Snowmix Video Transitions.

This guide explain how to use the transition library to create video transitions from one video feed to another. The transition library is a Snowmix Library distributed together with the Snowmix source. Currently the library will work with Snowmix version 0.5.1, but future versions will require a newer verseion.

Note that the Snowmix transition library is not distributed with the Snowmix version 0.5.1 distribution. However you can request the files through the Snowmix support forum.

The video below shows some of the transitions available through the library.

Overview

• Including the Library.
• Creating Place Holders.
• Creating a Transition.
• Listing Transition Functions.
• Reset Functions.
• Overlaying Transitions.

Including the library.

The transition library depends on other libaries to be include before the library itself. The libraries required are the basic-shapes.slib, the system.slib and the <í>feeds.slib. So to include the transition library, the following lines should appear in the ini file in the listed order.

When the library is included (loaded), the library will add a line to the macro associated with the overlay finish command. The library assumes that a macro has been assigned for the overlay finish command. If this is not the case, the library functions may not work correctly. After libraries have been included, the command command list Show can be used to list the content of the macro Show.

If the macro associated with overlay finish is replaced or changed, these lines should be included.

Creating Transition Place Holders.

To use transitions in Snowmix, at least one transition place holder must be created. This can be done using the following configuration.

The first argument is a unique transition ID number being an integer greater than 0. Usually the numbers 1, 2, 3 .... are used. The function TransitionCreate will create a transition place holder that can be used to display video feeds or images and can be used to perform transitions. When a transition place holder is created, it is by default disabled. To display or overlay the transition place holder, we will later have to enable it. The syntax for the function is as shown below:

The argument <x> and <y> is the the place of the upper left corner of the transition place holder and <width> and <height argument is the geometry of the place holder.

The transition place holder will by default have a Front and a Back source that need to be set. This is shown below here.

The example above sets the Front source to feed 1 and the Back source to feed 2. The command can be used any time later to stage a transition. Assuming the Front and Back sources are feed 1 and feed 2 and we want to transition to feed 3, we will execute the command:

This sets the Back source to feed 3 and we can then as shown later, request a transition from the Front source to the Back source.

Now we have set a Front and a Back source for the transition place holder, we can now enable it. This can be done as shown in the example below:

This enable transition place holder 1 and since we have set the Front source to be feed 1 the video feed 1 is now overlayed. This can seen using either the script output2screen executed in another terminal window. Alternatively the combination of using the script output2dummy executed on the command line in another window and the Snowmix command monitor on can also be used.

Creating a Transition.

Now we have created the transition place holder, enabled it and used one of the method listed above to see the result, we can now create the first transition.

The first transition we will create is a transition, where we will turn of the Back source and fade down the Front source. To do that we will use the command TransitionSetup. The function has the following syntax:

The first argument it the unique transition ID. The second argument is the number of steps or frame periods the transition will take. After this follow 3 arguments being a list of pre functions, a list of step functions and finally a list of post functions.

When the transition start, the functions listed in the pre function list will be executed once. Then for <steps> frame periods, the functions listed in the step function list will be executed. When the last step is reached, the functions list in the post function list will be executed once.

The code listed below will create the transition fading to black turning of the Back source and fading the Front source down.

We se we use the transition place holder id "1" and the number of steps are 30. This means the transition will take 1 second to execute if the system framerate has been set to 30. The pre funtion list contain the function name "Transition_SetBackOff". This will turn off the back source. If we did not do that, then fading down the front source would reveal the Back source. Nothing wrong in that, but that was not set as the scope of the transition.

The step funtion list contains the function Transition_FadeFrontDown. This will fade down the alpha for the front source in 30 small steps over the next 30 frame periods. The post function list is empty, so nothing will be done there.

Now we have setup the transition, we can now execute it. This is done in the example below:

The transition will now run for the next 30 frame periods. The two example lines can be concatenated to this:

It is now worth mentioning, that the transition place holder is in a mode, where both the Front and the Back source are faded down (ie. their alpha value is 0). We can now choose to fade up the Back source and switch front and back sources. This is shown below:

Here we see that as step function we have chosen to fade up the Back source. As a post function we will switch the Front source (faded down - alpha is 0) and the Back source (faded up - alpha is 1). After this we are back to normal, where the Front source is now displayed (alpha > 0) and the Back source is not (alpha is 0). The command examples could be combined to the following:

Listing Transition Functions.

In the previous section it was shown how to create a transition using the function TransitionSetup and lists of transitions functions. The command TransitionListFunctions can be used to list the transition functions available. Below is shown the functions available.

Please note that the functions above should be used in conjunction with the TransitionSetup function.

Reset Functions.

Reset functions purpose is to is to reset a transition place holder back to a known state.

Assume we have used a transition function that change the size of either the Front or the Back source, then as one of the post functions, we should add the ResetSize function. The functions such as ScaleFrontDown and ScaleBackUp will need ResetSize as one of the post functions.

If we have used a transition function that changes the position of either the Front or the Back source, then as one of the post functions we should add the ResetPosition function. All the MoveOut functions will need ResetPosition as one of its post functions.

If we have used a transition function that uses another clip geometry than a plain rectangle, then as one of the post functions, we should add the ResetRectClip function. The functions such as FrontCircle... will need ResetrectClip as one of its post functions.

While not a reset function, it is worth mentioning that many transitions that manipulate the Front source, could be position, clipping, alpha or other, to end displaying the Back source should add the SwitchFrontBack function to the list of post functions ending up switching the Front and the Back source and as a result of the transition ends up no longer displaying the original Front source but rather displaying the Back source now the Front source.

Overlaying Transitions.

All enabled transition place holder are automatically overlayed in the order they have been created. The order is reflected in the global variable transition(ids) and the functions to enable and disable a placeholder are TransitionEnable and TransitionDisable. It is permissable to reorder the global variable transition(ids) but numbers must be removed from the variable. If a transition place holder needs to be delete, the function TransitionDelete can be used.

Disabled (and for that matter also enabled) transition place holders can be manually overlayed using the tcl function TransitionOverlay from within the macro associated with the snowmix command overlay finish. If the function is used, it must be used with a number of arguments. These are shown below:

Here we see the macro Show is added at line 3 the line tcl eval TransitionOverlay 1 0 0 1920 1080 1. Assuming the macro Show is associated the overlay finish command, then when this line is executed, the transition place holder 1 is overlayed placing it at 0,0 and with the width 1920 and height 1080 and the alpha value 1. The values you use for placement, geometry and alpha can change for each frame or even when used many times for the same frame, change for each such usage.

Writing Your Own Transition Functions.

To be written.