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The reprint of this tutorial is authorized only with the active link: http://www.assistcg.com/ In this article you can learn how to edit 3d model's materials and final Render results using Node Editor.
1. Let's create any 3d model, for example – a pair of cherries. This model will consist of two cherries, petioles and two leaves. We'll make the berry by adding a NURBS Sphere, moving some of its vertices and scaling it to an appropriate size. In the middle of our berry put the stone that is made of the same NURBS Sphere (fig.1). The second berry is a duplicate of the first one with minor changes in rotation, size and location.
 We'll make the leaves using a mesh-surface, having subdivided it several times (fig.2). The petioles can be made by several methods: «skinning» creation of a surface by means of curves (see «Blender Documentation Volume I - User Guide») or simply by extruding vertices of a mesh-circle. In our case we choose the last described method as it is advisable to get a file of a small size.  2. Now it is necesary to add materials and textures to our created objects, add lighting. As these processes are well described in User Guide we won't describe them here. All necessary data are in the attached .blend file. At this stage it is advisable to pay special attention to lighting (tuning of specks and shadows). Basically, the image can be observed as a ready one (fig.3), as final color correction can be made in any raster editor. At the same time such a color correction is possible in Blender using Node Editor; by the way, its functionality reminds Shake from Apple.  3. Color Correction with the Help of Node Editor Having observed attentively the rendered image (fig.3) let's elicit the lacks. The image is too dark with the lack of color saturation. It is a good idea to accentuate the fragments of our foreground (berries). For this, we should blur the elements of the middle ground (leaves and petioles). We've defined the aims of color correction and now can open Node Editor. For more comfortable work, the following windows must be opened: Node Editor, UV/Image Editor, Buttons Window, 3d View. Node Editor works in two modes: Material Nodes and Composite Nodes. Let's examine these two modes.
Material Nodes With the help of this mode you can edit materials of your selected object. To work with Material Nodes you should select an object you want to edit (when you select an object you also select a material of the object), activate the «Material Nodes» and «Use Nodes» buttons in Node Editor, activate Buttons Window (F5 >> panel Links and PipeLine) and press on the «Add New» button (red stripe) or just choose the available materials and start to edit them in Node Editor. For editing use the «Add» menu, where you can choose the necessary node. If you want to delete a node, select it and press the «Delete» button on keyboard. You can observe the results of your materials' correction in 3d view with the help of Render Preview (menu View >> Render Preview) or just having rendered the image or part of the image. You can render any part you like, you should only set render border with Shift+B (in active camera view). Now let's try to add red color of reflection to all materials except cherries (fig.4). In this mode you can edit not only color, but and other parameters (examine the «Add» menu). I should note that nodes are very comfortable for materials' editing as it is so easy to return to the position you like: just delete the node you don't like or disactivate it; if you want to return to the first, initial position you can simply press on the «Use Nodes» button.  Composite Nodes To use this mode it is nessary to activate the «Do Composite» button (F10, the «Anim» panel), then to press the «Composite Nodes» and «Use Nodes» buttons in Node Editor. Such panels will appear: «Render Layers» and «Composite». Open UV/Image Editor and select «IM: RenderResult» (fig.5) (this option will help us to edit render results in real time). Let's come back to our Node Editor and in the «Render Layers» panel press on the small button in the right lower corner. Thus, we can render from Node Editor. Now we can set to work. Direct your attention to the «Add» menu and look at the available nodes. Choose the nodes you want to work with. 
This mode influences on all layers and if you want to edit one particular layer, then you should have a separate render for that particular layer. Afterwards you can combine your received images in raster editor (Gimp, Photoshop, etc.). Now let's return to our example. To make the image brighter add the «RGB Curves» node (Add >> Color >> RGB Curves). Edit brightness, contrast and color of your image. Also we may slightly blur the leaves using the «Defocus» node (Add >> Filter >> Defocus). Try to play with parameters and options; this is the best way of study.
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