Custom Filter

This guide explains how to create custom filters by inheriting from the Filter class in GPUPixel.

Overview

GPUPixel's filter system is built on OpenGL shaders. Each filter inherits from the base Filter class and implements specific image processing effects through shader programs.

Basic Structure

A typical custom filter class has the following structure:

class CustomFilter : public Filter {
 public:
  static std::shared_ptr<CustomFilter> create();
  bool init();
  virtual bool proceed(bool bUpdateTargets = true, int64_t frameTime = 0) override;

  // Custom parameter setters
  void setParameter(float value);

 protected:
  CustomFilter(){};

  // Filter parameters
  float _parameter;
};

Implementation Steps

1. Create Factory Method

Implement a static factory method that creates and initializes the filter:

std::shared_ptr<CustomFilter> CustomFilter::create() {
  auto ret = std::shared_ptr<CustomFilter>(new CustomFilter());
  if (ret && !ret->init()) {
    ret.reset();
  }
  return ret;
}

2. Initialize Filter

Implement the init() method to set up shader programs and register parameters:

bool CustomFilter::init() {
  // Initialize with shader string
  if (!initWithFragmentShaderString(kCustomFragmentShader)) {
    return false;
  }

  // Register parameters
  _parameter = 1.0f;
  registerProperty(
      "parameter",
      _parameter,
      "Description of the parameter",
      [this](float& value) { setParameter(value); });

  return true;
}

3. Define Shader Program

Create GLSL shader program for your filter effect:

const std::string kCustomFragmentShader = R"(
    precision highp float;
    uniform sampler2D inputImageTexture;
    uniform float parameter;
    varying highp vec2 textureCoordinate;

    void main() {
      // Implement your filter effect here
      vec4 textureColor = texture2D(inputImageTexture, textureCoordinate);
      gl_FragColor = textureColor;
    })"

4. Implement Processing Logic

Override the proceed() method to update shader uniforms and process the image:

bool CustomFilter::proceed(bool bUpdateTargets, int64_t frameTime) {
  // Update shader uniforms
  _filterProgram->setUniformValue("parameter", _parameter);
  
  // Call base class implementation
  return Filter::proceed(bUpdateTargets, frameTime);
}

Example: HueFilter

Here's a real example of the HueFilter that adjusts image hue:

class HueFilter : public Filter {
 public:
  static std::shared_ptr<HueFilter> create();
  bool init();
  virtual bool proceed(bool bUpdateTargets = true, int64_t frameTime = 0) override;

  void setHueAdjustment(float hueAdjustment);

 protected:
  HueFilter(){};
  float _hueAdjustment;
};

The HueFilter demonstrates:

• Parameter registration with callback
• Shader uniform updates in proceed()
• Value conversion in parameter setter

Best Practices

1. Parameter Management

   • Use meaningful parameter names
   • Provide clear parameter descriptions
   • Implement proper value validation

2. Shader Optimization

   • Keep shaders simple and efficient
   • Use appropriate precision qualifiers
   • Consider performance impact

3. Error Handling

   • Check initialization success
   • Validate parameter values
   • Handle resource cleanup

See Also

• Custom Input
• Custom Target