Computing normals from terrain elevation data in fragment shader

The application loads terrain elevation data into floating point textures. Textures are then rendered on a float color buffer (using *GL_ARB_color_buffer_float*).

The first pass is to get an overall texture (resulting from multiple textured quads rendering). To do this, I disable the color clamp, render each quad, and the copy the (float) frame buffer into a (float) texture: this texture is the base of multiple passes for processing the terrain elevation data.

There's no problem in rendering the float texture data using false colors: indeed the terrain elevation information is preserved during the first pass (no color clamp).

I'm getting in trouble by computing the terrain normals. Here is a snipper of the fragment shader:

// Required shader language version (compatibility profile)
#version 150 compatibility

//---------------------------------------------------------------------------
// SYSTEM INPUTS/OUTPUT
//---------------------------------------------------------------------------

// Fragment texture coordinate (in)
in vec4 gl_TexCoord[];
// Fragment color (out)
vec4 gl_FragColor;

    // DTED elevation data as texture.
// Value fetched by the texture if the altitude of the terrain, in meters.
uniform sampler2D rdr_ElevationData;

// Obtain fragment elevation
float GetFragmentElevation(const vec2 texcoord)
{
    // This is OK! Tested with false colors
    return (texture(rdr_ElevationData, gl_TexCoord[0].st).r);
}

vec3 GetFragmentNormal(const vec2 texcoord, const float sOffset, const float tOffset)
{
    const vec2
        texcoordN  = texcoord + vec2( 0.0,      +tOffset),
        texcoordNE = texcoord + vec2(+sOffset,  +tOffset),
        texcoordE  = texcoord + vec2(+sOffset,   0.0),
        texcoordSE = texcoord + vec2(+sOffset,  -tOffset),
        texcoordS  = texcoord + vec2( 0.0,      -tOffset),
        texcoordSW = texcoord + vec2(-sOffset,  -tOffset),
        texcoordW  = texcoord + vec2(-sOffset,   0.0),
        texcoordNW = texcoord + vec2(-sOffset,  +tOffset);
    float TerrainAroundHeights[9];

    // Ensuring clamped texture coordinates
    clamp(texcoordN , 0.0, 1.0);
    clamp(texcoordNE, 0.0, 1.0);
    clamp(texcoordE , 0.0, 1.0);
    clamp(texcoordSE, 0.0, 1.0);
    clamp(texcoordS , 0.0, 1.0);
    clamp(texcoordSW, 0.0, 1.0);
    clamp(texcoordW , 0.0, 1.0);
    clamp(texcoordNW, 0.0, 1.0);
    // Fetch terrain heights around the fragment
    TerrainAroundHeights[0] = GetFragmentElevation(texcoord);
    TerrainAroundHeights[1] = GetFragmentElevation(texcoordN);
    TerrainAroundHeights[2] = GetFragmentElevation(texcoordNE);
    TerrainAroundHeights[3] = GetFragmentElevation(texcoordE);
    TerrainAroundHeights[4] = GetFragmentElevation(texcoordSE);
    TerrainAroundHeights[5] = GetFragmentElevation(texcoordS);
    TerrainAroundHeights[6] = GetFragmentElevation(texcoordSW);
    TerrainAroundHeights[7] = GetFragmentElevation(texcoordW);
    TerrainAroundHeights[8] = GetFragmentElevation(texcoordNW);

    const float NormalLength = 0.001;

    vec3 v0 = vec3(0.0, 0.0, 0.0), v1 = vec3(0.0, NormalLength, 0.0), v2 = vec3(NormalLength, 0.0, 0.0);

    // Compute averaged heights for each fragment corner
    /*v0.z = (TerrainAroundHeights[0] + TerrainAroundHeights[7] + TerrainAroundHeights[6] + TerrainAroundHeights[5]) / 4.0;
    v1.z = (TerrainAroundHeights[0] + TerrainAroundHeights[1] + TerrainAroundHeights[8] + TerrainAroundHeights[7]) / 4.0;
    v2.z = (TerrainAroundHeights[0] + TerrainAroundHeights[5] + TerrainAroundHeights[4] + TerrainAroundHeights[3]) / 4.0;*/

    // MY TESTS... unable to understand what's going on
    if (TerrainAroundHeights[0] < (TerrainAroundHeights[5]))
        v0.z = 1.0;
    return (v0);

    // Compute terrain normal
    return (normalize(cross(v2 - v0, v1 - v0)));
}

void main()
{
    ivec2 rdr_ElevationData_Size = textureSize(rdr_ElevationData, 0);       // Texture size, in textel
    vec3 frag_Normal;                                                       // Fragment normal
    float rdr_ElevationData_S_CoordStep = 1.0 / /*rdr_ElevationData_Size[0]*/ 672.0 /* Tried but not working... * 4.0 */;
    float rdr_ElevationData_T_CoordStep = 1.0 / /*rdr_ElevationData_Size[1]*/ 672.0 /* Tried but not working... * 4.0 */;

    // Determine fragment normal
    frag_Normal = GetFragmentNormal(gl_TexCoord[0].st, rdr_ElevationData_S_CoordStep, rdr_ElevationData_T_CoordStep);

            // Test purpose... (always 0.0!!!)
    gl_FragColor = vec4(frag_Normal.z, 0.0, 0.0, 1.0);
}

You can see in GetFragmentNormal, I try to get the elevation data from the neightboor textels, but the values of TerrainAroundHeights seems alw开发者_如何学Goays equal!!!

Have I missed something huge? Is there a better way to fetch neightboor textel?

You compute the offset texcoord in GetFragmentNormal and then pass it in to GetFragmentElevation, but then you don't use it in the texture lookup -- you use the original texcoord before adding the offset.

float GetFragmentElevation(const vec2 texcoord)
{
    // This is OK! Tested with false colors
    return (texture(rdr_ElevationData, gl_TexCoord[0].st).r);
}

So not too suprisingly, you get the same elevation regardless of the offset. Change that gl_TexCoord[0] to be texcoord instead.