Add a param-origin extension

include/clap/ext/draft/param-origin.h

@@ -0,0 +1,61 @@
+#pragma once
+
+#include "../../plugin.h"
+
+static CLAP_CONSTEXPR const char CLAP_EXT_PARAM_ORIGIN[] = "clap.param-origin/1";
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/// This extension provides an optional value per parameter that lets the host draw a visual
+/// indication for the parameter's origin.
+///
+/// examples:
+/// - lowpass filter cutoff parameter with an origin equal to param_info.min_value
+///       [-------------->        ]
+///      60Hz                   20kHz
+///      min=origin              max
+/// - highpass filter cutoff parameter with an origin equal to param_info.max_value
+///       [        <--------------]
+///      60Hz                   20kHz
+///      min                     max=origin
+/// - (bipolar) parameter with a range from -1.0 to +1.0 with an origin of 0.0
+///       [     <------|          ]
+///     -1.0          0.0       +1.0
+///      min         origin      max
+/// - crossfade parameter without an origin
+///       [              o        ]
+///       A                       B
+///      min                     max
+
+typedef struct clap_plugin_param_origin {
+   // Get the origin value for a parameter.
+   // Returns false if the parameter has no origin, true otherwise.
+   // The host must not call this for params with CLAP_PARAM_IS_ENUM flag set.
+   //
+   // out_value constraints:
+   // - has to be in the range from param_info.min_value to param_info.max_value
+   // - has to be an integer value if CLAP_PARAM_IS_STEPPED flag is set
+   // [main-thread]
+   bool(CLAP_ABI *get)(const clap_plugin_t *plugin, clap_id param_id, double *out_value);
+} clap_plugin_param_origin_t;
+
+typedef struct clap_host_param_origin {
+   // Informs the host that param origins have changed.
+   //
+   // Note: If the plugin calls params.rescan with CLAP_PARAM_RESCAN_ALL, all previously scanned
+   // parameter origins must be considered invalid. It is thus not necessary for the plugin to call
+   // param_origin.changed in this case.
+   //
+   // Note: This is useful if a parameter origin changes on-the-fly. For example a plugin might want
+   // to change the origin of a filter cutoff frequency parameter when the corresponding filter type
+   // (LP/BP/HP) has changed.
+   //
+   // [main-thread]
+   void(CLAP_ABI *changed)(const clap_host_t *host);
+} clap_host_param_origin_t;
+
+#ifdef __cplusplus
+}
+#endif

include/clap/host.h

@@ -35,6 +35,12 @@ typedef struct clap_host {
    void(CLAP_ABI *request_process)(const struct clap_host *host);
 
    // Request the host to schedule a call to plugin->on_main_thread(plugin) on the main thread.
+   // This callback should be called as soon as practicable, usually in the host application's next
+   // available main thread time slice. Typically callbacks occur withink 33ms / 30hz.
+   // Despite this guidance, plugins should not make assumptions about the exactness of timing for
+   // a main thread callback, but hosts should endeavour to be prompt. For example, in high load situations
+   // the environment may starve the gui/main thread in favor of audio processing, leading to substantially
+   // longer latencies for the callback than the indicative times given here.
    // [thread-safe]
    void(CLAP_ABI *request_callback)(const struct clap_host *host);
 } clap_host_t;