Abstract
Evaluation of a protocol for automated extraction of morphometric measurements from avian eggs using digital photography As many ecomorphological studies are limited by the time required to gather manual measurement data, automatizing the process is an important focus of methodological innovations. We developed, implemented and validated a protocol for the semi–automated extraction of a set of morphometric variables of egg size and shape from digital pictures. The protocol was implemented in R language as a web app called OvometriK. After binarizing and calibrating images, this protocol uses geometric and trigonometric functions to calculate eleven egg variables. We tested calculations in several ways, assuming contour continuity or using voxel counts. Application was validated with geometric shapes and 30 manually–measured chicken eggs. Mathematical validation with spheres showed that the algorithm provided high precision diameter measures, with a correlation of 99.9 %. Average estimation error was 1.4 %. The mathematical volume estimation was underestimated by 27 %, while voxels were underestimated by only 6 %. Differences between manual egg measurements of diameters and those obtained from images was less than 3 mm (4 %). Correlation between estimated volume and measured by silica gel filling was higher than 90 % using the voxel count method. Neither inclination angle or picture resolution had significant effects on precision (3.2 % maximum difference). Measures showed high repeatability and represent a significant saving in processing time. This new protocol represents an improvement on previous programs regarding limitations of platform, accessibility and number of variables. Furthermore, its flexibility and openness means it can be adapted to other specific applications.
Highlights
Evaluation of a protocol for automated extraction of morphometric measurements from avian eggs using digital photography
We developed, implemented and validated a protocol for the semi–automated extraction of a set of morphometric variables of egg size and shape from digital pictures
A simple tool for calculating egg shape, volume and surface area from digital images
Summary
El protocolo de medición a partir de fotografías se basa en la extracción de las coordenadas del contorno del huevo, a partir de la modificación del algoritmo propuesto por Claude (2008), luego de binarizar la imagen y transformar las unidades de píxeles a milímetros por medio de una escala de referencia que debe aparecer en las fotografías. Para hacer una estimación más precisa de la superficie del huevo, se supuso que cada sección de 1 píxel de grosor era la proyección de un semicono cuyo diámetro mayor era igual al de la sección anterior (como sugieren Paganelli et al, 1974). El centro del huevo coincide con el punto medio del eje del diámetro mayor, y el centroide se calcula como la media de las coordenadas de todos los puntos que forman el contorno (que puede presentar irregularidades locales por defectos de la imagen). El número de armónicos se puede variar manualmente, pero por defecto se emplean cuatro, que según Denis (2014) son suficientes para describir las formas más frecuentes de los huevos de las aves. Dirigido a comprobar la efectividad de los algoritmos matemáticos de extracción de las variables, se realizó
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