Hello! I am creating a simple open mask. All my other masks are behaving like little angels, but two refuse to change their "first vertex"... the mask simply closes, linking with the vertex that I want to be the last. I am transforming this mask into a write-on, so it is important that it goes in the right direction. Thanks!
You can have problems using the selection tool to select vertices. This method always works:
Make sure the pen tool (g) is active
Select the mask path in the timeline panel
Hold down the Ctrl/Cmnd key to bring up the white selection pointer and drag a selection around the vertex you want to use as the first vertex
Use the menu Layer>Mask and Shape Path>Set first vertex
Set a mask keyframe for the path
Cut the mask keyframe using Ctrl/Cmnd + x
Add Write the Write On effect
Set a keyframe for Brush Position
Paste (Ctrl/Cmnd + v) to paste the mask path to brush position
If the path goes the wrong way you can Time Reverse keyframes by selecting the Brush Position property in the timeline then using the menu Animate>Keyframe Assistant>Time Reverse Keyframes.
FYI, I almost never use Write On. I use a shape layer path with Trim Paths added. You have a lot more control and a lot more options. Just do everything listed above but instead of adding the Write On effect, deselect all layers, use the pen tool (g) to set the first vertex anywhere in the comp panel and paste (Ctrl/Cmnd +v). This will give you a shape layer path in the same position as your mask providing the transform properties of the layer with the mask are all at their defaults. Now you can press the U key twice to reveal all modified properties of the path, change the stroke width, end caps, dashes, gaps, and then add trim paths to animate how the stroke draws on. The shape layer can then be used as a track matte. It's a much cleaner solution that retains the ability to continuously rasterize and add other shape layer animators.
For me, it has sometimes been a problem when the object was part of a group.
See, if the object lies within a group. If so, remember what layers were part of that group, then ungroup it. Ignore error messages telling you about expression errors. Try to set the first vertex now, then re-group.