RetroArch/gfx/drivers/rsx_shaders/modern_alpha_blend.fcg

void main
(
  uniform sampler2D texture,
  uniform float4 bgcolor,
  float2 texcoord : TEXCOORD0,
  float4 color : COLOR,
  out float4 oColor : COLOR
)
{
  if (bgcolor.a > 0.0)
    oColor = bgcolor;
  else
    oColor = tex2D(texture, texcoord) * color;
}