Using Alpha for Semi-Transparency with SDL_Color in SDL2

The SDL_Color struct includes an a member, which represents the alpha channel, controlling opacity or transparency:

1// R=255, G=0, B=0, Alpha=128
2SDL_Color SemiTransparentRed{255, 0, 0, 128};

An alpha value of 255 means fully opaque, while 0 means fully transparent. Intermediate values, like 128 (roughly half opacity), suggest semi-transparency.

However, simply storing a color with an alpha value less than 255 in an SDL_Color and passing it to SDL_MapRGB() for use with SDL_FillRect() on a standard window surface (obtained via SDL_GetWindowSurface()) will not typically result in a semi-transparent rectangle being drawn onto the existing background.

Why SDL_FillRect() Doesn't Blend By Default

There are several reasons why this direct approach usually fails for standard window surfaces:

1. Surface Format: The window surface might not have an alpha channel itself, or it might not be configured for blending operations. SDL_FillRect() often performs a direct pixel overwrite based on the mapped color, ignoring any alpha information in that mapped color value for blending purposes.
2. SDL_FillRect() Behavior: This function is primarily designed for fast, opaque fills. It doesn't inherently perform alpha blending calculations (combining the source color with the destination color based on alpha).

Achieving Alpha Blending

True alpha blending in SDL generally requires one of the following:

Using SDL_Renderer and Textures:

• You load images into SDL_Texture objects.
• You can set the texture's blend mode using SDL_SetTextureBlendMode(texture, SDL_BLENDMODE_BLEND).
• You can modulate the texture's alpha using SDL_SetTextureAlphaMod(texture, alpha_value).
• When you render the texture using SDL_RenderCopy(), SDL handles the blending against the renderer's target (e.g., the window).
• You can also draw primitive shapes with alpha using renderer draw functions after setting the blend mode on the renderer itself via SDL_SetRenderDrawBlendMode().

Using SDL_Surface Blending:

• You need two surfaces: a source surface (e.g., representing your semi-transparent rectangle) and a destination surface (e.g., the window surface or another off-screen surface).
• The source surface might need to be created specifically with per-pixel alpha (e.g., using SDL_CreateRGBSurfaceWithFormat()).
• You must enable blending on the source surface using SDL_SetSurfaceBlendMode().
• You then use SDL_BlitSurface() to copy/blend the source onto the destination. SDL's blitting function can perform alpha blending if the surfaces and modes are set up correctly.

In the context of this lesson, using SDL_FillRect() directly won't achieve transparency. Proper alpha blending techniques are covered in later parts of the course.