add optional interframe blending with m key

src/crab/display.cr

@@ -10,6 +10,8 @@ class Display
   @last_time = Time.utc
   @seconds : Int32 = Time.utc.second
 
+  @blend : Bool = false
+
   def initialize
     @window = SDL::Window.new(window_title(59.7), WIDTH * SCALE, HEIGHT * SCALE, flags: SDL::Window::Flags::OPENGL)
     setup_gl
@@ -22,6 +24,15 @@ class Display
     update_draw_count
   end
 
+  def toggle_blending : Nil
+    if @blend
+      LibGL.disable(LibGL::BLEND)
+    else
+      LibGL.enable(LibGL::BLEND)
+    end
+    @blend = !@blend
+  end
+
   private def window_title(fps : Float) : String
     "crab - #{fps.round(1)} fps"
   end
@@ -79,6 +90,8 @@ class Display
     puts "OpenGL version: #{String.new(LibGL.get_string(LibGL::VERSION))}"
     puts "Shader language version: #{String.new(LibGL.get_string(LibGL::SHADING_LANGUAGE_VERSION))}"
 
+    LibGL.blend_func(LibGL::SRC_ALPHA, LibGL::ONE_MINUS_SRC_ALPHA)
+
     vert_shader_id = compile_shader(File.read("src/crab/shaders/gba_colors.vert"), LibGL::VERTEX_SHADER)
     frag_shader_id = compile_shader(File.read("src/crab/shaders/gba_colors.frag"), LibGL::FRAGMENT_SHADER)
 

src/crab/keypad.cr

@@ -70,6 +70,7 @@ class Keypad
       when .r?          then @keyinput.r = bit
         # Extras
       when .tab? then @gba.apu.toggle_sync if event.pressed?
+      when .m?   then @gba.display.toggle_blending if event.pressed?
       else            nil
       end
     when SDL::Event::JoyHat

src/crab/shaders/gba_colors.frag

@@ -15,5 +15,5 @@ void main() {
                              30 * color.b + 230 * color.g +  10 * color.r,
                             220 * color.b +  10 * color.g +  50 * color.r) / 255,
                        vec3(1.0 / outGamma));
-  frag_color.a = 1.0;
+  frag_color.a = 0.5;
 }