CODEWITHSUNDEEP
opengltexturesglslfragment-shadermipmaps
Christopher Bruns
Christopher Bruns

Reputation: 9518

How to access automatic mipmap level in GLSL fragment shader texture?

How do I determine what mipmap level was used when sampling a texture in a GLSL fragment shader?

I understand that I can manually sample a particular mipmap level of a texture using the textureLod(...) method:

uniform sampler2D myTexture;

void main()
{
    float mipmapLevel = 1;
    vec2 textureCoord = vec2(0.5, 0.5);
    gl_FragColor = textureLod(myTexture, textureCoord, mipmapLevel);
}

Or I could allow the mipmap level to be selected automatically using texture(...) like

uniform sampler2D myTexture;

void main()
{
    vec2 textureCoord = vec2(0.5, 0.5);
    gl_FragColor = texture(myTexture, textureCoord);
}

I prefer the latter, because I trust the driver's judgment about appropriate mipmap level more than I do my own.

But I'd like to know what mipmap level was used in the automatic sampling process, to help me rationally sample nearby pixels. Is there a way in GLSL to access the information about what mipmap level was used for an automatic texture sample?

Upvotes: 19

Views: 28560

Answers (2)

genpfault
genpfault

Reputation: 52165

From here:

take a look at the OpenGL 4.2 spec chapter 3.9.11 equation 3.21. The mip map level is calculated based on the lengths of the derivative vectors:

float mip_map_level(in vec2 texture_coordinate)
{
    vec2  dx_vtc        = dFdx(texture_coordinate);
    vec2  dy_vtc        = dFdy(texture_coordinate);
    float delta_max_sqr = max(dot(dx_vtc, dx_vtc), dot(dy_vtc, dy_vtc));
    return 0.5 * log2(delta_max_sqr);
}

Upvotes: 15

Christopher Bruns
Christopher Bruns

Reputation: 9518

Below are three distinct approaches to this problem, depending on which OpenGL features are available to you:

  1. As pointed out by Andon M. Coleman in the comments, the solution in OpenGL version 4.00 and above is simple; just use the textureQueryLod function:

    #version 400

uniform sampler2D myTexture;
in vec2 textureCoord; // in normalized units
out vec4 fragColor;

void main()
{
    float mipmapLevel = textureQueryLod(myTexture, textureCoord).x;
    fragColor = textureLod(myTexture, textureCoord, mipmapLevel);
}

  2. In earlier versions of OpenGL (2.0+?), you might be able to load an extension, to similar effect. This approach worked for my case. NOTE: the method call is capitalized differently in the extension, vs. the built-in (queryTextureLod vs queryTextureLOD).

    #version 330

#extension GL_ARB_texture_query_lod : enable

uniform sampler2D myTexture;
in vec2 textureCoord; // in normalized units
out vec4 fragColor;

void main()
{
    float mipmapLevel = 3; // default in case extension is unavailable...
#ifdef GL_ARB_texture_query_lod
    mipmapLevel = textureQueryLOD(myTexture, textureCoord).x; // NOTE CAPITALIZATION
#endif
    fragColor = textureLod(myTexture, textureCoord, mipmapLevel);
}

  3. If loading the extension does not work, you could estimate the automatic level of detail using the approach contributed by genpfault:

    #version 330

uniform sampler2D myTexture;
in vec2 textureCoord; // in normalized units
out vec4 fragColor;

// Does not take into account GL_TEXTURE_MIN_LOD/GL_TEXTURE_MAX_LOD/GL_TEXTURE_LOD_BIAS,
// nor implementation-specific flexibility allowed by OpenGL spec
float mip_map_level(in vec2 texture_coordinate) // in texel units
{
    vec2  dx_vtc        = dFdx(texture_coordinate);
    vec2  dy_vtc        = dFdy(texture_coordinate);
    float delta_max_sqr = max(dot(dx_vtc, dx_vtc), dot(dy_vtc, dy_vtc));
    float mml = 0.5 * log2(delta_max_sqr);
    return max( 0, mml ); // Thanks @Nims
}

void main()
{
    // convert normalized texture coordinates to texel units before calling mip_map_level
    float mipmapLevel = mip_map_level(textureCoord * textureSize(myTexture, 0));
    fragColor = textureLod(myTexture, textureCoord, mipmapLevel);
}

In any case, for my particular application, I ended up just computing the mipmap level on the host side, and passing it to the shader, because the automatic level-of-detail turned out to be not exactly what I needed.

Upvotes: 37

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