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Blender has a kind of awesome nodes that we can use to pull mattes from all sorts of different images. To start with let's just go ahead and press E to execute the noodle network and I have all of the different matte nodes configured for a whole bunch of different sample images that we can process here. First is a Half Moon by Samara Iodice and it's an awesome image of the moon taken with her camera, and it's against the black background. So, if I want to pull a matte from this, I say oh, black-and-white, I want to use the Luminance Key.
The Luminance Key is added here just by pressing Space, Add > Matte and here's all the Matte Keys that we are going to be going through. For the Luminance Key we have two controls, one is the control, where we are going to pull the matte from? So, any pixel value that has a value somewhere between in this case 000 and .002 is going to be translated to Alpha 0. So, actually we are going to pull, then if we click on the matte we see the matte for that.
So, Alpha 0 or black for anything within that value. And notice I have scrunched it down so low and so narrow that I'm even pulling the glow from the moon. So, if I wanted to pull this in, and let's say use this as 1 and then subtract off another one where it's a really hard crisp edge, I can then composite in a different glow to the moon. Another kind of unexpected result is pulling a matte from an image like this.
You wouldn't think that just because it has this terrible white background with a crinkled shape and wrinkles and all sorts of different colors that you could pull a good matte, but by just adjusting the Low and the High values you get a pretty good matte from this one image. Moving onto the Chroma Key, the Chroma Key is the green screen node within Blender, pulls a key from the green screen. And just like the other Luminance Key it has a kind of a display here, a little mini display showing you what it's pulling from and with most green screens, the green screen doesn't cover the entire camera width so you are going to have to run this through a Crop node and then Alpha over it onto whatever background you want, in this case a nice neutral white. For this node what you want to do is you want to sample a Key Color so that it knows how to pull the matte, and it's a Color Swatch so you just click on the Swatch, and then what I do is usually sample using the Eyedropper sample from the actual image in the Input Image node.
Once I do that then it uses that Key Color, and then we have a bunch of other controls here to say okay, within variations basically variations of that green, what am I going to translate to Alpha 0. Moving on down the line and I'm holding Shift and middle mouse button, Mac users hold Shift+Option and then click-and -drag to do the same mouse action. What we have here is a Difference Key and this is me standing in my blue-shirt wagging my finger. What we are going to do now with a Difference Key is it takes the difference between two images or between your image and a particular color and pulls a key from it.
It is actually four nodes in one where you can make the choice of which channel set to operate on. And then within each channel set what the deviation is from that particular value to pull the matte. So, in this case I'm using Hue Saturation and Value, and so the first box here is the Hue, the second is the Saturation and the third is the Value. If I was using RGB channels, then it would be Red values in here, Green in here and Blue over here.
So, when we do that and we pull the key from that, we can look at the key. What happens is it's going to take all of the blue, that's that particular shade of blue, out of the picture. We can then Alpha Over and put me in a pink shirt if you want, or we can, let's be even a little more creative. If we take a background plate of the same shot without me in it, then we can use the Mix node with pulling that matte and erase my body. And so now I don't have a body and I'm just a talking head.
That's the Difference Key, and the Channel Key is a little much the same and that it uses the same sets of channel selectors available to select RGB, HSV, but it sort of combines the other node and that you get now arranged. So, what I have done with this very red car and I want to pull a matte from the car body, if you will. What I can do is select the Red Channel and then work on the Red Channel, and anything in the image that has of red value between these two values will be matted out, and you can see the matte that we have pulled.
The neat thing about this too with the Channel Key is that we can set up a range of values. So, if I pull this Low down what happens is I start getting a gray-scale of the matte, which is really cool. Because then if I want to blend in a different color and change the color of the car, I can do that and then shade that based on the amount of red that was originally in the image. So, if I want to come over here and I wanted to make my car pink, there we go.
Now, I have a pink Viper. The other kind of a neat thing to do, I just was doing this and you can download this image from NASA if you wish. This is a picture of a spiral galaxy and what I have done is I have selected yellow. I wanted to do some research on where there are yellow stars in this galaxy. And so by setting this value to .301, that's a particular shade of yellow, I can pull the matte and see where the yellow stars are in the galaxy, and we can see that they are in the center of the galaxy, they are kind of spread out.
So, kind of different ways that you can use all of the different matte nodes within Blender to pull a key so that you can then use that as a mask to mask over, whatever you want to do.
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