feat: trail fades out

include/sims.hpp

@@ -1,6 +1,7 @@
 #ifndef SIMS_HPP
 #define SIMS_HPP
 
+#include <SDL3/SDL_pixels.h>
 #include <SDL3/SDL_render.h>
 #include <utility>
 
@@ -10,10 +11,11 @@ private:
     float y;
     float vx = 1;
     float vy = 1;
-    std::pair<float, float> old_positions[100];
+    std::pair<float, float> old_positions[200];
 
 public:
     SDL_Renderer* renderer;
+    SDL_Color color = {255, 255, 255, 255}; // white
 
     Body(int x_pos, int y_pos);
 

src/main.cpp

@@ -62,6 +62,8 @@ int main() {
     if (renderer == NULL) {
         std::printf("SDL_CreateRenderer() failed");
     }
+    // enables alpha
+    SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
 
     SDL_Event e;
 

src/sims/two_bodies.cpp

@@ -5,6 +5,7 @@
 #include "../../include/gfx.hpp"
 #include <algorithm>
 #include <cmath>
+#include <cstdint>
 #include <iterator>
 #include <utility>
 
@@ -17,10 +18,19 @@ Body::Body(int x_pos, int y_pos) : x(x_pos), y(y_pos) {
 void Body::draw_tail() {
     size_t n = std::size(old_positions);
 
+    uint8_t initial_alpha = color.a;
+    SDL_SetRenderDrawColor(renderer, color.r, color.g, color.b, color.a);
     for (size_t i = 0; i < n; ++i) {
-        const auto& p = old_positions[i];
+        float f = 1.0f;
-        SDL_RenderPoint(renderer, p.first, p.second);
+        if (i > 100) {
+            // linear interpolation
+            f = 1.0f - float(i - 100) / float(n - 100);
         }
+        uint8_t a = uint8_t(color.a * std::clamp(f, 0.0f, 1.0f));
+        SDL_SetRenderDrawColor(renderer, color.r, color.g, color.b, a);
+        SDL_RenderPoint(renderer, old_positions[i].first, old_positions[i].second);
+    }
+
 
     for (size_t i = n - 1; i > 0; --i) {
         old_positions[i] = old_positions[i - 1];