Blend

In the fixed-function pipeline, we use blending to combine the color of a fragment (pixel) with the color already in the framebuffer.

For RGB channel:

finalColor_RGB = (sourceColor_RGB * sourceColorFactor) + (destColor_RGB * destColorFactor)

• sourceColor_RGB: the color output of the fragment
• destColor_RGB: the existing color already stored in the framebuffer
• sourceColorFactor and destColorFactor: values that control how much each color contributes to the final result

For alpha channel:

finalAlpha = (sourceAlpha * sourceAlphaFactor) + (destAlpha * destAlphaFactor)

• sourceAlpha is the alpha component of the source color
• destAlpha is the alpha component of the destination color
• sourceAlphaFactor and destAlphaFactor: similar to factors like sourceColorFactor and destColorFactor, but we usually use GL_ONE or GL_ZERO here

You can do

// usually tweaking sourceColorFactor and destColorFactor is enough
GlStateManager.blendFunc(
        sourceColorFactor, 
        destColorFactor);

which is basically playing with sourceColorFactor and destColorFactor.

Use the code below if you want to set sourceAlphaFactor and destAlphaFactor altogether.

GlStateManager.tryBlendFuncSeparate(
        sourceColorFactor, 
        destColorFactor, 
        sourceAlphaFactor, 
        destAlphaFactor);

Here are some common values you’ll use for sourceFactor and destFactor:

Constant	Meaning
GL_ZERO	0.0
GL_ONE	1.0
GL_SRC_ALPHA	Source fragment’s alpha value
GL_ONE_MINUS_SRC_ALPHA	1.0 - Source fragment’s alpha value
GL_DST_ALPHA	Destination alpha value
GL_ONE_MINUS_DST_ALPHA	1.0 - Destination alpha value

Here's a common blending example:

GlStateManager.blendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);

Don't forget to restore states if needed, but be aware of the trade-offs.

boolean blend = GL11.glIsEnabled(GL11.GL_BLEND);
GL11.glGetInteger(GL14.GL_BLEND_SRC_RGB, intBuffer);
int blendSrcRgb = intBuffer.get(0);
GL11.glGetInteger(GL14.GL_BLEND_DST_RGB, intBuffer);
int blendDstRgb = intBuffer.get(0);
GL11.glGetInteger(GL14.GL_BLEND_SRC_ALPHA, intBuffer);
int blendSrcAlpha = intBuffer.get(0);
GL11.glGetInteger(GL14.GL_BLEND_DST_ALPHA, intBuffer);
int blendDstAlpha = intBuffer.get(0);

// render

if (blend)
    GlStateManager.enableBlend();
else
    GlStateManager.disableBlend();
GlStateManager.tryBlendFuncSeparate(blendSrcRgb, blendDstRgb, blendSrcAlpha, blendDstAlpha);

Contributors

Nullpinter

Last edited about 1 hour ago

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