使用VRay双面材质制作写实枫树叶
http://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/c1c320df93aae926d22fc9b15a07d3f2.jpgHow to make leaf materials for 3d trees using the VRay2sidedMtl, in this case a sugar maple (in autumn). The tutorial uses Onyx Tree for the tree generation, but this material technique works just as well with other tree objects with low-poly opacity mapped leaves.在这篇教程中将教大家如何使用Vray双面材质制作红枫树叶。这篇教程使用Onyx Tree来生成树，当然这篇教程用到的技术同样适用于用其它方法制作的低分辨率使用透明贴图作为树叶的模型。http://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/af5c8c5bfdb07ba66b212ffc4372f41d.jpgExporting from Onyxtree.
导入Onyxtree树木模型I went with 4 polygons per leaf so that the leaves aren't just flat. Remember to set the dimensions you want the individual leaves, and change the units when you export. You can also make it export 3 different leaf IDs with varying sizes. I normally export as a .obj file我设置每片树叶4个多边形面，这样叶子看上去就不会是平的。不要忘记设置树叶的坐标（dimensions）和输出的尺寸单位。可以输出3个不同IDs和不同大小的树叶，然后输出为obj格式。http://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/f7376de48c066c654d36d3f3f8403a41.jpgOpacity mapping
透明贴图It's important to make sure the opacity map is just pure black or white, with a sharp edge. You should also turn off filtering in the bitmap loader options.想要透明效果渲染正确并且质量高，那么透明贴图应该是纯白和纯黑，并且保持锐利的编辑。同样还需要设置bitmap的过滤方式（filtering）为Nomehttp://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/4464d7b53220d6d9586c44bcfdd2815f.jpghttp://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/4fb72954734397f9d5caacdcba4ef886.jpgThe opacity map applied to a grey material in 3ds Max and assigned to the leaves.默认材质球连接上透明贴图后的枫叶效果VRay2sidedMtl
Vray双面材质V-Ray's 2 sided material works best with geometry that has no thickness, which is what onyxtree outputs. It is a very quick way of generating a SSS (sub-surface scattering, think candle wax, skin, milk etc) type look. The image belowis rendered using a VRay2sidedMtl on all leaves with grey submaterials and a hand drawn image for the vein skeleton.Vray双面材质在onyxtree软件输出的无厚度模型上（没有挤压出厚度的多边形面）效果非常棒。用这种方法可以快速生成SSS效果的树叶。下图就是使用Vray双面材质并连接一个树叶支脉纹理图片渲染出来的。http://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/a3d1a0104c7049d8a78afcdae12eec96.jpghttp://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/3f654a88c2b6d80e6abf7b6d621ff2de.jpgFront material正面材质The front side material is a basic vraymaterial with a diffuse map and a reflection map. The color correction map is to produce slightly different hues of leaf for each of the 3 sub leaf types (you should have got a mult-subobject material when you imported the .obj file). I usually make the first leaf and then copy and paste it to the 2nd and 3rd and just change the hue value slightly. The reflection map is a b&w copy of the diffuse map with levels adjusted to make it more contrasty.枫叶正面材质使用基本vray材质连接一张diffuse贴图和一张反射贴图表现的。连接在Diffuse通道上面的色彩校正贴图(color correction map)对3个不同类型的枫叶设置稍微不同的色相（hues）值。我一般喜欢先制作好第一个叶子，然后复制粘贴出其它的叶子，每个叶子我只是稍微调节Diffuse通道的色相（hues）值。枫叶的反射贴图是Diffuse贴图通过去色和增加对比度制作出来的。http://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/d65edb3ac6624941787a2e13159e2653.jpghttp://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/a12cf3c69549aac03cd685f00422f580.jpgRender showing front leaf material on both sides with no transparency. 无半透明效果下的双面材质渲染效果Back material背面材质The back material is a copy of the front material but with a different diffuse bitmap and no reflection. Note that I overlaid the veins skeleton jpg on top in photoshop as well. (without it, the veins looked too light when viewed from the underside as they took 100% of the lighter back material)背面材质是正面材质的拷贝，唯一不同的地方就是使用了不同的diffuse贴图以及没有用反射贴图。我在Photoshop中将正面的颜色贴图叠加上树叶经脉纹理。(without it, the veins looked too light when viewed from the underside as they took 100% of the lighter back material)（如果没有这一步的话，假使我们使用了较亮的背面材质，那么从背光面上看，我们的叶脉会过亮。）http://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/d41d580f65984224c3be03590bd8baa3.jpghttp://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/93acfa0814516c3a8e22395174a339ba.jpgRender showing 2sided material with correct front/back materials but no transparency.无半透明效果下色彩校正后的正面/背面材质渲染效果Finished result最终渲染效果http://www.cgmax.org/resource/users/2012-04/d6b84cb68dab383f7d42938a5301dbaa/images/201208/8ddd8098efca8db6d1c64fb53bd68335.jpg
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