Enhance a picture with compositing

Now that we have a raw render, it is time to learn how to improve it using the compositing tools of Blender.

Introduction to nodal compositing

Blender is a complete tool that also allows compositing. This is the ability to edit an image or a sequence after the rendering phase. You probably have already tried compositing, maybe unknowingly. For example, Adobe Photoshop© is a software that allows us to composite a single image. Unlike Adobe Photoshop©, Blender uses a nodal system that provides a great flexibility. We can make changes at any point without the loss of information. Let's try this:

1. For a first approach of nodal compositing, let's open a new Blender scene.
2. In order to access the compositing mode, you must open the Node Editor. This is the same as the Node Editor for materials.
3. We must then check the Compositing button near the Shader button in the Header options. It is a small icon button symbolizing an image over another.
4. We must also check the Use Nodes button.

We have now two nodes, a Render Layer node and a Composite node.

1. We can split our scene to open the 3D View and make a render of the cube in the middle of the scene (F12).
2. We can also add a Viewer node (press Shift + A and select Output | Viewer). This node will allow us to visualize the compositing result directly in the Node Editor. You only need to connect the Image output socket of the Render Layer node to the Image input socket of the Viewer node and check the Backdrop option of the Header.

Now the render image appears behind the nodes. It will be pretty useful to do compositing in full screen. If you want to move the render image, use Alt and MMB. Two other interesting short keys are V to zoom in and Alt + V to zoom out.

1. We will add a Color Balance (press Shift + A and select Color | Color Balance) and connect it between the Render Layers node (to the Image output socket) and the Viewer node (to the Image input socket).
2. If we change the lift color, the render image is directly updated.
   
3. We can also replace the render of the cube by any other picture, by adding an Image node (press Shift + A and select Input | Image), and connecting the Image output socket to the Image input socket of the Color Balance node.

Note

Looking at a texture node through the Viewer

There is a node that can help you to better visualize what the compositing looks like at a certain point. You can press Ctrl + Shift and right-click on any node to append a ViewNode and connect to it.

The possibilities of compositing in Blender are enormous. For instance, you can easily use keying techniques that are often needed in the movie industry. It consists of replacing a green or a blue screen behind an actor by a virtual set. Compositing is an art and it take too long to explain everything, but we will see some of its basic concepts so that we can improve the shots of our sequence.

Now, we are going to work on the first shot of the sequence:

1. As with any other image file, we must add an Image node (press Shift + A and select Input | Image) and connect it to the viewer.
2. We press the Open button of the Image node, and we take the corresponding OpenEXR Multilayer file. We also check Auto-refresh.

Depth Pass

The following is the combined output socket of the Image node, and there is a Depth output socket. This pass will allow us to simulate an atmospheric depth. It is an effect that can be observed when we look at a distant landscape and a kind of haze is formed. The Depth pass is a visual representation of the Z-Buffer on a grayscale. The objects near the camera will have a gray value close to black, unlike the distant elements, which will have a value close to white. This pass could serve for other things such as masking or blurring the focal depth. It all depends on the context. A controlled atmospheric effect may bring realism to the image.

1. We will start by adding a Normalize node (press Shift + A and select Vector | Normalize). This allows us to clamp all pixel values between 0 and 1. We cannot visualize the Depth pass without a Normalize node.
2. We will add RGB Curves (press Shift + A and select Color | RGB Curves) to change the contrast of the ZDepth pass and control its strength effect.
3. We will then add a Mix node (press Shift + A and select Color | Mix ) with a Mix blend mode.

Now that we have added the nodes, we are going to connect them as follows:

1. We will need to connect the Z output of the Image node to the input of the Normalize node and the output of the Normalize to the Image input of the RGB Curves node.
2. We will also connect the Image output of the RGB Curves node to the Fac input of the Mix node and the Combined output socket of the Image node to the first Image input socket.
3. We must adjust the RGB Curves node by adding another point to the curve. The point is located at X: 0.36667 and Y: 0.19375.
4. We will also need to change the color of the second Image input socket of the Mix node. The hex code of the color is D39881. It will color the white pixels.
   

   A render before and after the ZDepth pass

   

   Color correction of the shot

One of the most important aspects of compositing is color calibration. Fortunately, there are easy-to-use tools in Blender to do that.

1. In order to quickly change the hue, we will add a Color Balance node (press Shift +A and select Color | Color Balance). The hue corresponds to the color tint of the image.
2. We must be very careful to slightly change the value of Lift, Gamma, and Gain. They become strong quickly. The RGB values of Lift are R: 1.000, G: 0.981, and B: 0.971. The RGB values of Gamma are R: 1.067, G: 1.08, and B: 1.068. The RGB values of Gain are R:1.01, G: 0.998, and B: 0.959.

Note

There are two correction formulas: Lift/Gamma/Gain and Offset/Power/Slope. These are the two ways to get the same result. For each one, there are three color wheels and a value controller (Fac). You can modify the darker, the mid-tone, and the highlight values separately.

If you want more information about the color balance node in Blender, visit this link :

https://www.blender.org/manual/composite_nodes/types/color/color_balance.html

A render image with an adjustment of the Color Balance node will look like this:

Before finishing the compositing, let's add a few effects.

Adding effects

We will add a Filter node (press Shift + A and select Filter | Filter). We must connect the Image output socket of the Mix node to the Image input socket of the Soften node, and we will connect the image output socket of the Soften node to the Image input socket of the color balance node. We will keep the filter type to Soften with Fac of 0.500. This blends the pixels so that the image is less sharp. A photo is never perfectly sharp.

We will add then a Lens distortion node (press Shift + A and select Distort | Lens Distortion). We must connect the Image output socket of the Color Balance to the Image input socket of the Lens Distortion node and the Image output socket of the Lens Distortion node to the Image input socket of the Viewer node. We will check the Projector button. The distort option is at 0.000, and the dispersion option at 0.100. This node usually allows us to make a distortion effect such as a fish eye, but in our case, it will allow us to make a chromatic aberration. This adds a soft and nice effect.

The nodes of compositing

We have now completed the appearance of the shot.

A render with final compositing

Compositing rendering phase

We are now ready to make the render of our compositing:

1. In the Output options (Properties | Render | Output), we must change the Output path. We will write the following address: //Render\01\Compositing\. We will press Enter to validate the address.
2. We will also change the Output format to TGA.
3. It is a compositing render, so we will check the Compositing option in the Post Processing tab (Properties | Processing).
4. Now, we are ready to render the scene. We will use the same process for each scene.