How to do seamless stitching?

The right four images are transformed to the left one, I do this in the shaders.
The vertices are like this:

As you can see in the left image, there are seams in the red circles, so how to remove them in shaders.

gl version: gl es 3.0
raw image data format: rgb565

I want to apply alpha-blending, the progress will be like this .

But I will use this blend formula: