tests/gm/additive_blend.cpp

/*
 * Copyright 2026 Rive
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "gm.hpp"
#include "gmutils.hpp"
#include "rive/renderer.hpp"

#include "assets/nomoon.png.hpp"

#include <array>

using namespace rivegm;
using namespace rive;

// Exercises the additiveness paint value (a Rive extension), which
// interpolates the srcOver composite toward additive: 0 is normal srcOver,
// 1 is fully additive (result.rgb = src + dst, destination alpha preserved).
//
// Layout: 2 columns x 6 rows of 300x300 cells on a black background.
//
// Left column (srcOver references):
//   Row 0: opaque green square overlaid by a red square whose alpha ramps
//          from 1 at the top to 0 at the bottom.
//   Row 1: stroked variants of row 0.
//   Row 2: opaque gradient square overlaid by a gradient square whose alpha
//          ramps from 1 at the top to 0 at the bottom.
//   Row 3: image mesh overlaid by an image mesh whose opacity ramps from 1
//          at the top to 0 at the bottom (drawn as horizontal strips).
//   Row 4: drawImage (image paint on a path): base image at full opacity
//          under an overlapping image at 60% opacity.
//
// Right column (additive):
//   Row 0: filled RGB venn circles; overlaps accumulate to
//          yellow/cyan/magenta/white.
//   Row 1: stroked rings (additive applies to strokes independently).
//   Rows 2 & 3: same geometry as the left column, with the overlay drawn
//          additively, so left vs right isolates srcOver vs additive.
//
// Row 5: the additiveness slider (0 = srcOver, 1 = fully additive). An opaque
//        green square under five red strips with additiveness 0, .25, .5,
//        .75, 1 (srcOver to fully additive, left to right). Left column solid
//        color, right column gradient.
constexpr float CELL = 300.f;

// Draws one quad (2 triangles) of an image as a mesh.
static void draw_image_mesh_quad(
    Renderer* renderer,
    RenderImage* img,
    const AABB& dst,
    const AABB& uv,
    float additiveness,
    float opacity)
{
  Factory* factory = TestingWindow::Get()->factory();
  auto pts = factory->makeRenderBuffer(
      RenderBufferType::vertex,
      RenderBufferFlags::mappedOnceAtInitialization,
      4 * sizeof(Vec2D));
  memcpy(
      pts->map(),
      std::array<Vec2D, 4>{
          Vec2D{dst.left(), dst.top()},
          Vec2D{dst.right(), dst.top()},
          Vec2D{dst.left(), dst.bottom()},
          Vec2D{dst.right(), dst.bottom()}}
          .data(),
      pts->sizeInBytes());
  pts->unmap();
  auto uvs = factory->makeRenderBuffer(
      RenderBufferType::vertex,
      RenderBufferFlags::mappedOnceAtInitialization,
      4 * sizeof(Vec2D));
  memcpy(
      uvs->map(),
      std::array<Vec2D, 4>{
          Vec2D{uv.left(), uv.top()},
          Vec2D{uv.right(), uv.top()},
          Vec2D{uv.left(), uv.bottom()},
          Vec2D{uv.right(), uv.bottom()}}
          .data(),
      uvs->sizeInBytes());
  uvs->unmap();
  auto indices = factory->makeRenderBuffer(
      RenderBufferType::index,
      RenderBufferFlags::mappedOnceAtInitialization,
      6 * sizeof(uint16_t));
  memcpy(
      indices->map(),
      std::array<uint16_t, 6>{0, 1, 2, 1, 3, 2}.data(),
      indices->sizeInBytes());
  indices->unmap();
  renderer->drawImageMesh(
      img,
      ImageSampler::LinearClamp(),
      pts,
      uvs,
      indices,
      4,
      6,
      BlendMode::srcOver,
      opacity,
      additiveness);
}

