Michael Gormish Prepared by:Emin Sinan Mimaroglu ICON BASED ERROR CONCEALMENT FOR JPEG AND JPEG 2000 IMAGES Published in the 2003 International Conference on Image Processing (ICIP 2003) Michael Gormish Prepared by:Emin Sinan Mimaroglu

Outline Introduction Jpeg error concealment Jpeg 2000 introduction Summary

Introduction Small errors can have devastating effects on compressed files for this reason, 1)Jpeg provides restart markers which cause encoding and decoding processes to be reset after some amount of coded data. 2)Jpeg 2000 encodes image portions independently to allow for random access but also several options were included to improve the ability to detect and recover from errors like :”segmentation symbols”,”termination on every coding pass ”.

Error??? Figure 1 shows several images taken with the digital camera and transferred to a computer while the camera had a low battery.

Error It is obvious that there is useful data available for the lower portion of the image although the position and the color is wrong.

Error

How the Error Occurs? In figure2 the first region appears correct. Then the “error region “begins which is solid color band. Decoder resynchronizes with the code stream and produces image in region 2. Because decoder gets the DC value of the previous block’s DC.

Error Concealment There are 3 important things to check in error concealment: 1) Determining the location. 2) Determining the correct DC values to use in the error regions. 3) Estimating values to use in the error regions.

Jpeg Error Concealment

Jpeg Error Concealment Algorithms 1)Align compressed data correctly 2)Adjust DC value of full image to match the icon’s DC value

Align Compressed Data Correctly Every 8x8 block in the error code stream begins with a DC value which has been subtracted from the previous blocks DC value and then Huffman coding the difference. These difference DC values may have more or less DC values than the number 8x8 block in the image because the errors could cause insertion or deletion of code blocks.

Align Compressed Data Correctly N is the number of sample which overlap. I(b) is the differential DC value of the bth block of. The scaled icon image. E(b-s) is the differential DC value of the error image. M(s) indicates the amount of shift caused by errors in the compressed data.

Align Compressed Data Correctly Cont. T end C(t)=m1(i)+ m2(i) I=0 i=t This estimate(c(t)) is sufficient for computing the correct DC offset to use after the error

After the Alignment…

Adjust DC Value of Full Image to Match the Icon’s DC Value Each region of the corrected image receives pixel data from a different place. The first region can be decoded normally until the first error. The region after the error is filled in by decoding and shifting the blocks by the amount determined from the align compressed data algorithm which is c(t). The absolute DC values for all color components(y,Cb,Cr) must be determined.This can be done by determining the overall average DC value for blocks in the region in both the error image and the icon image.

Adjust DC Value of Full Image to Match the Icon’s DC Value Cont. The average luminance DC value in the error image assuming the previous absolute DC value was 0 is 1 t2 Oy= ————E(b-s2) t2-t1 b=t1 Apply this equation also for Cb and Cr components by summing the differential DC values.

Adjust DC Value of Full Image to Match the Icon’s DC Value Cont. The average Luminance DC value in the icon image is 1 t2 Ay= ————DC(b-s2) t2-t1 b=t1 The corrected image should use Ay-Oy as the previous DC values and apply the differences decoded from the shifted codestream as usual.

Jpeg 2000

Jpeg 2000 Jpeg 2000 is wavelet based image compression standard. In JPEG 2000 the blocks are compressed individually without reference to the adjoining blocks. Jpeg 2000 can operate at higher compression ratios without generating ‘blocky and blurry ‘artifacts of the DCT based jpeg standard

Jpeg 2000 The JPEG 2000 standard works on image tiles . The source image is partitioned into rectangular non-overlapping blocks in a process called tiling. All operations, including component mixing, wavelet transform, quantization, and entropy coding, are performed independently on each different tile. Tiling is done to reduce memory requirements, and since each tile is reconstructed independently, they can be used to decode specific parts of the image, rather than the whole image.

Jpeg 2000

Jpeg 2000 Each tile component is then decomposed using the DWT into a series of decomposition levels which each contain a number of subbands.

Advantages of Jpeg2000 Jpeg 2000 has lots of advantages: 1) Low bit rate compression performance 2)Offers both Lossy and lossless compression 3) Random codestream access and processing, also referred as region of interest(ROI)

Jpeg 2000 Error Concealment

Determine Error Location Wavelet subbands from the presumed valid icon can be compared with corresponding subbands from the image. If the wavelet coefficients from a scaled icon differ significantly for any code block,an error can be declared for that code block.

Replace coded data with data from icon In the event of an error,data can be replaced on a code block basis with data from the scaled icon. If this is done on a compressed codestream,packet headers will have to be rewritten.

Example

Conclusion The methods presented here are particularly useful for as long as there are a few errors in the image.