Added SDL2 Experiment 5

A simple gravity "simulation" inspired by Spacewar. Values are not physically accurate. It's fun to resize the window, as the star will remain in the middle of the screen.

SDL2-Bits-And-Pieces/SDL2-Experiment-05.c

@@ -0,0 +1,186 @@
+// SDL Experiment 05, Barra Ó Catháin.
+// ===================================
+#include <SDL2/SDL.h>
+#include <SDL2/SDL_timer.h>
+#include <math.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+void DrawCircle(SDL_Renderer * renderer, int32_t centreX, int32_t centreY, int32_t radius)
+{
+   const int32_t diameter = (radius * 2);
+
+   int32_t x = (radius - 1);
+   int32_t y = 0;
+   int32_t tx = 1;
+   int32_t ty = 1;
+   int32_t error = (tx - diameter);
+
+   while (x >= y)
+   {
+	 //  Each of the following renders an octant of the circle
+      SDL_RenderDrawPoint(renderer, centreX + x, centreY - y);
+      SDL_RenderDrawPoint(renderer, centreX + x, centreY + y);
+      SDL_RenderDrawPoint(renderer, centreX - x, centreY - y);
+      SDL_RenderDrawPoint(renderer, centreX - x, centreY + y);
+      SDL_RenderDrawPoint(renderer, centreX + y, centreY - x);
+      SDL_RenderDrawPoint(renderer, centreX + y, centreY + x);
+      SDL_RenderDrawPoint(renderer, centreX - y, centreY - x);
+      SDL_RenderDrawPoint(renderer, centreX - y, centreY + x);
+
+      if (error <= 0)
+      {
+         ++y;
+         error += ty;
+         ty += 2;
+      }
+
+      if (error > 0)
+      {
+         --x;
+         tx += 2;
+         error += (tx - diameter);
+      }
+   }
+}
+
+int main(int argc, char ** argv)
+{
+	SDL_Event event;
+	bool quit = false;
+	int width = 0, height = 0;
+	uint32_t rendererFlags = SDL_RENDERER_ACCELERATED;
+	double posX = 0, posY = 0;
+	long positionX = 0, positionY = 0;
+	double velocityX = 20, velocityY = 0, gravityX = 0, gravityY = 0, gravityMagnitude = 0, gravityAcceleration = 0;
+
+	// Initialize the SDL library, video, sound, and input:
+	if (SDL_Init(SDL_INIT_EVERYTHING) != 0)
+	{
+		printf("SDL Initialization Error: %s\n", SDL_GetError());
+	}
+
+	if (SDL_Init(SDL_INIT_EVERYTHING) != 0)
+	{
+		printf("SDL Initialization Error: %s\n", SDL_GetError());
+	}
+	
+	// Create an SDL window and rendering context in that window:
+	SDL_Window * window = SDL_CreateWindow("SDL_TEST", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 640, 0);
+	SDL_Renderer * renderer = SDL_CreateRenderer(window, -1, rendererFlags);
+	
+	// Enable resizing the window:
+	SDL_SetWindowResizable(window, SDL_TRUE);
+
+	while (!quit)
+	{
+		// Check if the user wants to quit:
+		while (SDL_PollEvent(&event))
+        {
+            switch (event.type)
+            {
+                case SDL_QUIT:
+				{
+					quit = true;
+					break;
+				}
+				default:
+				{
+					break;
+				}
+            }
+        }
+
+        // Store the window's current width and height:
+		SDL_GetWindowSize(window, &width, &height);
+
+		// Calculate the gravity vector:
+		// Calculate the vector between the star and ship:
+		gravityX = (width/2 - posX);
+		gravityY = (height/2 - posY);
+
+		// Make it into a unit vector:
+		gravityMagnitude = sqrt(pow(gravityX, 2) + pow(gravityY, 2));
+		gravityX /= gravityMagnitude;
+		gravityY /= gravityMagnitude;
+		
+		// Calculate the gravity between them and scale the vector:
+		if(gravityMagnitude > 15)
+		{
+			gravityAcceleration = 2 * (2500 / pow(gravityMagnitude, 2));
+			gravityX *= gravityAcceleration;
+			gravityY *= gravityAcceleration;
+		}
+		else
+		{
+			gravityAcceleration = 0.02 * (2500 / pow(gravityMagnitude, 2));
+			gravityX *= gravityAcceleration;
+			gravityY *= gravityAcceleration;
+		}
+
+		// Wrap the position if the ship goes off-screen:
+		if(posX > width + 15)
+		{
+			posX = 0;
+			velocityX *= 0.6;
+		}
+		if(posY > height + 15)
+		{
+			posY = 0;
+			velocityY *= 0.6;
+		}
+		if(posX < -15)
+		{
+			posX = width;
+			velocityX *= 0.6;
+		}
+		if(posY < -15)
+		{
+			posY = height;
+			velocityY *= 0.6;
+		}
+
+		// Calculate the new current velocity:
+		velocityX += gravityX;
+		velocityY += gravityY;
+
+		// Calculate the new position:
+		posX += velocityX;
+		posY += velocityY;
+
+		positionX = (long)posX;
+		positionY = (long)posY;
+
+		// Set the colour to black:
+		SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
+
+		// Clear the screen, filling it with black:
+		SDL_RenderClear(renderer);
+
+		// Set the colour to green:
+		SDL_SetRenderDrawColor(renderer, 0, 255, 0, 255);
+
+		// Draw a circle "ship" around the current position:
+		DrawCircle(renderer, positionX, positionY, 15);
+		
+		// Set the colour to yellow:
+		SDL_SetRenderDrawColor(renderer, 255, 255, 0, 255);
+
+		// Draw a circle in the center as the star:
+		DrawCircle(renderer, width/2, height/2, 30);
+
+		// Draw a line representing the velocity:
+		SDL_RenderDrawLine(renderer, positionX, positionY, (long)((posX + velocityX * 3)), (long)((posY + velocityY * 3)));
+		
+		// Present the rendered graphics:
+		SDL_RenderPresent(renderer);
+
+		// Delay enough so that we run at 60 frames:
+		SDL_Delay(1000 / 60);
+	}
+	return 0;
+}
+// ===========================================================================================
+// Local Variables:
+// compile-command: "gcc `sdl2-config --libs --cflags` SDL2-Experiment-05.c  -lm"
+// End: