Triplanar texturing in glsl - opengl

Question

I followed a paper called "GPU Based Algorithms for Terrain Texturing" and it says the following:
The main algorithm to apply triplanar texturing is fairly simple.
First, we check whether the slope is relatively large in the same way
that we do with slope based texturing. These regions with high slope
will be the only regions aected by the algorithm. We then check what
the larger component of the normal is, out of x and z. If x is the
larger component, we use the geometry z coordinate as the texture
coordinate s, and the geometry y coordinate as the texture coordinate
t. If z is the larger component, we use the geometry x coordinate as
the texture coordinate s, and the geometry y coordinate as the texture
coordinate t.
So I tried to implement it. This is my heightmap:
Note that I added white lines in the borders just for the experiment, so now I have maximum-height walls surrounding my map.
Now following the articles, here's the implementation in the vertex shader:
#version 430
uniform mat4 ProjectionMatrix;
uniform mat4 CameraMatrix;
uniform vec3 scale;
layout(location = 0) in vec3 vertex;
layout(location = 1) in vec3 normal;
out vec3 fsVertex;
out vec3 fsNormal;
out vec2 fsUvs;
void main()
{
fsVertex = vertex;
fsNormal = normalize(normal);
if(fsNormal.y < 0.75) {
if(fsNormal.x > fsNormal.z)
fsUvs = vertex.zy * scale.zy;
else
fsUvs = vertex.xy * scale.xy;
}
else
fsUvs = vertex.xz * scale.xz;
gl_Position = ProjectionMatrix * CameraMatrix * vec4(vertex * scale, 1.0);
}
Here's the fragment shader, if it helps.
This is what I get:
Here's a further look, for proportion.
The top and left walls (of the heightmap) are rendered ok, and the bottom and right walls still suffer from stretching. I also get these weird stretches spots next to the beginning of the walls.
What could be the cause of this?

Answer 1

If you want to check if the normal's x or z coordinate are longer, you should use the abs function:
if(abs(fsNormal.x) > abs(fsNormal.z))
Furthermore, the y > 0.75 seems like a coarse approximation, which is probably good enough in most cases. Actually, the maximum of abs(x), abs(y), abs(z) gives you the correct plane.

Answer 2

Here is a DX11/HLSL implementation i used. GLSL conversion should be easy.
With the exponent value you can tune the blending speed at the borders. i used something like 3.
float3 SampleTriplanarTexture(Texture2D<float4> tex1, Texture2D<float4> tex2, Texture2D<float4> tex3, float3 normal, float3 pos, float exponent)
{
//triplanar projection
float mXY = pow(abs(normal.z), exponent);
float mXZ = pow(abs(normal.y), exponent);
float mYZ = pow(abs(normal.x), exponent);
float total = 1.0f / (mXY + mXZ + mYZ);
mXY *= total;
mXZ *= total;
mYZ *= total;
return tex1.SampleLevel(linearSampler2, pos.xz, 0) * mXZ +
tex2.SampleLevel(linearSampler2, pos.xy, 0) * mXY +
tex3.SampleLevel(linearSampler2, pos.yz, 0) * mYZ;
}