Implementació d'un sistema per al control de la qualitat d'imatges JPEG

L'estàndard JPEG és el format gràfic per a imatges fixes de to continu que s'utilitza com a mitjà de publicació en la web. La bona relació entre la raó de compressió obtinguda i la qualitat de la imatge comprimida i l'àmplia difusió entre els usuaris d'Internet, l'han convertit en l'estàndard per a la compressió d'imatges. L'usuari, però, només pot controlar la qualitat final pel mètode d'assaig-error i aquest procés té un cost computacional elevat ja que la imatge s'ha de comprimir, descomprimir i després calcular l'error quadràtic mitjà. Aquest treball proporciona una llibreria de rutines bàsiques que permet modificar la matriu de quantificació en funció de les característiques de la imatge, de la percepció de l'observador i del nivell de qualitat desitjat per l'usuari, sense penalitzar la raó de compressió i amb costos computacionals raonables.El estándar JPEG es el formato gráfico para imágenes fijas de tono continuo que se utiliza como medio de publicación en la web. La buena relación entre la razón de compresión obtenida y la calidad de la imagen comprimida y la amplia difusión entre los usuarios de Internet, lo han convertido en el estándar para la compresión de imágenes. Sin embargo, el usuario solo puede controlar la calidad final por el método de ensayo-error y este proceso tiene un coste computacional elevado puesto que la imagen se tiene que comprimir, descomprimir y después calcular el error cuadrático medio. Este trabajo proporciona una librería de rutinas básicas que permite modificar la matriz de cuantificación en función de las características de la imagen, de la percepción del observador y del nivel de calidad deseado por el usuario, sin penalizar la razón de compresión y con costes computacionales razonables.The standard JPEG is the graphic format for fixed footage of continual key that utilises as a mode of publication at the web. The nice account among the reason of compression obtained and the quality of the image compressed and the wide diffusion among the users of Internet, have converted it at the standard for the compression of footage. The user, but, only can control the final quality for the method of assay-error and this process has an overhead computational cost since the image has to compress , decompress and next compound the half quadratic error. This work provides a bookshop of basic routines that permits to modify the matrix of quantization at function of the characteristics of the image, of the perception of the observer and of the level of quality desired by the user, without penalising the reason of compression and with reasonable computational costs

JPEG standard uniform quantization error modeling with applications to sequential and progressive operation modes

In this paper we propose a method for computing JPEG quantization matrices for a given mean square error or PSNR. Then, we employ our method to compute JPEG standard progressive operation mode definition scripts using a quantization approach. Therefore, it is no longer necessary to use a trial and error procedure to obtain a desired PSNR and/or definition script, reducing cost. Firstly, we establish a relationship between a Laplacian source and its uniform quantization error. We apply this model to the coefficients obtained in the discrete cosine transform stage of the JPEG standard. Then, an image may be compressed using the JPEG standard under a global MSE (or PSNR) constraint and a set of local constraints determined by the JPEG standard and visual criteria. Secondly, we study the JPEG standard progressive operation mode from a quantization based approach. A relationship between the measured image quality at a given stage of the coding process and a quantization matrix is found. Thus, the definition script construction problem can be reduced to a quantization problem. Simulations show that our method generates better quantization matrices than the classical method based on scaling the JPEG default quantization matrix. The estimation of PSNR has usually an error smaller than 1 dB. This figure decreases for high PSNR values. Definition scripts may be generated avoiding an excessive number of stages and removing small stages that do not contribute during the decoding process with a noticeable image quality improvement.En aquest treball es proposa un mètode per calcular les matrius de quantització JPEG per a un determinat error mitjà quadrat o PSNR.En este trabajo se propone un método para calcular las matrices de cuantización JPEG para un determinado error medio cuadrado o PSNR

JPEG standard uniform quantization error modeling with applications to sequential and progressive operation modes

In this paper we propose a method for computing JPEG quantization matrices for a given mean square error or PSNR. Then, we employ our method to compute JPEG standard progressive operation mode definition scripts using a quantization approach. Therefore, it is no longer necessary to use a trial and error procedure to obtain a desired PSNR and/or definition script, reducing cost. Firstly, we establish a relationship between a Laplacian source and its uniform quantization error. We apply this model to the coefficients obtained in the discrete cosine transform stage of the JPEG standard. Then, an image may be compressed using the JPEG standard under a global MSE (or PSNR) constraint and a set of local constraints determined by the JPEG standard and visual criteria. Secondly, we study the JPEG standard progressive operation mode from a quantization based approach. A relationship between the measured image quality at a given stage of the coding process and a quantization matrix is found. Thus, the definition script construction problem can be reduced to a quantization problem. Simulations show that our method generates better quantization matrices than the classical method based on scaling the JPEG default quantization matrix. The estimation of PSNR has usually an error smaller than 1 dB. This figure decreases for high PSNR values. Definition scripts may be generated avoiding an excessive number of stages and removing small stages that do not contribute during the decoding process with a noticeable image quality improvement.En aquest treball es proposa un mètode per calcular les matrius de quantització JPEG per a un determinat error mitjà quadrat o PSNR.En este trabajo se propone un método para calcular las matrices de cuantización JPEG para un determinado error medio cuadrado o PSNR