Texture2D<float> In_BaseMaskTex;
Texture2D<float> In_HeightTex;
Texture2D<float> RemapTex;
RWTexture2D<float> OutputTex;

float EffectStrength;
float4 TerrainDimensions;
int RemapTexRes;
float4 TextureResolution;

#define EPSILON TextureResolution.z
#define TERRAIN_ASPECT_SCALE TextureResolution.w
#define TWO_OVER_PI 0.6366197723675f

float3 gradient(uint2 id, uint2 offset) {

    uint upY = max(0, id.y - offset.y);
    uint downY = min((uint)TextureResolution.y, id.y + offset.y);
    uint rightX = max(0, id.x - offset.x);
    uint leftX = min((uint)TextureResolution.x, id.x + offset.x);

    float dzdx = ((In_HeightTex[uint2(rightX, upY)] + 2 * In_HeightTex[uint2(rightX, id.y)] + In_HeightTex[uint2(rightX, downY)]) -
        (In_HeightTex[uint2(leftX, upY)] + 2 * In_HeightTex[uint2(leftX, id.y)] + In_HeightTex[uint2(leftX, downY)])) / 8.0f * TerrainDimensions.y * TextureResolution.x / TerrainDimensions.x;
    float dzdy = ((In_HeightTex[uint2(leftX, downY)] + 2 * In_HeightTex[uint2(id.x, downY)] + In_HeightTex[uint2(rightX, downY)]) -
        (In_HeightTex[uint2(leftX, upY)] + 2 * In_HeightTex[uint2(id.x, upY)] + In_HeightTex[uint2(rightX, upY)])) / 8.0f * TerrainDimensions.y * TextureResolution.x / TerrainDimensions.z;

    float mag = length(float2(dzdx, dzdy)); // "steepness"
    return float3(dzdx / mag, dzdy / mag, mag * TERRAIN_ASPECT_SCALE);
}


#pragma kernel GradientMultiply
[numthreads(1, 1, 1)]
void GradientMultiply(uint3 id : SV_DispatchThreadID)
{
    uint2 offset = uint2(EPSILON, EPSILON);

    float3 g = gradient(id.xy, offset);

    float angle = saturate(TWO_OVER_PI * atan2(g.z, EPSILON * TextureResolution.x));

    uint remapIdx = (uint)(angle * (float)(RemapTexRes - 1));
    float remappedGradient = RemapTex[uint2(remapIdx, 0)];

    OutputTex[id.xy] = lerp(1.0f, remappedGradient, EffectStrength) * In_BaseMaskTex[id.xy];
}