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'Ageing' a model is one of the more tricky aspects of 3D, making it looked 'used' in such a way that it appears natural and random, like its falling to bits under its own weight; normally this would mean some extensive work in edit mode, pushing and pulling groups of vertices which can often look a little forced or contrived. Fortunately Blender 3Ds "Sculpt Mode" tool comes in handy as an alternative and relatively quick way of doing just what's needed.
A basic understanding of Blender is necessary to get the most from the following material. Although this tutorial was originally written for 2.49 or below, the same principles and most actions can be used and/or applied in Blender 2.5/2.6 and beyond. Any changes in this respect are marked in the below using the "#" ("Hash") symbol.
Before actually doing anything to the mesh it's best to be mindful of two main issues;
Vertex, polygon & edgeloop distribution
Making sure there are enough polygons and 'edgeloops' present to allow the mesh to deform problem free.
Coincidental position of vertexes
The results of deformation won't always work the way its expected when elements are not aligned correctly.
What both of these mean is that where ever possible the mesh should be built in a 'optimised' fashion for editing rather than for it's use in a game engine, there is a difference. Optimising for editing means the mesh is constructed in such a way as to allow for easy manipulation and changes to the object, it should be relatively easy to add or remove details on such a mesh. On the other hand, a model optimised for game/interactive media use is usually stripped right down to its essential elements, which often mean that further editing can be difficult due to having to work around the often limited shapes and numbers of polygons a model contains (usually triangles' which further complicate additional editing). Bear this in mind when doing the initial build work on the modeled object, game optimisation can come later.
As mentioned above it's important to keep in mind when building a model the eventual goal in terms of what is going to be done with the mesh physically and not just what its going to look like rendered in game. What this means is that as a mesh is constructed, vertex, face and edgeloop distribution has to take into account any eventual mesh deformation; it's no good adding a subdivision to the roof of a building, for example, without adding a complimentary division in a lower wall, preventing gaps from appearing where faces don't meet up. The image below shows this by way of a series of edgeloops that travel down the building from top to bottom; because the loops do this, manipulating the area between roof and wall will result in uniform distortion between the two sections.
The results of deforming don't always go as planned, especially where prior care hasn't been taken to ensure that edgeloops follow the general contours of the mesh (top to bottom, side to side). What this means is that as mentioned above, edges, but more important, the vertexes that compose the different sections of a mesh, need to match up - a 'wall' and 'roof' vertex should occupy the same co-ordinates in space; not doing this now (as best as possible) results in gaps forming because of edge length and vertex position differences.
There are several ways to snap vertices together, the most common being to select a 'parent' (or 'master') vertex first, then a 'child' (single vertex) or 'children' (multiple vertices), pressing Alt+M to bring up the "Snap to & Weld" (#"Merge"#) pop up - various options are presented, click "Snap to First" (#"At First"#)to snap and weld/merge selected vertexes to the first one in the series (the 'master'). Or... Select a vertex and press "Shift+S" to open the "Snap Cursor To" (#"Snap"#) pop up. Various options are presented, clicking "Snap Cursor to Selected" (#"Cursor to Selected"#) will move the 3D cursor to the selected object. Select another vertex or vertices and press "Shift+S" again to open the Snap pop up selecting "Snap Selected to Cursor" (#"Selected to Cursor"#) this time; the selected objects will snap to the cursor but will not be welded/merged into a single object. Be sure then, to snap vertices together (weld or snap-to-cursor) before proceeding with any further editing of the mesh using Blender's sculpt tool.
Design note : making sure vertices are in the same place beforehand also reduces the extra work that may need doing to the UVW map after distorting the mesh with the sculpt tool or moving vertices around.
If not done already, it's best to join the mesh into one single object at this point with vertexes "'remDoubled' (#"Remove Doubles"#) - co-positional vertexes are merged into a single vertex. Make sure that material assignment is done in such a way as to allow for easy break up of the model - if needed - once sculpt manipulation is finished; entering 'sculpt' mode now means that manipulations will move connected vertexes whilst keeping distortions uniform where-ever possible.
Changing to 'sculpt mode' is simply a matter of selecting "Sculpt Mode" from the 'mode' drop down list in the header bar of the 3DView port as shown above (the header may be at the bottom or top of the view port). Additionally, if the "Transform Properties" (#"View/Transform Properties"#) panel isn't already visible in the 3DView, press "N" to display it - the buttons and features should change when going into sculpt mode providing localised control for a number of common sculpt tools parameters. For mesh distortion, "Grab" is a good starting point, click and 'paint' (LMB+drag) with the tool over various areas of the mesh to distort the model.
Despite best efforts, problems will arise where areas of the mesh that were overlooked or weren't prepped properly which result in various forms of mismatch between different elements of the object (see image below);
Window edge wasn't split to match the number of divisions/polygons of the window unit
Polygons pulled too much out of shape due to being a little heavy handed
Gaps caused by not subdividing handrails more efficiently
Gap caused by deforming the porch roof but not the support upright
Each of these problems can be fixed by reediting the mesh in 'edit' mode and tidying them up. Although this may seem a disadvantage, it should be viewed in context of the control gained using the sculpt mode tools as an editing device instead of 'selection/proportional falloff' whilst in edit mode.
Depending on how extreme the sculpted distortions are, it may mean the UVW map needs to be checked and adjusted to reduce any stretching visible on the mesh; it is possible to leave minor problems 'as is' due to the way they can sometimes emphasise the manipulations that have been done, major problems will need to be fixed however. With that in mind, it's always better to do this kind of sculpt distortion before any final texture work has been done for the object. Looking at the images below for example, area #1 below shows a problem area where a previously 'hidden' dark patch on the texture is now visible, this sort of problem is the result of mesh elements now being in different places than previously (due to being physically moved by sculpt manipulation) as well as an "and/or" result of distortions to the mesh that have had a direct effect on the UVW layout itself.
The Halloween House has had a set of ambient occlusion maps had been previously rendered before distorting the mesh so it had both a functional UVW map and a base set of textures. However, as a result of sculpt editing, and with the mesh distorting the way it has, the previous ambient occlusion baked textures now need to be redone because they're out of place; because ambient occlusion relies on, and is relative to, the physical characteristics of a model, any changes mean re-rendering if either the mesh and/or UVW map have been altered ; problems may not be as noticeable with 'flat' textures, that is, texture that are 'tiled' over a model - it's usually an issue associated with 'unique' textures made to fit a specific feature, model or section.
Design note : it's usually a good idea to have access to a set of 'grid maps' - a set of textures that have a simple checker or line grid on them - to make looking for, finding and fixing distortion errors easier to do.
Areas of texture distortion as a result of using sculpt to edit the mesh. These usually need correcting.
Before and after showing area #1 updated after re-baking the ambient occlusion maps.
Test render on the broken 'house' section to check the ambient occlusion renders (and resulting textures) are no longer distorted
Although this sort of manipulation can be done with the various mesh tools available in edit mode, using sculpt mode and its tools do allow for a quick method of altering objects with a decent amount of control over what's being done. After the model has been 'repaired' (the gaps in the mesh closed as mentioned above), the UVW maps tweaked and any textures re baked or reedited in an image editing application, the model will be ready for final output as a model. Although this technique is shown on the low poly model of a wooden house, using the sculpt tools in this capacity really comes into its own when dealing with higher poly meshes that have a much denser distribution of vertexes; manually manipulating those in edit mode can be frustrating and time consuming.