如果您想要透明图像，SDL2中有两种方法：
1.（静态法）
使用图像编辑软件，直接改变那里的alpha值，它将继续到SDL2。
1.（动态方法）

// This sets the texture in blendmode.
SDL_SetTextureBlendMode(texture, SDL_BLENDMODE_BLEND);

// This section should be where you alter the alpha. 
// You can make fade in-fade out effects, etc... Just put the changes here.
Uint8 alpha = 0x7F; 

// Sets the alpha into the texture.
SDL_SetTextureAlphaMod(texture, alpha); 

// Redraws the image with a fresh, new alpha.
SDL_RenderCopy(renderer, texture, NULL, &rect);