impeller/entity/shaders/rrect_blur.frag

// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

precision highp float;

#include <impeller/gaussian.glsl>
#include <impeller/types.glsl>

uniform FragInfo {
  f16vec4 color;
  vec2 rect_size;
  float blur_sigma;
  float corner_radius;
}
frag_info;

in vec2 v_position;

out f16vec4 frag_color;

const int kSampleCount = 4;

float16_t RRectDistance(vec2 sample_position, vec2 half_size) {
  vec2 space = abs(sample_position) - half_size + frag_info.corner_radius;
  return float16_t(length(max(space, 0.0)) + min(max(space.x, space.y), 0.0) -
                   frag_info.corner_radius);
}

/// Closed form unidirectional rounded rect blur mask solution using the
/// analytical Gaussian integral (with approximated erf).
float RRectBlurX(vec2 sample_position, vec2 half_size) {
  // Compute the X direction distance field (not incorporating the Y distance)
  // for the rounded rect.
  float space =
      min(0.0, half_size.y - frag_info.corner_radius - abs(sample_position.y));
  float rrect_distance =
      half_size.x - frag_info.corner_radius +
      sqrt(max(0.0, frag_info.corner_radius * frag_info.corner_radius -
                        space * space));

  // Map the linear distance field to the approximate Gaussian integral.
  vec2 integral = IPVec2FastGaussianIntegral(
      float(sample_position.x) + vec2(-rrect_distance, rrect_distance),
      float(frag_info.blur_sigma));
  return integral.y - integral.x;
}

float RRectBlur(vec2 sample_position, vec2 half_size) {
  // Limit the sampling range to 3 standard deviations in the Y direction from
  // the kernel center to incorporate 99.7% of the color contribution.
  float half_sampling_range = frag_info.blur_sigma * 3.0;

  float begin_y = max(-half_sampling_range, sample_position.y - half_size.y);
  float end_y = min(half_sampling_range, sample_position.y + half_size.y);
  float interval = (end_y - begin_y) / kSampleCount;

  // Sample the X blur kSampleCount times, weighted by the Gaussian function.
  float result = 0.0;
  for (int sample_i = 0; sample_i < kSampleCount; sample_i++) {
    float y = begin_y + interval * (float(sample_i) + 0.5);
    result +=
        RRectBlurX(vec2(sample_position.x, sample_position.y - y), half_size) *
        IPGaussian(float(y), float(frag_info.blur_sigma)) * interval;
  }

  return result;
}

void main() {
  frag_color = frag_info.color;

  vec2 half_size = frag_info.rect_size * 0.5;
  vec2 sample_position = v_position - half_size;

  if (frag_info.blur_sigma > 0.0) {
    frag_color *= float16_t(RRectBlur(sample_position, half_size));
  } else {
    frag_color *= float16_t(-RRectDistance(sample_position, half_size));
  }
}