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Automated detection of filaments on full disk H images

Nicolas Fuller and Jean Aboudarham

Meudon Observatory / LESIA

October 2003

EGSO WP5

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Automated Detection of Filaments / NF & JA 2003

Original imageOriginal image

StandardizationStandardization

CleaningCleaning

Seeds detectionSeeds detection

Region growingRegion growing

Shape descriptionShape description

Catalogue parametersCatalogue parameters

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Cleaning Process

Darkening Dust lines

Automated Detection of Filaments / NF & JA 2003

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Cleaning process / Intensity normalization 1

• Need to compute the slow variations of the background• Use of a large median filter on a resized image• The first approximation is influenced by large bright plages and large filaments

1. Resize the image I to a smaller scale -> Is2. Apply a median filter with a large window to Is3. Resize to original scale (-> B) and subtract from I -> I’4. From I’define 2 thresholds to roughly locate filaments and bright plages5. Replace their value with the corresponding values in B -> I”6. Apply step 1 to 3 to I” and get the final background and the normalize image

Automated Detection of Filaments / NF & JA 2003

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Cleaning process / Intensity normalization 2

- =

- =

Automated Detection of Filaments / NF & JA 2003

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Cleaning process / Dust lines removal 1

Need to compute a binary image with most of theNeed to compute a binary image with most of theline points set to 1 and the background to 0 :line points set to 1 and the background to 0 :• ThresholdThreshold• Thinning morphological operatorThinning morphological operator

Automated Detection of Filaments / NF & JA 2003

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Cleaning process / Dust lines removal 2

Original Threshold Thinning

Hough transform

Houghbackprojection

Threshold

Line pixelslocations

Pixels valuescorrection

Automated Detection of Filaments / NF & JA 2003

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Automated Detection of Filaments / NF & JA 2003

Image enhancement

To enhance the image sharpness we use a Laplacian filter• Filaments contours are better defined• Allows to detect the thinnest parts of the filaments more efficiently

g(x,y) = f(x,y) –2f(x,y) where 2f = 2f/x2 + 2f/y2

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Digital implementation:

Before and after enhancement

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Automated Detection of Filaments / NF & JA 2003

Region Growing

Definition: “procedure that groups pixels into larger regions based on predefined criteria. It starts with a set of ‘seed’ points and from these grow regions by appending to each seed those neighboring pixels that have properties similar to the seed”

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Automated Detection of Filaments / NF & JA 2003

Region Growing / Seeds detection

To find the seed points we apply a windowed threshold:The pixels statistics in each window (200*200) are computed and the

threshold is given by: Twin = Mwin – x win

M : Mean

: constant

: standard deviation

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Automated Detection of Filaments / NF & JA 2003

Region Growing

For each seed we define an intensity range which is a criteria to append connected pixels to the seed: [ 0, Tbr ]

where Tbr = Mbr – x br

( br stands for Bounding Rectangle )A minimum region size is also defined

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Automated Detection of Filaments / NF & JA 2003

Region growing / BBSO

The process has been tested on other H full disk observations : Big Bear Solar Observatory example

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Automated Detection of Filaments / NF & JA 2003

Shape analysis / Morphological operators

Morphological closing

Morphological thinning / pruning

Skeleton Length/centre/Chain Code…

Chain codedirection numbers

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Automated Detection of Filaments / NF & JA 2003

Parameters : examples

GRAV_C_CAR_LAT DOUBLE -18.297280 GRAV_C_CAR_LON DOUBLE 337.64961 BRPIX_X_LL DOUBLE 562.00000 BRPIX_Y_LL DOUBLE 418.00000 BRPIX_X_UR DOUBLE 574.00000 BRPIX_Y_UR DOUBLE 430.00000 SAMPLECOUNT LONG 58 AREA DOUBLE 2.8334533 SKE_LEN_DEG DOUBLE 2.254509 ELONG DOUBLE 0.90625000 MEAN_INT_RATIO DOUBLE 0.83272228 FEAT_MAX_INT DOUBLE 978.00000 FEAT_MIN_INT DOUBLE 689.00000 FEAT_MEAN_INT DOUBLE 869.36206 ENC_MET STRING 'CHAINCODE' COD_PIX_X DOUBLE 572.00000 COD_PIX_Y DOUBLE 417.00000 COD_SKE_PIX_X DOUBLE 574.00000 COD_SKE_PIX_Y DOUBLE 418.00000 SKE_CHAIN STRING '33332233334343' BND_CHAIN STRING '00123322222234433443444566707777677076'

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Automated Detection of Filaments / NF & JA 2003