Several SDL-related features and enhancements.

- Render the scene in a thread. This allows the main thread to
  run an event loop and detect buttons being pressed (which is used
  to abort the rendering at any time).
- Pressing "F12" takes a screenshot
- Clicking on the rendered image traces a special "debugging" ray
  (can be used to debug issues).

src/bitmap.h

@@ -49,6 +49,11 @@ class Bitmap {
 	bool loadEXR(const char* filename); //!< Loads an EXR file
 	bool saveEXR(const char* filename); //!< Saves the image into the EXR format, preserving the dynamic range, using Half for storage.
 
+	enum OutputFormat { /// the two supported writing formats
+		outputFormat_BMP,
+		outputFormat_EXR,
+	};
+
 	virtual bool loadImage(const char* filename); //!< Loads an image (the format is detected from extension)
 	virtual bool saveImage(const char* filename); //!< Save the bitmap to an image (the format is detected from extension)
 };

src/main.cpp

@@ -296,6 +296,7 @@ void render()
 
 	for (int y = 0; y < frameHeight(); y++) {
 		for (int x = 0; x < frameWidth(); x++) {
+			if (wantToQuit) return;
 			Color avg(0, 0, 0);
 			for (int i = 0; i < samplesPerPixel; i++) {
 				if (scene.camera->stereoSeparation == 0) {
@@ -338,6 +339,13 @@ void render()
 	printf("Frame took %d ms\n", elapsed);
 }
 
+int renderSceneThread(void* /*unused*/)
+{
+	render();
+	rendering = false;
+	return 0;
+}
+
 bool parseCmdLine(int argc, char** argv)
 {
 	if (argc == 1) return true;
@@ -350,6 +358,17 @@ bool parseCmdLine(int argc, char** argv)
 	}
 }
 
+void debugRayTrace(int x, int y)
+{
+	// trace a test ("debugging") ray through a clicked pixel on the screen
+	Ray ray = scene.camera->getScreenRay(x, y);
+	ray.flags |= RF_DEBUG;
+	if (scene.settings.gi)
+		pathtrace(ray, Color(1, 1, 1), getRandomGen());
+	else
+		raytrace(ray);
+}
+
 int main(int argc, char** argv)
 {
 	if (!parseCmdLine(argc, argv)) return -1;
@@ -358,9 +377,9 @@ int main(int argc, char** argv)
 	initGraphics(scene.settings.frameWidth, scene.settings.frameHeight);
 	scene.beginRender();
 	scene.beginFrame();
-	render();
+	renderScene_threaded();
 	displayVFB(vfb);
-	waitForUserExit();
+	if (!wantToQuit) waitForUserExit();
 	closeGraphics();
 	printf("Exited cleanly\n");
 	return 0;

src/sdl.cpp

@@ -28,8 +28,13 @@
 #endif // _WIN32
 #include <stdio.h>
 #include "sdl.h"
+#include "bitmap.h"
 
 SDL_Surface* screen = NULL;
+volatile bool rendering; // used in main/worker thread synchronization
+SDL_Thread *render_thread;
+SDL_mutex *render_lock;
+bool render_async, wantToQuit = false;
 
