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DFX Tutorial 4 - Keying
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Often times, we need to combine various visual elements in the course of a production. Some areas need to be transparent, to allow the background to show through, and some need to be solid, like the live action. The keying tools in DFX combine these elements together in a seamless fashion.

In this tutorial, you will be using one of the most powerful tools in DFX - the Chroma Keyer.

Chroma Keying is the process of creating a matte based on certain ranges of colors or hues. Using the tool's controls, define what colors and hues are allowed to be transparent and which are not.


Step 1
Maximize DFX by clicking on the maximize button in the top right hand corner. From the tool bar, drag a Loader Tool (LD) down onto the Flow.

Step 2
On the File tab of Loader 1, click on the file folder on the right. Locate a folder called Footage. Open the sub-folder called Med_Back2. Select any one of the files in it. (MedXXXX.dft)

Step 3
Just below Loader 1, drag a new Loader from the toolbar. Click on the File tab, and select one of the GuysXXXX.dft files from the Trooper_1 directory in the Footage folder.

We now have our foreground and background footage. We are going to composite the soldiers over the background footage that we loaded with our first loader. Unfortunately, as is often the case, we don't have an existing Alpha matte for the soldiers. This is where the Chroma Keyer tool will come in. This footage is excellent to practice on, because, as in the industry, the bluescreen background is not perfect, and will require some tweaking to achieve the effect we want.

Step 4
From the Tools menu at the top of the screen, pull down to Matte and select the Chroma Key tool. This should now connect to Loader 2.

If it does not, drag a Cky tool from the toolbar onto the Flow just to the right of Loader 2.

Step 5
Double click on the ChromaKey tool to open its Control Header in the right area of the screen. The first tab contains four range controllers. Each of these control the range of acceptable color values for each channel, Red, Green Blue, and Luminance.

Step 6
Because the original footage was shot against a bluescreen, it is with this range that we will start. Place the cursor over the Chroma Key tool and right click.

Select View On and click Large. As was demonstrated in the previous tutorial, Masks, Paths & Animation, the button bar beneath each view controls which color channel shown. Double click on the Alpha channel button to view it.

Step 7
Before proceeding further, grab the time arrow, and scrub forward to frame 50. This will ensure that the actors are in frame when attempting to key them out of the background.

Step 8
All files are given an Alpha channel in DFX. In this case, however, unless they had one before, the Alpha will be transparent (black). View Loader 2 on the Large View and view the Chroma Keyer on the Small View, with only the Alpha channel visible. By box selecting areas on the Large View, tell the Chroma Keyer which colors to remove from the Alpha channel. As you do so, notice the white areas appearing on the Small View.

These colors are being allowed by the tool, and represent solid areas in the Alpha Channel.

Step 9
Continue to box select, but this time use the Alpha channel visible on the Small View. After having roughly established the matte, click on the Soft Range slider to fine-tune the edges of the matte. Finally, click on the Image tab of the Chroma Keyer and adjust the spill suppression all the way to the right. The drop down box above it must be set to the color of the screen that you are keying out.

As with all things, practice with mattes makes perfect, so find the worst footage possible and do your best to it.

Step 10
When satisfied with the matte, it is time to join the foreground and background images together. For this we will use the Merge tool.

Step 11
From the tool bar, drag the Mrg tool onto the Flow one square to the right of the Chroma Keyer. At this point in time, it won't connect to anything. This means that we will have to drag connections from our other tools onto the Merge tool.

Step 12
If you hold your cursor over the tool inputs of the Merge for a moment, a pop up will appear to tell you whether that is the foreground or background input of the Merge. This is very important when layering elements, for, if it is not done properly, the results will not be what you want. Once you have identified the background input on Merge 1, click and drag from the output of Loader 1 to the background input of Merge 1.

Step 13
Click on Merge 1 with the cursor to highlight it. Now press the 1 key on the keyboard. This is the shortcut to display a tool on the Large View. 2 will display it on the Small View and 3 will display it on a monitor attached to your system, if it is attached to the outputs from the card.

Step 14
Click on the output of your Chroma Keyer tool and drag to the foreground input of the Merge 1 tool. The two elements and now merged, completing this first keyed composite.

Notice that on the Large View the composited image is tinted blue. Click on the arrow on the Control Header of Merge 1 to expand it. Move the slider marked Subtractive/Additive all the way to the left to see the tint disappear.

Step 15
Step 15 By adding a Saver tool, render out the sequence that was just composited, or, if you prefer, save the Flow as tutorial 04.flw and proceed to the next tutorial.

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