// Approximates a vertical opacity ramp (1 at the top, 0 at the bottom) by
// drawing the image as a stack of mesh strips with stepped opacity.
static void draw_image_mesh_alpha_ramp(
    Renderer* renderer,
    RenderImage* img,
    const AABB& dst,
    float additiveness)
{
  constexpr int strips = 256;
  for (int i = 0; i < strips; ++i)
  {
    float t0 = float(i) / strips;
    float t1 = float(i + 1) / strips;
    AABB stripDst = {
        dst.left(),
        dst.top() + t0 * dst.height(),
        dst.right(),
        dst.top() + t1 * dst.height()};
    AABB stripUV = {0.f, t0, 1.f, t1};
    draw_image_mesh_quad(
        renderer,
        img,
        stripDst,
        stripUV,
        additiveness,
        1.f - (t0 + t1) * .5f);
  }
}

DEF_SIMPLE_GM_WITH_CLEAR_COLOR(additive_blend, 0xff000000, 600, 1800, renderer)
{
  Factory* factory = TestingWindow::Get()->factory();
  constexpr float stops[2] = {0.f, 1.f};

  // Base square and overlapping overlay square within a cell at (ox, oy).
  auto baseRect = [](float ox, float oy) {
    return AABB{ox + 40.f, oy + 40.f, ox + 190.f, oy + 190.f};
  };
  auto overlayRect = [](float ox, float oy) {
    return AABB{ox + 110.f, oy + 110.f, ox + 260.f, oy + 260.f};
  };

  // Rows 0 & 1, left: opaque green square under a red square whose alpha
  // ramps 1 -> 0 top to bottom (via a gradient that only varies alpha).
  auto squaresCell = [&](float ox,
                         float oy,
                         RenderPaintStyle style,
                         float thickness,
                         BlendMode overlayBlend) {
    AABB base = baseRect(ox, oy);
    Paint basePaint;
    basePaint->style(style);
    if (style == RenderPaintStyle::stroke)
    {
      basePaint->thickness(thickness);
    }
    basePaint->color(0xff00ff00);
    renderer->drawPath(PathBuilder::Rect(base), basePaint);

    AABB overlay = overlayRect(ox, oy);
    ColorInt overlayColors[2] = {0xffff0000, 0x00ff0000};
    Paint overlayPaint;
    overlayPaint->style(style);
    if (style == RenderPaintStyle::stroke)
    {
      overlayPaint->thickness(thickness);
    }
    overlayPaint->shader(factory->makeLinearGradient(
        0,
        overlay.top(),
        0,
        overlay.bottom(),
        overlayColors,
        stops,
        2));
    overlayPaint->blendMode(overlayBlend);
    renderer->drawPath(PathBuilder::Rect(overlay), overlayPaint);
  };
  squaresCell(0, 0, RenderPaintStyle::fill, 0.f, BlendMode::srcOver);
  squaresCell(0, CELL, RenderPaintStyle::stroke, 20.f, BlendMode::srcOver);

  // Rows 0 & 1, right: classic RGB venn arrangement, all circles additive.
  // Two-circle overlaps accumulate to yellow/cyan/magenta, all three to
  // white.
  auto vennCell =
      [&](float ox, float oy, RenderPaintStyle style, float thickness) {
        struct Circle
        {
          float cx, cy;
          ColorInt color;
        };
        constexpr float radius = 80.f;
        const Circle circles[] = {
            {ox + 150.f, oy + 110.f, 0xffff0000}, // red, top
            {ox + 110.f, oy + 185.f, 0xff00ff00}, // green, bottom-left
            {ox + 190.f, oy + 185.f, 0xff0000ff}, // blue, bottom-right
        };
        for (const Circle& c : circles)
        {
          Paint paint;
          paint->style(style);
          if (style == RenderPaintStyle::stroke)
          {
            paint->thickness(thickness);
          }
          paint->color(c.color);
          paint->additiveness(1.f);
          renderer->drawPath(PathBuilder::Circle(c.cx, c.cy, radius), paint);
        }
      };
  vennCell(CELL, 0, RenderPaintStyle::fill, 0.f);
  vennCell(CELL, CELL, RenderPaintStyle::stroke, 40.f);

  // Row 2: opaque horizontal gradient square under a vertical gradient
  // square that fades opaque red -> transparent yellow. Left srcOver,
  // right additive.
  auto gradientCell = [&](float ox, float overlayAdditiveness) {
    constexpr float oy = CELL * 2;
    AABB base = baseRect(ox, oy);
    ColorInt baseColors[2] = {0xff00ff00, 0xff0000ff};
    Paint basePaint;
    basePaint->shader(factory->makeLinearGradient(
        base.left(),
        0,
        base.right(),
        0,
        baseColors,
        stops,
        2));
    renderer->drawPath(PathBuilder::Rect(base), basePaint);

