majordomo/imagegen/faceswap.go

package imagegen

import "context"

// FaceSwapRequest transfers an identity from Source into Target.
//
// This is a DIFFERENT OPERATION from Edit, not a better-tuned one. Measured
// against the instruction-edit models on 2026-07-31, asking a diffusion model
// to put a specific person's face into a photo does not work by any route —
// by name, by description, or by supplying the portrait as a reference image.
// Face swapping is a dedicated detect/align/blend pipeline; a provider that
// cannot do it should not pretend Edit is a substitute.
type FaceSwapRequest struct {
	// Target is the photo to edit — the pose, expression, lighting and
	// everything outside the face are preserved from it.
	Target Image

	// Source is a photo of the face to put in. Only the identity travels;
	// the source's own pose and expression do not.
	Source Image

	// Index selects WHICH face in Target, in the provider's documented
	// ordering (llamaswap: left to right by box centre, as reported by
	// ListFaces). nil = the largest face, which is right for a portrait and
	// wrong for a group — enumerate first when it matters.
	Index *int

	// All swaps every detected face and ignores Index.
	All bool
}

// FaceSwapOption mutates a FaceSwapRequest before it is sent.
type FaceSwapOption func(*FaceSwapRequest)

// WithFaceIndex selects which face in the target to replace.
func WithFaceIndex(i int) FaceSwapOption { return func(r *FaceSwapRequest) { r.Index = &i } }

// WithAllFaces swaps every detected face.
func WithAllFaces() FaceSwapOption { return func(r *FaceSwapRequest) { r.All = true } }

// Apply returns a copy of the request with all options applied.
func (r FaceSwapRequest) Apply(opts ...FaceSwapOption) FaceSwapRequest {
	for _, opt := range opts {
		opt(&r)
	}
	return r
}

// DetectedFace is one face located in an image, in PIXEL coordinates.
type DetectedFace struct {
	// Index is the face's position in the provider's stable ordering, and
	// the value FaceSwapRequest.Index expects.
	Index int
	// Box is [x0, y0, x1, y1].
	Box [4]int
	// Score is the detector's confidence, 0-1.
	Score float64
}

// Size returns the box dimensions. Derived rather than stored: carrying
// width/height alongside Box is two sources of truth for one fact, and the
// pair can disagree after any transform.
func (f DetectedFace) Size() (w, h int) {
	return f.Box[2] - f.Box[0], f.Box[3] - f.Box[1]
}

// SwappedFace is the MEASURED outcome for one face the provider replaced.
//
// It exists because "the call returned an image" and "the likeness
// transferred" are different claims that look identical from outside, and a
// caller that cannot tell them apart will go looking for another way to
// check. The one it reaches for — asking a vision model who the result looks
// like — is wrong in exactly the cases that matter: a VLM shown a jogger in a
// Georgetown cap holding McDonald's cups answers "Bill Clinton" whoever's
// face is on him, so it reports failure on a correct swap.
type SwappedFace struct {
	// Index is the face's position in the provider's left-to-right ordering.
	Index int

	// Width, Height are the replaced face's pixel size in the TARGET.
	// Meaningful only against ImageWidth/ImageHeight: a 138px face is large
	// in a 400px picture and nearly invisible in a 2000px one, and it is the
	// ratio, not the absolute size, that decides whether a person notices.
	Width, Height int

	// ImageWidth, ImageHeight are the target image's dimensions, repeated on
	// every entry so a single face is self-describing without the caller
	// holding onto the rest of the response.
	ImageWidth, ImageHeight int

	// Yaw is how far the head is turned from camera, in degrees, or nil when
	// the provider does not report pose. The best single predictor of whether
	// a swap will READ as the source person: past roughly ±45° the features
	// carrying identity are edge-on and the result looks like a generic
	// person rather than a specific one.
	Yaw *float64

	// IdentitySimilarity is cosine similarity between the source face and the
	// face actually present in the result, 0-1, or nil when the provider
	// could not measure it. Above ~0.5 the identity transferred; a LOW value
	// is the only evidence that a swap genuinely failed.
	IdentitySimilarity *float64
}

// FractionOfImage is the swapped face's width as a share of the image's, 0-1.
// The number that predicts whether a person will SEE the change: the swap
// that prompted all this replaced a 138px face in a 1010px-wide photo — 14%,
// correct by every measure and invisible at a glance — while the same code on
// a 168px face in a 385px picture (44%) is unmistakable. Returns 0 when the
// dimensions are unknown.
func (f SwappedFace) FractionOfImage() float64 {
	if f.ImageWidth <= 0 || f.Width <= 0 {
		return 0
	}
	return float64(f.Width) / float64(f.ImageWidth)
}

// FaceSwapper is the optional face-transfer surface. Separate interface so
// existing providers keep compiling; callers type-assert.
type FaceSwapper interface {
	// ListFaces enumerates the faces in an image, in the SAME ordering
	// FaceSwapRequest.Index uses. Exposed because a caller asked to change
	// "the man on the right" needs a way to name one face and to check its
	// own choice against pixel boxes.
	ListFaces(ctx context.Context, img Image) ([]DetectedFace, error)

	// FaceSwap transfers Source's identity into Target.
	FaceSwap(ctx context.Context, req FaceSwapRequest, opts ...FaceSwapOption) (*Result, error)
}