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Analysis of angular reading distortions of photographic images Análise das distor es em leituras angulares de imagens fotográficas

Keywords: fotogrametria , análise de imagem assistida por computador , instrumenta o , estudos de valida o , photogrammetry , image processing computer-assisted , instrumentation , validation studies

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Abstract:

BACKGROUND: Although photogrammetry is a widespread technique in the health field, despite of the methodological efforts distortions in the angular readings of the images are common. OBJECTIVE: To measure the error of angular measurements in photo images with different digital resolutions in an object with pre-determined angles. METHODS: We used a rubber ball with 52 cm in circumference. The object was previously marked with angles of 10°, 30°, 60° and 90° degrees. The photographic records were performed with the focal axis of the camera perpendicular and three meters away from the object, without the use of optical zoom and a resolution of 3, 5 and 10 Megapixels (Mp). All photographic records were stored and a previously trained experimenter using the computer program ImageJ analyzed the angular values of each photo. The measurements were performed twice within a fifteen-days interval. Subsequently, we calculated the accuracy, relative error and error in degrees values, precision and the Intraclass Correlation Coefficient (ICC). RESULTS: When analyzing the angle of 10°, the average accuracy of measurements was higher for those records of 3 Mp resolution compared to 5 and 10 Mp resolutions. The ICC was considered excellent for all resolutions. With regards to the analyzed angles in photographic records, it was possible to verify that the 90-degree angle photographs were more accurate, had lower relative error and error in degrees, and were more precise, regardless of image resolution. CONCLUSION: The photographs records that were taken with a 3 Mp resolution provided great accuracy and precision measurements and lower errors values, suggesting to be the proper resolution to generate image of angles of 10o and 30o. CONTEXTUALIZA O: A biofotogrametria é uma técnica difundida na área da saúde e, apesar dos cuidados metodológicos, há distor es nas leituras angulares das imagens fotográficas. OBJETIVO: Mensurar o erro das medidas angulares em imagens fotográficas com diferentes resolu es digitais em um objeto com angulos pré-demarcados. MéTODOS: Utilizou-se uma esfera de borracha com 52 cm de circunferência. O objeto foi previamente demarcado com angulos de 10o, 30o, 60o e 90o, e os registros fotográficos foram realizados com o eixo focal da camera a três metros de distancia e perpendicular ao objeto, sem utiliza o de zoom óptico e com resolu o de 3, 5 e 10 Megapixels (Mp). Todos os registros fotográficos foram armazenados, e os valores angulares foram analisados por um experimentador previamente treinado, utilizando o pro grama ImageJ. As aferi es das

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