uniform mat4 u_matrix; uniform float u_extrude_scale; uniform float u_opacity; uniform float u_intensity; in vec2 a_pos; out vec2 v_extrude; #pragma mapbox: define highp float weight #pragma mapbox: define mediump float radius // Effective "0" in the kernel density texture to adjust the kernel size to; // this empirically chosen number minimizes artifacts on overlapping kernels // for typical heatmap cases (assuming clustered source) const highp float ZERO = 1.0 / 255.0 / 16.0; // Gaussian kernel coefficient: 1 / sqrt(2 * PI) #define GAUSS_COEF 0.3989422804014327 void main(void) { #pragma mapbox: initialize highp float weight #pragma mapbox: initialize mediump float radius // unencode the extrusion vector that we snuck into the a_pos vector vec2 unscaled_extrude = vec2(mod(a_pos, 2.0) * 2.0 - 1.0); // This 'extrude' comes in ranging from [-1, -1], to [1, 1]. We'll use // it to produce the vertices of a square mesh framing the point feature // we're adding to the kernel density texture. We'll also pass it as // a out, so that the fragment shader can determine the distance of // each fragment from the point feature. // Before we do so, we need to scale it up sufficiently so that the // kernel falls effectively to zero at the edge of the mesh. // That is, we want to know S such that // weight * u_intensity * GAUSS_COEF * exp(-0.5 * 3.0^2 * S^2) == ZERO // Which solves to: // S = sqrt(-2.0 * log(ZERO / (weight * u_intensity * GAUSS_COEF))) / 3.0 float S = sqrt(-2.0 * log(ZERO / weight / u_intensity / GAUSS_COEF)) / 3.0; // Pass the out in units of radius v_extrude = S * unscaled_extrude; // Scale by radius and the zoom-based scale factor to produce actual // mesh position vec2 extrude = v_extrude * radius * u_extrude_scale; // multiply a_pos by 0.5, since we had it * 2 in order to sneak // in extrusion data vec4 pos = vec4(floor(a_pos * 0.5) + extrude, get_elevation(floor(a_pos * 0.5)), 1); gl_Position = u_matrix * pos; }