Face Alignment by Explicit Shape Regression

Xudong Cao Yichen Wei Fang Wen Jian Sun

Microsoft Research Asia

OutlineIntroductionFace Alignment by Shape RegressionImplementation detailsExperimentsDiscussion and Conclusion

IntroductionA face shape S = [x1, y1, ..., x, y ]T consists

of Nfp facial landmarks. Given a face image, the goal of face

alignment is to estimate a shape S that is as close as possible to the true shape

Introductionmost alignment approaches can be

classified into two categoriesoptimization-based

minimize another error function that is correlated to instead

regression-based learn a regression function that directly maps

image appearance to the target output

Introductionthe shape constraint is essential in all

methodsMost previous works use a parametric shape

model to enforce such a constraint

a novel regression-based approach without using any parametric shape models the regressor realizes the shape constraint in an

non-parametric manner

Introductiona boosted regressor to progressively

infer the shape the early regressors handle large shape

variations and guarantee robustness the later regressors handle small shape

variations and ensure accuracy

two-level boosted regressionshape-indexed featurescorrelation-based feature selection method

Face Alignment by Shape Regressionuse boosted regression to combine T weak

regressors (R1, ...Rt, ...,RT ) in an additive manner

Given a facial image I and an initial face shape S0

each regressor computes a shape increment δS from image

Face Alignment by Shape Regression

where the tth weak regressor Rt updates the previous shape St−1 to the new shape St

Given N training examples , the regressors (R1, ...Rt , ...,RT) are sequentially learnt until the training error no longer decreases

Where is the estimated shape in previous stage

Face Alignment by Shape RegressionTwo-level cascaded regression learn each weak regressor Rt by a second

level boosted regression

the shape-indexed image features are fixed in the second level

they are indexed only relative to St−1 and no longer change when those r’s are learnt

Face Alignment by Shape RegressionPrimitive regressoruse a fern as primitive regressor r

δSb : regression output

Ωb : alignment error

Si : the estimated shape in the previous step

Face Alignment by Shape RegressionPrimitive regressor (Cont.)

The solution for above is the mean of shape differences

To overcome over-fitting in the case of insufficient training data in the bin, a shrinkage is performed as

where β is a free shrinkage parameter

Face Alignment by Shape RegressionNon-parametric shape constraint the final regressed shape S can be expressed as

the initial shape S0 plus the linear combination of all training shapes

as long as the initial shape S0 satisfies the shape constraint, the regressed shape is always constrained to reside in the linear subspace constructed by all training shapes

Face Alignment by Shape Regression

Face Alignment by Shape RegressionShape-indexed (image) features use simple pixel-difference features, i.e., the

intensity difference of two pixels in the imageA pixel is indexed relative to the currently

estimated shape rather than the original image coordinates

for each weak regressor Rt in the first level, randomly sample P pixels, P2 pixel-difference features are generated

Face Alignment by Shape Regressionn-best randomly generating a pool of ferns and selecting

the one with minimum regression error

Correlation-based feature selection exploit the correlation between features and the

regression target

Face Alignment by Shape RegressionCorrelation-based feature selection (Cont.)a good fern should satisfy two properties

each feature in the fern should be highly discriminative to the regression target

correlation between features should be low so they are complementary when composed

Face Alignment by Shape RegressionTo find features satisfying such properties

1. Project the regression to a random direction to produce a scalar

2. Among P2 features, select a feature with highest correlation to the scalar

3. Repeat steps 1. and 2. F times to obtain F features

4. Construct a fern by F features with random thresholds

Implementation detailsTraining data augmentationMultiple initializations in testing

Running time performance

Parameter settings

ExperimentsBioID It consists of 1,521 near frontal face images

captured in a lab environmentLFPW Its images are downloaded from internet and

contain large variationsLFW87 The images mainly come from the LFW dataset has 87 annotated landmarks,much more than

that in BioID and LFPW

ExperimentsComparison with previous workComparison to [1] on LFPW

Comparison to [12] on LFW87

[1] Localizing parts of faces using a concensus of exemplars. In CVPR, 2011[12] Face alignment via component-based discriminative search. In ECCV, 2008

ExperimentsComparison with previous work (Cont.)Comparison to previous methods on BioID

[20] Fully automatic facial feature point detection using gabor feature based boosted classifiers[5] Feature detection and tracking with constrained local models. BMVC, 2006[14] Locating facial features with an extended active shape model. ECCV, 2008[19] Facial point detection using boosted regression and graph models. In CVPR, 2010

ExperimentsAlgorithm validation and discussions Two-level cascaded regression

Shape indexed feature mean error of local index method, which is much smaller

than the mean error of global index method

Feature selection

ExperimentsAlgorithm validation and discussions (Cont.) Feature range

the distance between the pair of pixels normalized by the distance between the two pupils

Discussion and ConclusionBy jointly regressing the entire shape and

minimizing the alignment error, the shape constraint is automatically encoded. The resulting method is highly accurate, efficient, and can be used in real time applications such as face tracking

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