Making the Diffusion Map
First of all, what is diffusion? Diffusion is a property witch controls how much light is reflected back to the camera and how much of that light is absorbed by that object, it basically controls how much of the surface's color we can see but not to get confused with specular reflections witch are very different, specular reflections control how much an object reflects a "light source", unlike diffusion witch controls the reflection of a surface's "color". In real life the more reflective an
object is the less the diffuse or the less color is reflected of its own, a very reflective object would not show much color simply because the color you see are reflections of other objects surrounding it. For example a chrome metal surface has a very high reflection value of about 85% with would make the diffuse value very low 15%, witch would result in a very dark color because its reflective very little color of its own and a object such as the bark of a tree is not very reflective witch results in a high diffuse level. Even if an object has the most vibrant colors once you make that object say 60 percent reflective then the less amount of those colors were going to see, what we will see are mostly that object's surroundings.
Another example would be a mirror, its 100% percent reflective therefore the diffusion value would be set to 0, because a mirror does not show any color of it's own.
Why use a diffusion map? If you don't use diffusion chances are that when you place that given object in a scene that it will look over lit or over saturated in areas that come in direct contact with light or areas were the specular reflections are more noticeable, other areas often become desaturated or lacking in color, this is partially because that object is showing 100 percent of its diffuse + the specular amount lets say 40 now you can say the object has a value of 140 because that object is reflecting all of its own color and its reflecting at 40 percent witch will produce an unrealistic interaction with
light, the highlights on that object will look brighter than wee need and its true colors are not going to show up properly, that could be solved if that object was 60 percent diffuse. This is a guideline you can use to get about the right amount of diffusion in your surface but having a slightly off balance diffusion value is not wrong if it looks better at the end. Diffusion isn't used mainly to keep a balance with specularity it is also used to maintain color information in our color maps more in tact or true to there color even with harsher lighting setups by using similar color value information in the color map to the diffusion map to make sure light's don't change the original color tones much because as we know diffusion controls the amount of color that is reflected by our objects and therefore making darker areas in the color map also darker in the diffuse map would result in a black color that not only is black but absorbs light as a true black color would thus creating a more vivid color and adding depth to the colors, its called color depth because not only will you see the color
changes but the colors will absorb and reflect virtual light accordingly, what you would normally do in a diffusion map is start of with a gray version of the color map and combine it with the specular map or at times like in this case were going to use an inverted version of our specular map with minor modifications for our diffusion map.
But why not use information for the color map into the diffusion map in this case? We don't want the leaf to keep all of its colors!, because a leaf is very thin therefore lighting changes its color dramatically, if you put a red light behind a leaf what's going to happen is that the whole leaf is going be red, here's a little look at the future the first leaf is using a diffusion map that uses information from the color map thus maintaining its colors under several circumstances:
As you can see it looks fake because diffusion is not allowing the colors to change but this is good in other object but not in this case because like I said a leaf is so thin that lighting and other information such as translucency and luminosity maps would change it dramatically witch a diffusion map that uses color value information from our color map does not allow. On the other hand here's a leaf with a more subtle diffusion map, the diffusion map does indeed allow light, translucency and
luminosity maps to change it accordingly for the leaf effect that were looking for, but like I said, keeping its colors may be a good thing for other objects but its better too keep it subtle for thin translucent objects such as a leaf. Here's the leaf with a more subtle diffusion map.
For the reasons that I just mentioned were going to simply invert the specular map and make some adjustments, therefore the diffusion map will be the quickest one to create.
Make sure your specular map is showing and the bump and color maps are hidden, hide the brown damage layer in the specular set for now, control click on the specular level layer to select the leaf, press shift+ctrl+c to execute the copy merged command, create a new set and rename it to diffusion map, make a new layer inside the diffusion set and rename it to diffusion now paste the specular map into this layer, now wee need to add the brown damage layer we hidden earlier so make a duplicate of it and move it to the diffusion set above the diffusion layer, merge it down to make a single diffusion map. We added the damage layer after because we want to keep the areas that were painted outside the selection.
With our diffusion map in tact invert the colors of that image, right off the bat this image would not look that good as our diffusion map because the diffusion effect is going to look to exaggerated, like the dry fingerprints would have a brighter color tone than the areas around it, we want to keep this effect but we want to make it more subtle so we need to make the diffusion map more of the same color or to wash it out a little, go to image>adjustments>curves, click on the left point and change the output to 45 and give the right point an output value of 215 and apply. Now this image would defiantly create a more subtle diffusion effect, here is a screen of the diffusion map.
Here's a render with the diffusion map applied, its not an extreme difference, in this case diffusion made a subtle but yet essential difference on our surface by bringing out some of its color and details such as the scratches and by making specular reflections look more subtle.