Eyeon:Manual/Tool Reference/Primatte/Part4
From VFXPedia
Contents
Part 1
2. Accessing Primatte from Fusion
Part 2
7. External Garbage Matte Input
Part 3
8. Primatte Tools and Buttons Descriptions
- 8a. Primatte Tab
- 8b. Fine Tuning Tab
- 8c. Replace Tab
- 8d. Degrain Tab
- 8e. Matte Tab
- 8f. Sampling Selectors
Part 4
9. More About the Primatte Polyhedral Slicing Algorithms...
9. More About the Primatte Polyhedral Slicing Algorithms...
- There are three Primatte algorithms. Here is a chart that shows the main differences between them...
- For a description of the Primatte algorithm, go to Section 9a. or click here.
- For a description of the Primatte RT+ algorithm, go to Section 9b. or click here.
- For a description of the Primatte RT algorithm, go to Section 9c. or click here.
9a. Explanation of how Primatte works...
- The Primatte chromakey algorithm is a sophisticated method of color space segmentation that can be easily explained to help a user achieve maximum effectiveness with the tool. Basically Primatte segments all the colors in the foreground image into one of four separate categories. The result is a 'spill suppressed' foreground image and a matte which is used to apply the modified foreground to a suitable background.
- Primatte works in 3D RGB color space. Here is a visual representation of the Primatte algorithm after an image has been processed.
- By operating the Primatte interface, the user essentially creates three concentric, multi-faceted polyhedrons. These can be pictured as three globes (or polyhedrons or polys), one within the other, which share a common center point. The creation of these polyhedrons separates all possible foreground colors into one of four regions; inside the small polyhedron (1), between the small and medium polyhedrons (2), between the medium and the large polyhedrons (3) and outside the large polyhedron (4).
- The four regions created are described as follows:
- Region 1 (inside the small polyhedron) - This region contains all of the foreground image colors that are considered 100% background. These are the green or blue or whatever colors that were used as the backing color of the foreground image.
- Region 2 (between the small and medium polyhedrons) - This region contains all the foreground colors that are at the edges of the foreground object(s), in glass, glass reflections, shadows, sheets of water and other transparent and semi-transparent color regions. These color regions also have spill suppression applied to them to remove color spill from the backing screen.
- Region 3 (between the medium and large polyhedrons) - This region contains all the foreground image colors that are 100% foreground but have spill suppression applied to them to remove color spill from the backing screen. Otherwise they are 100% solid foreground colors.
- Region 4 (outside the large polyhedron) - This region contains all the 100% foreground image colors that are not modified from the original foreground image. There is no spill suppression applied to these colors.
- In the first step in using Primatte (Select Background Color), the user is asked to indicate the backing color on the original foreground image. The sample should usually be taken from a 'medium shaded' area near the foreground object. By 'medium shaded' area, it is meant that if green is the backing color and the green area of the foreground image has many shades of green ranging from very pale green to almost black, a shade of green in-between these extreme ranges should be chosen. If good results are not obtained using this sample, Primatte should be reset and another sample taken using a slightly darker or lighter shade of green. The first sample of Primatte often determines the final result as the center point of all three polyhedrons is created based on this first sample.
- A single pixel may be selected or a range of pixels (snail trail or rectangular sample). If a range of pixels is taken, the sample will be averaged to get a single color sample. This single pixel or averaged color sample then becomes the center of the small polyhedron. A few other shades around that color are included in the original small polyhedron.
- NOTE: It is recommended that a single pixel be selected as the first sample as you then have some idea where the center point of the polyhedrons is located. If a box sample or a long snail trail sample is made. You can only guess at the average color that ends up being the center point. You can get an idea how this sample affects the algorithm by resetting the Primatte plug-in, going to the Matte View and clicking around on the green or blue screen area while in the Select Background Color Operation Mode. You can immediately see the results of the initial settings of the polyhedrons in this way. After making a sample of the backing screen color in the first step, the result is a small golf ball-shaped poly as shown in the following image.
- The second step in using Primatte is to clean up the backing color area by adding additional shades of green or blue to the small poly. This second step (Clean Background Noise) is usually executed while viewing the black and white Matte View.
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| Before Background noise removal | After Background noise removal |
- While in the Clean Background Noise sampling mode, the user samples the white milky regions as shown in the left-hand image above. As the user samples these regions, they turn to black as shown in the right-hand image above.
- What is happening in the Primatte algorithm is that these new shades of green (the white milky areas) are added to the small poly where all the shades of green or blue are moved. The next two images show the new pixels sampled (white dots) in relation to the small poly and the image next to it shows how the small poly extends outward to encompass the newly sampled colors into the small poly.
- The advantage of this technique is that the polyhedron distorts to enclose only the shades of green that are in the backing screen. Other shades of green around these colors are left undisturbed in the foreground. Other chromakeyers expand from a golf ball-sized shape to a baseball to a basketball to a beach ball. Since it expands in all directions, many shades of green are relegated to 100% background making it hard to get good edges around the foreground objects.
- Now that the user has created a small polyhedron, he must shape the medium and large polys. A default medium and large poly are both automatically created and are then modified based on the next couple of Primatte operations. The third Primatte step (Clean Foreground Noise) is to sample and eliminate gray areas in the 100% foreground area of the image.
