When talking about «transitions», Kdenlive actually lumps up three different things into a single word: crossfades, blending, and compositing of two clips.
In some more detail...
Crossfades
Fades between two clips: dissolve, slide, and wipe are perfect examples. Please note that Kdenlive yet cannot crossfade between two clips on the same track, so you need always two tracks for crossfading.
- It's important to note that crossfades don't have any keyframable settings. All settings are time-invariant.
- However, the crossfade itself is strictly depending on time, over which you have no control.
- Crossfades can be «reversed» in the sense that the crossfade either starts with the upper or lower track clip first.
- This property that controls this behavior is (at least to me) totally counter-intuitively. It probably alienates most Kdenlive users, even old hands like me (in fact, Kdenlive should automatically take care of this whenever possible):
- reverse on: crossfading from upper to lower track clip.
- reverse off: crossfading from lower to upper track clip.
- A better name for the reverse property would probably a selection instead of a check box, which can be set to either «from upper to lower clip» or «from lower to upper clip».
- Albeit affine could be considered to be a crossfade too (a dissolve without any wipe), it belongs into a different category, namely compositing.
Blending
- Blending doesn't offer any keyframable settings, all settings are invariant of time.
Compositing
- The important difference to crossfades and blending is that compositing parameters can be keyframed. That is, they are allowed to vary over time.
- A typical example are soft fade-ins and fade-outs at the beginning and end of a title clip over a video clip.
- While compositing can be used also for simple crossfades, compositing offers more functionality over crossfades, such as scaling, rotating, and moving the upper track clip before compositing it with the lower track clip.
