Function InterpolateColorCode

src/pdf/PDFGradientShader.cpp:144–195

* Assumes t - startOffset is on the stack and does a linear interpolation on t between startOffset * and endOffset from prevColor to curColor (for each color component), leaving the result in * component order on the stack. It assumes there are always 3 components per color. */

 * component order on the stack. It assumes there are always 3 components per color.
 */
void InterpolateColorCode(float range, Color beginColor, Color endColor,
                          const std::shared_ptr<MemoryWriteStream>& result) {
  // Linearly interpolate from the previous color to the current. Scale the colors from 0..255 to
  // 0..1 and determine the multipliers for interpolation.
  // C{r,g,b}(t, section) = t - offset_(section-1) + t * Multiplier{r,g,b}.

  // Figure out how to scale each color component.
  constexpr int ColorComponents = 3;
  float multiplier[ColorComponents];
  for (int i = 0; i < ColorComponents; i++) {
    multiplier[i] = (endColor[i] - beginColor[i]) / range;
  }

  // Calculate when we no longer need to keep a copy of the input parameter t. If the last component
  // to use t is i, then dupInput[0..i - 1] = true and dupInput[i .. components] = false.
  bool dupInput[ColorComponents];
  dupInput[ColorComponents - 1] = false;
  for (int i = ColorComponents - 2; i >= 0; i--) {
    dupInput[i] = dupInput[i + 1] || multiplier[i + 1] != 0;
  }

  if (!dupInput[0] && multiplier[0] == 0) {
    result->writeText("pop ");
  }

  for (int i = 0; i < ColorComponents; i++) {
    // If the next components needs t and this component will consume a
    // copy, make another copy.
    if (dupInput[i] && multiplier[i] != 0) {
      result->writeText("dup ");
    }

    auto colorComponent = static_cast<uint8_t>(beginColor[i] * 255);
    if (multiplier[i] == 0) {
      PDFUtils::AppendColorComponent(colorComponent, result);
      result->writeText(" ");
    } else {
      if (multiplier[i] != 1) {
        PDFUtils::AppendFloat(multiplier[i], result);
        result->writeText(" mul ");
      }
      if (colorComponent != 0) {
        PDFUtils::AppendColorComponent(colorComponent, result);
        result->writeText(" add ");
      }
    }

    if (dupInput[i]) {
      result->writeText("exch ");
    }
  }
}

void WriteGradientRanges(const GradientInfo& info, const std::vector<size_t>& rangeEnds, bool top,
                         bool first, const std::shared_ptr<MemoryWriteStream>& result) {