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An image gradient is a directional change in the intensity or color in an image. The gradient of the image is one of the fundamental building blocks in image processing . For example, the Canny edge detector uses image gradient for edge detection .
The gradient is obtained from an existing image and modified for image editing purposes. Various operators, such as finite difference or Sobel, can be used to find the gradient of a given image. This gradient can then be manipulated directly to produce several different effects when the resulting image is solved for.
In mathematical morphology and digital image processing, a morphological gradient is the difference between the dilation and the erosion of a given image. It is an image where each pixel value (typically non-negative) indicates the contrast intensity in the close neighborhood of that pixel.
The images below illustrate the change in the direction of the gradient on a grayscale circle. When the sign of and are the same the gradient's angle is positive, and negative when different. In the example below the red and yellow colors on the edge of the circle indicate positive angles, and the blue and cyan colors indicate negative angles.
Following the differential geometric way of expressing the requirement of non-maximum suppression proposed by Lindeberg, [4] [18] let us introduce at every image point a local coordinate system (,), with the -direction parallel to the gradient direction. Assuming that the image has been pre-smoothed by Gaussian smoothing and a scale space ...
SSIM has also been used on the gradient of images, making it "G-SSIM". G-SSIM is especially useful on blurred images. [19] The modifications above can be combined. For example, 4-G-r* is a combination of 4-SSIM, G-SSIM, and r*. It is able to reflect radiologist preference for images much better than other SSIM variants tested. [20]
A linear, or axial, color gradient. In color science, a color gradient (also known as a color ramp or a color progression) specifies a range of position-dependent colors, usually used to fill a region. In assigning colors to a set of values, a gradient is a continuous colormap, a type of color scheme.
This flooding process is performed on the gradient image, i.e. the basins should emerge along the edges. Normally this will lead to an over-segmentation of the image, especially for noisy image material, e.g. medical CT data. Either the image must be pre-processed or the regions must be merged on the basis of a similarity criterion afterwards.