surface. Such rivets are typically only used on aircraft skins but I felt an inaccurate rivet was better than no rivet at all. Here is the a screen render of the model. It's a simple lathed, two part model with a flat button head and a dimpled surrounding surface with a slight distortion around the rivet. As usual I built the rivet full scale to be about 0.5" (12 mm) in diameter.
The next step is to apply a gradient material so the changes in Y values in the model are shaded proportionally; that is, high areas are brighter and low areas are darker. While viewing the model from the side, turn on the rulers and mark the highest and lowest points. In the case of this rivet, I wanted the surrounding surface to have a middle grey value of 128 so I put the rulers an equal distance on either side. The gradient material has an upper RGB value of 256, 256, 256 and a lower value of 0, 0, 0. It's start and end Y positions should have the same values as the limiting ruler values. Group the model, apply the gradient material and screen render a top view.
The left image is the result. But this can't be right! My monitor is set to 16 million colours and I can't even get 256 levels of grey? I assume the problem is due to the height of the model and the numerical accuracy of the gradient algorithm. It simply can't properly deal with a height differential of only 0.04" (1 mm).
The solution is to scale up the model considerably (1000%) and adjust the gradient limits accordingly. The right image is the resulting screen render. There's still some strangeness in the surrounding surface but we can fix that in a paint app.
After some retouching, massaging the brightness and contrast and some cut and pasting, the final, full size decal is a column of flush rivets with a simulated edge of overlapping sheet metal that I added in PSPro. The result can be seen in these images. I found that I preferred the look of the rivets with a bump percentage of more than 100%.