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All right, let's talk about mixing and combining images and portions of images with one another. So, go ahead and press F12 to go ahead and refresh and for the Mac users, just come on over here and click Render, does the same things and this pulls a frame from our Compositing noodle. Go ahead and minimize that and let's talk about the Mix node. The Mix node takes an image from the top socket and mixes it with the bottom. Pretty simple concept. It has couple of different modes though.
One is we can mix the two images together in which case the pixels from each image are blended together according to the factor that gives a weight, 0 being more towards the top socket and then 1 being more towards the bottom socket. We can take the pixels from the Base layer and add on more pixel colors and values to the resulting image. We can Subtract, Multiply, Screen Overlay, Divide pixels out if we want to divide let's say an Alpha map by half to cut everything in half, we can divide that.
We can take the Difference between two images, if we are trying to compare two images to see how they have changed from when we shot the live action shot to maybe after we have done some compositing on them. We can take the differences between two images, we can use one image to darken or lighten another one. If you are an old darkroom guy, then you are familiar with dodging and burning certain areas of the image and usually you use those with a mask to control the factor of where in the image you do the dodging and the burning. The Alpha Over node over here uses the Alpha channel of the image or a render layer to determine where to blend one image over another.
So, we have an example down here, I'm doing a Shift+middle mouse button to pan this whole view. Here in this Viewer node, we have the option of where we have taken the Background Plate here and we have threaded it to the top socket of this Alpha Over node and then we have threaded in this Scene Input that has a blank Alpha background. It's colored though. So, we have to use the Premultiply to get rid of that color ring. We are going to combine this CG over this live action plate based on the Alpha channel and the resulting output comes in down here.
The Z Combine node combines two images based on their distance from camera. So, it helps you re-sequence or re- layer images in Post Pro and we'll over that a little bit here under Color, Z Combine. We have the two images that come in from the top and one comes in from the bottom and then we can feed the Z Factor socket here, by clicking and dragging and connecting these two sockets or we can manually set the Z Value, so that all of these pixels in this image are now going to be, let's say 0.8 units from camera.
The resulting or combined image then has these pixels selected from each of the images based on the distance of the camera. So, if this pixel is closer to the camera than 0.8, than it's going to show up on the combined output, otherwise the other socket is used. So, that's how you use the Mix nodes to blend images together or the AlphaOver node to layer images on top of one another in the Blending Compositor.
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