OpenGL: Terrain deformation using a heightmap in the vertex shader - opengl

Question

I have been trying to implement a heightmap to my terrain shader, but the terrain remains flat. The texture is properly loaded in the vertex shader, and I try to use the greyscale values of the texture based on the mesh's uvs to adjust the vertex height:
//DIFFUSE VERTEX SHADER
#version 330
uniform mat4 projectionMatrix;
uniform mat4 viewMatrix;
uniform mat4 modelMatrix;
in vec3 vertex;
in vec3 normal;
in vec2 uv;
uniform sampler2D heightmap;
out vec2 texCoord;
void main( void ){
vec3 _vertex = vertex;
_vertex.y = texture(heightmap, uv).r * 2.f;
gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4(_vertex, 1.f);
texCoord = uv;
}
Fragment: (the splatmap works so ignore that)
uniform sampler2D splatmap;
uniform sampler2D diffuse1;
uniform sampler2D diffuse2;
uniform sampler2D diffuse3;
uniform sampler2D diffuse4;
in vec2 texCoord;
out vec4 fragment_color;
void main( void ) {
///Loading the splatmap and the diffuse textures
vec4 splatTexture = texture2D(splatmap, texCoord);
vec4 diffuseTexture1 = texture2D(diffuse1, texCoord);
vec4 diffuseTexture2 = texture2D(diffuse2, texCoord);
vec4 diffuseTexture3 = texture2D(diffuse3, texCoord);
vec4 diffuseTexture4 = texture2D(diffuse4, texCoord);
//Interpolate between the different textures using the splatmap's rgb values (works)
diffuseTexture1 *= splatTexture.r;
diffuseTexture2 = mix (diffuseTexture1, diffuseTexture2, splatTexture.g);
diffuseTexture3 = mix (diffuseTexture2,diffuseTexture3, splatTexture.b);
vec4 outcolor = mix (diffuseTexture3, diffuseTexture4, splatTexture.a);
fragment_color = outcolor;
}
Some additional info:
All textures are loaded like this in my terrain material and passed to the shader (works properly):
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, heightMap->getId());
glUniform1i (_shader->getUniformLocation("heightMap"),0);
...
The plane mesh uvs are mapped like this:
(0,1) (1,1)
(0,0) (1,0)
I guess I am doing something horribly wrong, but I can't figure out what. Any help is appreciated!

Answer 1

Does your writing this:
The plane mesh uvs are mapped like this:
(0,1) (1,1)
(0,0) (1,0)
… mean that your mesh consists of just 4 vertices? If so, then that's your problem right there: The Vertex shader can not magically create "new" vertices, so your heightmap texture is sampled at only 4 points (and nothing in between).
And because you sample the texture coordinates at integer values and your texture coordinates and are at 0 and 1, you're effectively sampling the very same texture coordinate, so you're going to see the same displacement for all four vertices.
Solution: Tesselate your base mesh so that there are actually vertices available to displace. A tesselation shader is perfectly fine for that.
EDIT:
BTW, you can simplyfiy your vertex shader a bit: For the attributes make it a
in vec2 vertex;
which requires just 2/3 of the space of vec3, since you're not using the z component anyway.
float y = texture(heightmap, uv).r * 2.f;
gl_Position =
projectionMatrix
* viewMatrix
* modelMatrix
* vec4(vertex.x, y, vertex.y, 1.f);