Chroma Key with a Web cam in Stop MotionMaker Advance

Stop MotionMaker Advances  chroma key system is able to generate good composites from  even video web cam images.  The is possible due to specialised processing and functions only available on this stop motion specific application.

Background Information

To give the user a better understanding of the process, some background on each control is given before the chromakey tutorial.

Initial Range Normally the first slider to be adjusted, works from either finding prominent green or blue (depending on setting) pixels from the background plate and replacing them with pixels from the chromakey image.   It starts from light colours and works to dark.   The higher the setting the darker the colour that will be chromakeyed.

1 to 8 Range This slider controls the loaded  chromakey colours (from the main image)   1 to 8.  This adjustment controls how far either side of the loaded colour will be replaced by chromakey pixels.

9 to 15 Range The second slider controls the loaded  chromakey colours (from the main image)   9 to 15.   This adjustment controls how far either side of the loaded colour will be replaced by chromakey pixels.

Spill Range This slider adjust how far either side of the selected spill colour will be effected by the ‘Spill Adjust Colour’ control.  Once again this will only work 7 pixels in from the chromakey edge

Spill Adjust Colour This slider adjust how much the green or blue component of the select colour will be reduced by.      Note: If it is reduced too far, the select green colour will turn to red.

Transparency This slider controls how much grey background image is mixed with the main image.   This function used with a smooth glass cup and a bright/strong colour image, can give the illusion of chromakeying through glass.

Blur Chroma Image This slider is used to blur the chromakey background image to change the ‘depth of field’ or to match a high resolution image to a video image.  It will only effect the background chromakey image.

Add Chroma Mat When selected, this function will force the pixels draw upon in the main image to have the chromakey image on.  This is also used to paint over support wires etc.

Subtract Chroma Mat When selected, this function will force the pixels drawn upon in the main image to have the chromkey off.  Use to stop problem colour parts of the figure from being ‘eaten’ or replace by the chromakey image when it is not wanted.

Reset Chroma Mat When selected, this function will reset the pixels drawn upon back to a normal state (not forced on or off).

Shrink to fit via Sliders When selected, the chromakey image will reduce in size to match the position of the x/y sliders.

Lock mat When chromakeying live, will lock the set chromakey mat to reduce video noise when using a video chromakey background and the figure is not moving.

Feather Inner Edge Selects how deep a blur effect will be used on the chromakey inner edge. Used to smooth up the digitally produced chroma edge.

Feather Outer Edge Selects how far out a blur effect will be used on the chromakey outer edge.  Used to smooth up the digitally produced chroma edge.

The following tutorial uses four lights, two on the background plate and two on the subject. A USB webcam was used (Logitec 4000).

First connect your camera to the computer and open Stop MotionMaker Advance. Then select ‘New ‘ and then take a frame for system use.

 Movement Activated The chromakey system uses the fact that stop motion is the art of using still figures, to allow video noise to be reduced when the image is stationary or not being adjusted.

Tutorial Requirements

Computer: (3Ghz - 1 Gb RAM was used with Windows XP) + Stop MotionMaker Advance V1.87

Camera:  Logitec 4000 Webcam. (If you use a camera with better optics it will give improved results).

Lights: Two incandescent lights on the figure (60W) + two halogens (50 W) on the green background plate.

Chromakey Image: A chromakey image - either a jpg, bmp or avi video

Please note that the following images have been compressed for the web.

Don’t forget to setup your camera so that it is suitable for stop motion. i.e Manually set for conditions and lighting. You can save the camera setting under ‘User Defaults’. Note that a darker green felt was used under the figure to reduce colour spill.

Select the frames per second to be used and then select ‘B/Chromakey’ to open the ‘Chromakey (Background) Viewer’.

Take one frame for the application to reference to.

Select ‘B/Chromakey’ from the ‘Functions’ menu

The Chromakey (Background) viewer will now open and then green around the object will now turn to grey if it is the default green.

If you are using a blue background select ‘Blue Screen’ in the viewer window.

Import a image - in this case we will load a jpeg image.
Select ‘Open Jpeg’ and use the ‘Open’ dialog to import a image.

Now adjust the ‘Initial Range’ slider to control which colours are keyed over (from lighter to darker) in this case any pixels that have a predominate green to their RGB colour.  As there is basically no pixels in the object that are predominately green then we are able to move this slider to maximum (255).

Select the ‘Spill’ button and then with the help of the ‘Zoom’ window load a spill colour. Adjust the ‘Spill Range’ and ‘Spill Adjust Colour ‘ for best results.   The ‘Spill Range’ controls how far either side of the loaded spill colour other similar colours will be effected and the ‘Spill Adjust Colour’ controls how much green is taken out of the selected colours.  If you move it too far the colours will normally turn red.

Note:  The ‘Feather Inner Edge’, Feather Outer Edge’ and Colour Spill will reset to zero when the load buttons are selected to ensure only true colours are imported.   Selecting ‘Reset’ will reset the controls or they can be adjusted manually.

Adjust the ‘Initial Range’ slider control again (if necessary) to ensure none of the figure is being ‘eaten’ by the background key image.

Now load the first fine tune chroma colour by selecting ‘1’ and left click over some green pixels to be removed in the main screen. This loads the colour and displays it next to the number ‘1’.

Note. The ‘Zoom’ window will automatically open to aid in selection.

Note. When the number button is selected the default other chromakey settings will set to zero to ensure a unmodified colour is loaded.   Selecting the ‘Reset’ button after loading the colour will revert these settings back to their last position, or you can set them back manually.

Adjust the relevant slider control to increase or decrease the range of effect (replaced by the chromakey pixels) either side of that pixel colour.  If it seems that you are effecting the figure then load another colour against the next number (up to 15) and then enable the colour by selecting the checkbox next to it.

The image is now ready to use, but in this case we have decided to shrink and move the live image to another position within the Chromakey image.

Select the Adjust Main Image Size page and move the sliders as required.

Note.  The ‘Zoom’ slider in V1.95 will only adjust the Chromakey video preview size.

To make the image look more realistic we will hide the objects foot as would be the case in a real photo.  The ‘Add Chroma Mat’ selection is used to force the pixles ‘drawn’ on to the chromakey image.  Note that the ‘Subtract Chroma Mat’ selection can be used to force the pixels to the main or live image. Select the ‘Clear Garbage Mat’ to clear the mat settings.

We are now ready to take a frame, but first to improve the image quality we will select Frame averageing function in the ‘Video Source Setup’ Panel.  Select ‘Video Setup’ to display this panel.

A frame has now been taken and the object can now be move and another frame taken as is the normal work flow.  Note that the ‘onionskin’ or ‘loop last’ functions can be used to help alignment.