There are three systematic problems that occur in nearly all segmentation tasks of color fundus images:
• Nonuniform illumination
• Poor contrast
For the attenuation of noise, we cannot propose filters that are applicable in general, because the exigency on such filters depends on the segmentation task. If big features are to be detected (e.g. the macula), strong filters can be used whereas algorithms dedicated to the detection of small details (e.g. microa-neurysms) must rely on filters that preserve even small dark details.
In this section, we present an algorithm for contrast enhancement and shade correction. First, we propose a simple global contrast enhancement operator. Applying this operator locally enhances the contrast and corrects nonuniform illumination in one step.
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