从片段着色器中的地形高程数据计算法线

Posted 2023-02-22

【中文标题】从片段着色器中的地形高程数据计算法线

【英文标题】：Computing normals from terrain elevation data in fragment shader

【发布时间】：2011-06-06 17:11:33

【问题描述】：

应用程序将地形高程数据加载到浮点纹理中。然后在浮点颜色缓冲区上渲染纹理（使用 *GL_ARB_color_buffer_float*）。

第一遍是获取整体纹理（由多个纹理四边形渲染产生）。为此，我禁用了颜色钳制，渲染每个四边形，并将（浮动）帧缓冲区复制到（浮动）纹理中：此纹理是处理地形高程数据的多个通道的基础。

使用假颜色渲染浮动纹理数据没有问题：确实在第一次通过时保留了地形高程信息（无颜色钳位）。

我在计算地形法线时遇到了麻烦。下面是片段着色器的截图：

// 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);

您可以在 GetFragmentNormal 中看到，我尝试从 neightboor 文本中获取高程数据，但 TerrainAroundHeights 的值似乎总是相等的！！！

我错过了什么大事吗？有没有更好的方法来获取 neightboor textel？

【参考方案1】：

您在 GetFragmentNormal 中计算偏移 texcoord，然后将其传递给 GetFragmentElevation，但随后您不会在纹理查找中使用它——您在添加偏移之前使用原始 texcoord。

float GetFragmentElevation(const vec2 texcoord)

    // This is OK! Tested with false colors
    return (texture(rdr_ElevationData, gl_TexCoord[0].st).r);

所以不要太奇怪，不管偏移量如何，你都会得到相同的高度。将 gl_TexCoord[0] 改为 texcoord。

【讨论】：

哦，天哪...我需要一些结对编程！谢谢！