 /// try to create a frame window with the given dimensions
 bool initGraphics(int frameWidth, int frameHeight)
@@ -105,3 +110,126 @@ int frameHeight(void)
 	if (screen) return screen->h;
 	return 0;
 }
+
+// find an unused file name like 'fray_0005.bmp'
+static void findUnusedFN(char fn[], const char* suffix)
+{
+	char temp[256];
+	int idx = 0;
+	while (idx < 10000) {
+		sprintf(temp, "fray_%04d.bmp", idx);
+		FILE* f = fopen(temp, "rb");
+		if (!f) {
+			sprintf(temp, "fray_%04d.exr", idx);
+			f = fopen(temp, "rb");
+		}
+		if (!f) break; // file doesn't exist - use that
+		fclose(f);
+		idx++;
+	}
+	sprintf(fn, "fray_%04d.%s", idx, suffix); 
+}
+
+bool takeScreenshot(const char* filename)
+{
+	extern Color vfb[VFB_MAX_SIZE][VFB_MAX_SIZE]; // from main.cpp
+
+	Bitmap bmp;
+	bmp.generateEmptyImage(frameWidth(), frameHeight());
+	for (int y = 0; y < frameHeight(); y++)
+		for (int x = 0; x < frameWidth(); x++)
+			bmp.setPixel(x, y, vfb[y][x]);
+	bool res = bmp.saveImage(filename);
+	if (res) printf("Saved a screenshot as `%s'\n", filename);
+	else printf("Failed to take a screenshot\n");
+	return res;
+}
+
+bool takeScreenshotAuto(Bitmap::OutputFormat fmt)
+{
+	char fn[256];
+	findUnusedFN(fn, fmt == Bitmap::outputFormat_BMP ? "bmp" : "exr");
+	return takeScreenshot(fn);
+}
+
+static void handleEvent(SDL_Event& ev)
+{
+	switch (ev.type) {
+		case SDL_QUIT:
+			wantToQuit = true;
+			return;
+		case SDL_KEYDOWN:
+		{
+			switch (ev.key.keysym.sym) {
+				case SDLK_ESCAPE:
+					wantToQuit = true;
+					return;
+				case SDLK_F12:
+					// Shift+F12: screenshot in EXR; else, do it in (gamma-compressed) BMP.
+					if (ev.key.keysym.mod & (KMOD_LSHIFT | KMOD_RSHIFT))
+						takeScreenshotAuto(Bitmap::outputFormat_EXR);
+					else
+						takeScreenshotAuto(Bitmap::outputFormat_BMP);
+					break;
+				default:
+					break;
+			}
+			break;
+		}
+		case SDL_MOUSEBUTTONDOWN:
+		{
+			// raytrace a single ray at the given pixel
+			extern void debugRayTrace(int x, int y);
+			debugRayTrace(ev.button.x, ev.button.y);
+			break;
+		}
+		default:
+			break;
+	}
+}
+
+class MutexRAII {
+	SDL_mutex* mutex;
+public:
+	MutexRAII(SDL_mutex* _mutex)
+	{
+		mutex = _mutex;
+		SDL_mutexP(mutex);
+	}
+	~MutexRAII()
+	{
+		SDL_mutexV(mutex);
+	}
+};
+
+bool renderScene_threaded(void)
+{
+	render_async = true;
+	rendering = true;
+	extern int renderSceneThread(void*);
+	render_thread = SDL_CreateThread(renderSceneThread, NULL);
+
+	if(render_thread == NULL) { //Failed to start for some bloody reason
+		rendering = render_async = false;
+		return false;
+	}
+
+	while (!wantToQuit) {
+		{
+			MutexRAII raii(render_lock);
+			if (!rendering) break;
+			SDL_Event ev;
+			while (SDL_PollEvent(&ev)) {
+				handleEvent(ev);
+				if (wantToQuit) break;
+			}
+		}
+		SDL_Delay(100);
+	}
+	rendering = false;
+	SDL_WaitThread(render_thread, NULL);
+	render_thread = NULL;
+
+	render_async = false;
+	return true;
+}

src/sdl.h

@@ -26,9 +26,13 @@
 #include "color.h"
 #include "constants.h"
 
+extern volatile bool rendering; // used in main/worker thread synchronization
+extern bool wantToQuit;
+
 bool initGraphics(int frameWidth, int frameHeight);
 void closeGraphics(void);
 void displayVFB(Color vfb[VFB_MAX_SIZE][VFB_MAX_SIZE]); //!< displays the VFB (Virtual framebuffer) to the real one.
 void waitForUserExit(void); //!< Pause. Wait until the user closes the application
 int frameWidth(void); //!< returns the frame width (pixels)
 int frameHeight(void); //!< returns the frame height (pixels)
+bool renderScene_threaded();