    AABB overlay = overlayRect(ox, oy);
    ColorInt overlayColors[2] = {0xffff0000, 0x00ffff00};
    Paint overlayPaint;
    overlayPaint->shader(factory->makeLinearGradient(
        0,
        overlay.top(),
        0,
        overlay.bottom(),
        overlayColors,
        stops,
        2));
    overlayPaint->additiveness(overlayAdditiveness);
    renderer->drawPath(PathBuilder::Rect(overlay), overlayPaint);
  };
  gradientCell(0, /*overlayAdditiveness =*/0.f);
  gradientCell(CELL, /*overlayAdditiveness =*/1.f);

  // Row 3: image mesh under an overlapping image mesh whose opacity ramps
  // 1 -> 0 top to bottom. Left srcOver, right additive.
  auto img = LoadImage(assets::nomoon_png());
  if (img != nullptr)
  {
    auto imageCell = [&](float ox, float overlayAdditiveness) {
      constexpr float oy = CELL * 3;
      draw_image_mesh_quad(
          renderer,
          img.get(),
          baseRect(ox, oy),
          {0.f, 0.f, 1.f, 1.f},
          /*additiveness =*/0.f,
          1.f);
      draw_image_mesh_alpha_ramp(
          renderer,
          img.get(),
          overlayRect(ox, oy),
          overlayAdditiveness);
    };
    imageCell(0, /*overlayAdditiveness =*/0.f);
    imageCell(CELL, /*overlayAdditiveness =*/1.f);

    // Row 4: drawImage (image paint on a path): base image at full
    // opacity under an overlapping image at 60% opacity. Left srcOver,
    // right additive.
    auto drawImageRect =
        [&](const AABB& rect, float additiveness, float opacity) {
          renderer->save();
          renderer->translate(rect.left(), rect.top());
          renderer->scale(
              rect.width() / img->width(),
              rect.height() / img->height());
          renderer->drawImage(
              img.get(),
              ImageSampler::LinearClamp(),
              BlendMode::srcOver,
              opacity,
              additiveness);
          renderer->restore();
        };
    auto drawImageCell = [&](float ox, float overlayAdditiveness) {
      constexpr float oy = CELL * 4;
      drawImageRect(baseRect(ox, oy), /*additiveness =*/0.f, 1.f);
      drawImageRect(overlayRect(ox, oy), overlayAdditiveness, .6f);
    };
    drawImageCell(0, /*overlayAdditiveness =*/0.f);
    drawImageCell(CELL, /*overlayAdditiveness =*/1.f);
  }

  // Row 5: the additiveness slider. An opaque green square under five red
  // strips with additiveness 0, .25, .5, .75, 1 (srcOver to fully additive,
  // left to right). Left cell solid color, right cell gradient.
  auto additivenessSweepCell = [&](float ox, bool gradient) {
    constexpr float oy = CELL * 5;
    AABB base = baseRect(ox, oy);
    Paint basePaint;
    basePaint->color(0xff00ff00);
    renderer->drawPath(PathBuilder::Rect(base), basePaint);

    AABB overlay = overlayRect(ox, oy);
    constexpr int strips = 5;
    for (int i = 0; i < strips; ++i)
    {
      AABB strip = {
          overlay.left() + overlay.width() * i / strips,
          overlay.top(),
          overlay.left() + overlay.width() * (i + 1) / strips,
          overlay.bottom()};
      Paint paint;
      if (gradient)
      {
        // Opaque red -> half-transparent red, top to bottom.
        ColorInt colors[2] = {0xffff0000, 0x80ff0000};
        paint->shader(factory->makeLinearGradient(
            0,
            strip.top(),
            0,
            strip.bottom(),
            colors,
            stops,
            2));
      }
      else
      {
        paint->color(0xc0ff0000);
      }
      paint->additiveness(float(i) / (strips - 1));
      renderer->drawPath(PathBuilder::Rect(strip), paint);
    }
  };
  additivenessSweepCell(0, /*gradient =*/false);
  additivenessSweepCell(CELL, /*gradient =*/true);
}