Smoothing shaderMaterial

Hello
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The ShaderMaterial I made myself uses 3 images.
Each image is used as one mask and two blended images.
As you can see from the above example, if the mask image resolution is low, it will look like a brick.
Of course increasing the resolution of the mask image will make it softer, but I’m going to use this for a large terrain. Used for a minimum of 256m x 256m.
In other words, you cannot apply a large resolution to the mask image.
Please tell me how to figure out the center point of the pixel so that the blending images blend smoothly.

------------------------ Below is the attached source file
source.zip (453.3 KB)

I solved it by multiplying the transparency of the mask.

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