cocos-creator-template/assets/chunks/common/lighting/light-map.chunk

// for legacy effects
void SampleAndDecodeLightMapColor(out vec3 lightmapColor, out float dirShadow, out float ao, sampler2D lightingMap, vec2 luv, float lum, vec3 worldNormal)
{
	// has radiant basis
#if CC_LIGHT_MAP_VERSION > 2
	// has high precision data
#elif CC_LIGHT_MAP_VERSION > 1
	vec4 dataLow = texture(lightingMap, luv);
	vec4 dataHigh = texture(lightingMap, luv + vec2(0.5, 0.0));
	lightmapColor.xyz = dataLow.xyz + dataHigh.xyz * 0.00392156862745098;
    lightmapColor.rgb *= lum;
	dirShadow = dataLow.a;
	ao = dataHigh.a;
#else
    vec4 lightmap = texture(lightingMap, luv);
    lightmapColor = lightmap.rgb * lum;
	dirShadow = lightmap.a;
	ao = 1.0;
#endif
}

// for surface shader
void GetLightMapColor(out vec3 lightmapColor, out float dirShadow, out float ao, sampler2D lightingMap, vec2 luv, float lum, vec3 worldNormal)
{
	vec4 lightmap;
	vec2 occlusion;
	SampleAndDecodeLightMapColor(lightmapColor, dirShadow, ao, lightingMap, luv, lum, worldNormal);

#if CC_USE_HDR
    // convert from standard camera exposure parameters to current exposure value
    // baked in LDR scene still regarded as exposured with standard camera parameters
    lightmapColor.rgb *= cc_exposure.w * cc_exposure.x;
#endif
}