Blender 3D has an amazing node base compositor. The examples below are just 1 of many examples on pulling a matte in an open source application like Blender. The image is using a split screen node to show how the advanced keyer is pulling a screen matte on the left and and source alpha matte on the right. This viewing example help you visualize noise such as grain and other fire flies in your matte.
This image is a process using nodes to pull a Difference Key Matte / Spill Suppression. Some color keys, linear keys and chroma-keys nodes may not have enough color information based on the hue and chroma sampling in the image. Their algorithms may be coded to key certain range of colors. I have learned this process from the great Steve Wright. He is a VFX GURU. Using the math node and color ramp node to subtract one channel from another and then clamping my whites and blacks, I was able to get a decent HI-CON matte.
After your all your keys has been pulled, the final is to sell the shot with what is called an edge matte. It is also know as doing a lightwrap. All objects in nature has some sort of absorption or reflection around its edges as well as on it’s surfaces. You use the edge matte (lightwrap) as a mask to let some of the background image pixels bleed through the foreground objects. When pulling a key it can sometime produce hard edges or edges that make to foreground seam out place. Lightwraps are your friend when its comes to green,blue,red screen keying.
Edge Matte (lightwrap) final comp with basic grading.
Blender 3D also has a clip assembler from the scene modeler, animation and the compositor. It’s main name is called the “VSE”. In the VSE you can edit and arrange video, still images, audio and color correct / color grading for your finishing.
Sometime in the future I may revisit this process with higher quality images in stead of the internet’s low-res JPEGs. I will also demo the 2d/3d camera motion tracker, masking and rotoscoping features in blender.