Ipseity-Demo/Assets/_Content/Shaders/EnkiBilal/Include/Mask.hlsl

#ifndef MASK_INCLUDED
#define MASK_INCLUDED

/*
    MASK - MEDIUM COMPATIBLE VERSION

    Goal:
    Keep the original portal / projection behavior while avoiding unnecessary work
    in cases where the mask is not supposed to affect the object.

    Important compatibility point:
    In the original shader, when all three masks are active, MaskID 0 is affected:
        Alpha = (MaskID <= 1) ? 1 - mask : mask;

    The previous Medium version incorrectly skipped MaskID 0 entirely.
    That could break portal logic.

    Changes from the original version:
    - Preserves the original Alpha logic.
    - Keeps rounded rectangle SDF to avoid changing portal shape.
    - Adds safer early returns only when they do not change original behavior.
    - Samples only active masks outside the "all three active" special case.
*/

float ProjectionMask(
    float3 PositionWS,
    float3 CamPositionWS,
    float3 RectPositionWS,
    float3 RectNormalWS,
    float3 RectUpWS,
    float2 RectSize,
    float Oval
)
{
    /*
        Build the local projection frame.
    */
    float3 N = normalize(RectNormalWS);
    float3 U = normalize(RectUpWS);
    float3 R = normalize(cross(N, U));

    /*
        Recompute U to guarantee an orthogonal frame.
    */
    U = normalize(cross(R, N));

    /*
        Ray from camera to fragment.
    */
    float3 camToFrag = PositionWS - CamPositionWS;
    float distToFrag = length(camToFrag);

    if (distToFrag <= 1e-5)
        return 0.0;

    float3 rayDir = camToFrag / distToFrag;

    /*
        Ray-plane intersection.
    */
    float denom = dot(rayDir, N);

    if (abs(denom) < 1e-6)
        return 0.0;

    float t = dot(RectPositionWS - CamPositionWS, N) / denom;

    /*
        Reject intersections behind the camera or behind the current fragment.
    */
    if (t < 0.0 || t > distToFrag + 1e-4)
        return 0.0;

    /*
        Compute hit point on the projection plane.
    */
    float3 hitPoint = CamPositionWS + rayDir * t;

    /*
        Convert hit point to rectangle local coordinates.
    */
    float3 localVec = hitPoint - RectPositionWS;
    float localU = dot(localVec, R);
    float localV = dot(localVec, U);

    float halfW = RectSize.x * 0.5;
    float halfH = RectSize.y * 0.5;

    if (halfW <= 1e-5 || halfH <= 1e-5)
        return 0.0;

    /*
        Original rounded rectangle SDF preserved.

        The previous Medium version replaced this with a hard rectangle / ellipse.
        That was cheaper, but it could change the portal shape.
    */
    float radius = saturate(Oval) * min(halfW, halfH);

    float2 uv = float2(localU, localV);
    float2 ext = float2(halfW, halfH);
    float2 q = abs(uv) - ext + radius;

    float sdf = length(max(q, 0.0)) - radius;

    return sdf <= 0.0 ? 1.0 : 0.0;
}

float SampleProjectionMask(float3 PositionWS, float4x4 M)
{
    /*
        M[0].w is used as the active flag.
    */
    if (M[0].w <= 0.5)
        return 0.0;

    return ProjectionMask(
        PositionWS,
        M[0].xyz,               // Camera world position
        M[1].xyz,               // Rectangle world position
        M[3].xyz,               // Rectangle normal in world space
        M[2].xyz,               // Rectangle up vector in world space
        float2(M[2].w, M[3].w), // Rectangle size
        M[1].w                  // Rounded shape amount
    );
}

void Mask_float
(
    float3 PositionWS,
    float MaskID,
    float4x4 Mask0,
    float4x4 Mask1,
    float4x4 Mask2,
    out float Alpha
)
{
    float active0 = Mask0[0].w > 0.5 ? 1.0 : 0.0;
    float active1 = Mask1[0].w > 0.5 ? 1.0 : 0.0;
    float active2 = Mask2[0].w > 0.5 ? 1.0 : 0.0;

    float activeCount = active0 + active1 + active2;

    /*
        No active mask case.

        Preserves original behavior:
        - MaskID 1 is hidden.
        - Other IDs remain visible.
    */
    if (activeCount <= 0.0)
    {
        Alpha = (MaskID == 1) ? 0.0 : 1.0;
        return;
    }

    /*
        Original special case:
        when all three masks are active, MaskID 0 is also affected by the mask.

        This is probably what your portal depends on.
    */
    if (active0 > 0.5 && active1 > 0.5 && active2 > 0.5)
    {
        float mask = 0.0;

        mask += SampleProjectionMask(PositionWS, Mask0);
        mask += SampleProjectionMask(PositionWS, Mask1);
        mask += SampleProjectionMask(PositionWS, Mask2);

        mask = saturate(mask);

        Alpha = (MaskID <= 1.0) ? (1.0 - mask) : mask;
        return;
    }

    /*
        Safe optimization:
        In the original shader, when not all three masks are active,
        MaskID 0 is always fully visible.

        So only here it is safe to skip all projection work for MaskID 0.
    */
    if (MaskID == 0)
    {
        Alpha = 1.0;
        return;
    }

    /*
        Partial active mask case.

        Only sample masks that are active.
    */
    float maskPartial = 0.0;

    if (active0 > 0.5)
        maskPartial += SampleProjectionMask(PositionWS, Mask0);

    if (active1 > 0.5)
        maskPartial += SampleProjectionMask(PositionWS, Mask1);

    if (active2 > 0.5)
        maskPartial += SampleProjectionMask(PositionWS, Mask2);

    maskPartial = saturate(maskPartial);

    /*
        Original convention preserved:
        - MaskID 1 is visible outside the mask.
        - MaskID > 1 is visible inside the mask.
    */
    Alpha = (MaskID == 1.0) ? (1.0 - maskPartial) : maskPartial;
}

#endif