SDL2.0でアルファブレンディング

コンパイル時に-DSAMPLE_WITH_ALPHAを指定するかしないかでアルファブレ ンディングの有り無しを切り替えます。

#ifdef SAMPLE_WITH_ALPHA
    SDL_SetTextureAlphaMod(sprite, SAMPLE_ALPHA);
#endif

以下のようなMakefileを用意し、alpha.cppに対して、-DSAMPLE_WITH_ALPHA の指定がないバイナリとあるバイナリを作成します。

TARGET := $(patsubst %.cpp,%,$(wildcard *.cpp))
UNAME := $(shell uname)

ifeq ($(UNAME),Linux)
CXXFLAGS += $(shell sdl2-config --cflags)
LDLIBS   += $(shell sdl2-config --libs) -lSDL2_ttf -lSDL2_image
else
CXXFLAGS += -framework SDL2 -framework SDL2_ttf -framework SDL2_image
CXXFLAGS += -I/Library/Frameworks/SDL2.framework/Headers/
CXXFLAGS += -I/Library/Frameworks/SDL2_image.framework/Headers/
CXXFLAGS += -I/Library/Frameworks/SDL2_ttf.framework/Headers/
endif

CXXFLAGS += -std=c++0x $(EXTRA_CXXFLAGS)

alpha:
  $(CXX) $(CXXFLAGS) alpha.cpp -o without-alpha $(LDLIBS)
  $(CXX) $(CXXFLAGS) alpha.cpp -o with-alpha \
    -DSAMPLE_WITH_ALPHA $(LDLIBS)

all: $(TARGET)

clean:
  rm -rf $(TARGET)

alpha.cppの内容は以下の通りです。

#include <iostream>
#include <sys/time.h>
#include <SDL.h>
#include <SDL_ttf.h>
#include <SDL_image.h>

#define SAMPLE_SPRITE "sample.png"
#define SAMPLE_REPEAT_TIME (1024)
#define SAMPLE_ALPHA (128)

static SDL_Window *gWindow;
static int gWindowWidth;
static int gWindowHeight;

static SDL_Renderer *gRenderer;

static bool initialize(int width, int height)
{
  if (SDL_Init(SDL_INIT_EVERYTHING) < 0)
    return false;

  if (SDL_CreateWindowAndRenderer(width, height, 0,
                                  &gWindow, &gRenderer) < 0)
    goto err;

  SDL_GetWindowSize(gWindow, &gWindowWidth, &gWindowHeight);
  return true;

 err:
  SDL_Quit();
  return false;
}

static void finalize()
{
  SDL_DestroyRenderer(gRenderer);
  SDL_DestroyWindow(gWindow);
  SDL_Quit();
}

SDL_Texture *createSprite(const char *spriteName)
{
  SDL_RWops *rwops;
  SDL_Surface *surface;
  SDL_Texture *texture;

  rwops = SDL_RWFromFile(spriteName, "rb");
  if (rwops == nullptr)
    return nullptr;

  surface = IMG_LoadPNG_RW(rwops);
  if (surface == nullptr)
    goto err;

  texture = SDL_CreateTextureFromSurface(gRenderer, surface);
  SDL_FreeSurface(surface);
 err:
  SDL_RWclose(rwops);
  return texture;
}

unsigned long getCurrentUsec()
{
  struct timeval timeval;
  gettimeofday(&timeval, NULL);
  return timeval.tv_sec * 1e6 + timeval.tv_usec;
}

int main()
{
  SDL_Texture *sprite;
  int ret = 1;
  unsigned long start, end;
  double fps;

  if (!initialize(400, 400)) {
    std::cerr << "Initialize error" << std::endl;
    return 1;
  }

  sprite = createSprite(SAMPLE_SPRITE);
  if (sprite == nullptr) {
    std::cerr << "Sprite error" << std::endl;
    goto err;
  }

  start = getCurrentUsec();

  for (size_t time = 0; time < SAMPLE_REPEAT_TIME; ++time) {
#ifdef SAMPLE_WITH_ALPHA
    SDL_SetTextureAlphaMod(sprite, SAMPLE_ALPHA);
#endif
    SDL_RenderClear(gRenderer);
    SDL_RenderCopy(gRenderer, sprite, nullptr, nullptr);
    SDL_RenderPresent(gRenderer);
  }

  end = getCurrentUsec();
  fps = 1e6 * SAMPLE_REPEAT_TIME / (end - start);
  std::cout << fps << std::endl;
  ret = 0;

  SDL_DestroyTexture(sprite);
 err:
  finalize();
  return ret;
}