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| Before Foreground noise removal | After Foreground noise removal |
- Again, the user makes several samples on the dark, grayish areas on the foreground object until it is solid white in color. Primatte is shaping the large polyhedron with each color region that is sampled. Care should be taken in both this and the previous steps to not sample too close to the edges of the foreground object. Getting too close to the foreground object's edges will result in hard edges around the foreground object. Primatte uses these samples to modify and shape the medium and large polys to the desired shape. At this point, the matte or key has been created and would allow the foreground objects to be composited into a new background image.
- If the user changes the display mode from the black and white Matte View to the color Composite View, there is usually 'color spill' on the edges (and sometimes the center) of the foreground objects. When on the edges of the foreground object, this spill comes from where the edges of the foreground object blended into the backing color. If it is on the center of the foreground object, it usually results from reflected color from the backing screen. The next Primatte step, either Spill Sponge, Fine Tuning or Spill(-), can now be used to eliminate this spill color.
- Let's take a look at what is happening in the Primatte algorithm while this next step is performed. Here is what the various tools in Primatte do to the Polyhedrons when they are used:
- As you can see above, the Spill Sponge bulges the large polyhedron in the color region specified. A color region is specified by clicking on the image in a particular area with spill present. For example, if the user clicks on some spill on the cheek of a foreground person, Primatte goes to the section of the large polyhedron closest to that particular flesh tone and bulges the polyhedron there. As a result, the flesh tones move from outside the large poly to in-between the medium and large polys. This is Region 3 and, if you remember, is 100% foreground with spill suppression. As a result of the suppression, the spill is removed from that cheek color and all other shades of that color on the foreground. The user would then continue to sample areas of the image where spill exists and each sample would remove spill from another color region.
- When all spill has been removed, the user should have a final composite. As a last step, he should go back to the Matte View and make sure that gray, transparent areas have not appeared in the foreground area. If there are any, the Matte Sponge Operation Mode should be selected and those gray pixels are sampled until they have all turned white again.
- The Matte Sponge and Spill Sponge tools bulge or dent the polyhedrons a pre-selected amount. If the desired results are not achieved or the results are too extreme for the image, a manual method can be applied. The user should choose the Fine Tuning sliders, select a color region of interest and then move the appropriate slider to get the desired results.
- For example, to remove spill, select a region of the composite image with spill on it. Move the spill or large poly slider to the right a little bit, the large poly will bulge and the spill should disappear. Move it a little more, if necessary. Moving this slider to the right removes spill (moves the colors from outside the large poly to between the medium and large polyhedrons) and moving it to the left, dents the large poly and moves that color region to outside the large poly.
- If the user samples a foreground object shadow and then moves the Matte or medium poly slider to the right, the shadow will become more transparent. This is useful for matching composited shadows to shadows on the plate photography. It can also be used to make clouds or smoke more transparent.
- If some foreground detail disappears during the composite, the user can select where the detail should be and then move the detail or small poly slider to the left. This dents the small poly in that color region and releases the detail pixels from the small poly into the visible region between the small and medium polyhedrons.
- The Spill Sponge and Matte Sponge tools are 'shortcut tools' that automatically move the sliders a pre-selected amount as a timesaving step for the user. Other 'shortcut tools' include the Make Foreground Trans. tool and the Restore Detail tool.
- These 'shortcut tools' are one-step operations where the user clicks on a color region of interest and Primatte performs a pre-calculated operation. Hopefully, most operations using Primatte would only require these tools, but the manual operation of the sliders is always an option. The Spill(-) tool bulges the large poly a small amount incrementally in the color region that is clicked on and the Spill(+) tool dents it a small amount with each click. The Matte(-) and Matte(+) tools do the same to the medium poly and the Detail(-) and Detail(+) do it to the small poly.
9b. Explanation of how Primatte RT+ works...
- The Primatte RT+ algorithm differs from the Primatte algorithm in that it has a six surface color separator instead of the 127-faceted polyhedrons. This makes the Primatte RT+ algorithm much simpler and, therefore, faster to calculate. The results and performance of Primatte RT+ falls in between the Primatte and Primatte RT options. Where the Primatte RT+ algorithm might not work well is with less saturated backing screen colors and it also does not support the Complement Color spill suppression method (which is the spill suppression method that delivers the best detail). For a well-lit and photographed image or clip, this algorithm will produce good results and render quickly. Here is what a visual representation of the Primatte RT algorithm looks like after an image has been processed...
9c. Explanation of how Primatte RT works...
- Primatte RT is the simplest algorithm and, therefore, the fastest. It uses only a single planar surface to separate the 3D RGB colorspace and, as a result, does not have the ability to separate out the foreground from the backing screen as carefully as the above Primatte algorithm. Like the Primatte RT+ algorithm, Primatte RT might not work well with less saturated backing screen colors and it too does not support the Complement Color spill suppression method (which is the spill suppression method that delivers the best detail).. For a well-lit and photographed image or clip, this algorithm will produce good results and render very quickly.
- Here is what a visual representation of the Primatte RT algorithm looks like after an image has been processed...
10. If you have trouble…
Please contact eyeon Software or the Imagica technical support person (below):
Primate Technical Support:
Scott Gross
Phone: +1-530-677-9980
FAX: +1-530-677-9981
Cell: +1-530-613-3212
E-mail: sgross@imagica-la.com
- Note: Standard support times are between the hours of 7AM and 6PM (Pacific/California Time), but he will try to help you anytime, if possible. Leave a message or a return phone number at the phone number of 1-530-677-9980 and he will get back to you as soon as possible.
