The purpose of a bump map is to provide the engine with a gray levels representation of height, which will be translated into a a fake appearance of surface hills and slopes.
One of the most common mistake is to simply use the color map (converted to grayscale or not, this doesn't matter) as bump.
The problem is that color maps are often sharp and full of crisp details for better quality rendering of the material. Sharp, crisp and bump mapping don't go well together.
In the example below we present a simple wood material without bump.
Then we simply converted the color texture to grayscale to use as bump with a 50% bump value.
There seem to be bumping but it's not very clear, light has no evident direction and it overall looks messy.
The reason is that for bump mapping the engine expects a small gradient of gray values that represents the low to high transition, without this information, the following happens (representation of a very small bump map detail):
|Hard bump detail||Soft bump detail|
The hard black to white transition leaves no room for a smooth
|A smooth black to white transition ensure that the engine has sufficient|
information to perform a smooth interpolation, providing better results.
To problem is similar to rendering a cube or any hard surface object with hard edges and one with filleted edges.
To solve this issue the quickest way is to simply blur the bump map in photoshop, by 1 to 5% depending the map resolution.
In this example a Gaussian blur of 1.5% in Photoshop gave the best details/sharpness ratio. Nothing else has been changed: Same bump amount, same everything.
To go further you can process your bump maps in a better way, for example by attenuating the pure white and pure black areas, duplicating the layer in screen mode and then blurring it even more.
The resulting map like the one shown below provides both better height representation while keeping small details: