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◆ main openmsx / src / video / Layer.hh
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#ifndef LAYER_HH
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#define LAYER_HH
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#include <cstdint>
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#include <utility>
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namespace openmsx {
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class Display;
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class OutputSurface;
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/** Interface for display layers.
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  */
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class Layer
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{
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public:
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	/** Determines stacking order of layers:
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	  * layers with higher Z-indices are closer to the viewer.
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	  */
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	enum class ZIndex : uint8_t {
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		BACKGROUND,
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		MSX_PASSIVE,
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		MSX_ACTIVE,
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		OSDGUI,
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		IMGUI,
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	};
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	[[nodiscard]] friend auto operator<=>(ZIndex x, ZIndex y) { return std::to_underlying(x) <=> std::to_underlying(y); }
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	/** Describes how much of the screen is currently covered by a particular
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	  * layer.
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	  */
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	enum class Coverage : uint8_t {
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		/** Layer fully covers the screen: any underlying layers are invisible.
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		  */
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		FULL,
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		/** Layer partially covers the screen: it may cover only part of the
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		  * screen area, or it may be (semi-)transparent in places.
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		  */
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		PARTIAL,
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		/** Layer is not visible, that is completely transparent.
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		  */
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		NONE
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	};
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	virtual ~Layer() = default;
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	/** Paint this layer.
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	  */
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	virtual void paint(OutputSurface& output) = 0;
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	/** Query the Z-index of this layer.
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	  */
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	[[nodiscard]] ZIndex getZ() const { return z; }
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	[[nodiscard]] bool isActive() const { return getZ() == ZIndex::MSX_ACTIVE; }
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	/** Query the coverage of this layer.
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	 */
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	[[nodiscard]] Coverage getCoverage() const { return coverage; }
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	/** Store pointer to Display.
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	  * Will be called by Display::addLayer().
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	  */
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	void setDisplay(Display& display_) { display = &display_; }
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protected:
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	/** Construct a layer. */
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	explicit Layer(Coverage coverage_, ZIndex z_)
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		: coverage(coverage_), z(z_)
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	{
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	}
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	/** Changes the current coverage of this layer.
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	  */
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	void setCoverage(Coverage coverage_) { coverage = coverage_; }
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	/** Changes the current Z-index of this layer.
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	  */
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	void setZ(ZIndex z);
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private:
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	/** The display this layer is part of. */
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	Display* display = nullptr;
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	/** Inspected by Display to determine which layers to paint. */
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	Coverage coverage;
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	/** Inspected by Display to determine the order in which layers are
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	  * painted.
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	  */
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	ZIndex z;
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	friend class ScopedLayerHider; // for setCoverage()
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};
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class ScopedLayerHider
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{
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public:
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	explicit ScopedLayerHider(Layer& layer);
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	ScopedLayerHider(const ScopedLayerHider&) = delete;
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	ScopedLayerHider(ScopedLayerHider&&) = delete;
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	ScopedLayerHider& operator=(const ScopedLayerHider&) = delete;
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	ScopedLayerHider& operator=(ScopedLayerHider&&) = delete;
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	~ScopedLayerHider();
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private:
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	Layer& layer;
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	Layer::Coverage originalCoverage;
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};
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} // namespace openmsx
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#endif