ios - Performant outline around UIImage edges

Question

I'm trying to draw an outline around the edges of a UIImage - achieving an outline effect like so:

As much as I understood, the best solution is to create a replica of the original image, filled with the desired outline color and drawing it behind the original image, and potentially looping through the image angles and re-drawing to round out rough edges.
Currently, I'm using the solution proposed here (by @Stéphane de Luca), but the problem is that this solution is very slow for my needs as I need to control the outline width with a slider.

So, basically I need to either find a different outlining algorithm or find a way to optimize the performance of the current solution.
What I did try to optimize performance:

• Scaling down the image - helps but not enough as drawing still happens many times for high quality
• Drawing for high quality once and re-render in lower quality to draw fewer times - getting unexpected, inconsistent outline results
• I'm thinking of creating the colorized image once and scale it somehow according to the outline width value but I'm not sure how exactly to approach it.

I'll appreciate any help, whether it's proposing a different algorithm / outlining technique or potential ways to optimize the current solution for high performance and fast value changes.

标签： iosswiftgraphicsuiimagecore-graphics