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< h1 class = "text-center" > Uniquely Label Connected Regions< / h1 >
< p class = "lead magick-description" > Connected-component labeling (alternatively connected-component analysis, blob extraction, region labeling, blob discovery, or region extraction) uniquely labels connected components in an image. The labeling process scans the image, pixel-by-pixel from top-left to bottom-right, in order to identify connected pixel regions, i.e. regions of adjacent pixels which share the same set of intensity values. For example, let's find the objects in this image:< / p >
< ul >
< a href = "../images/objects.gif" > < img src = "../images/objects.gif" width = "256" height = "171" class = "image-slices" alt = "purse" / > < / a >
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< p > To identify the objects in this image, use this command:< / p >
< ul > < pre class = "highlight" > < code > convert objects.gif -connected-components 4 -auto-level -depth 8 objects.png< / code > < / pre > < / ul >
< p > The detected objects are uniquely labeled. Use auto leveling to visualize the detected objects:< / p >
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< a href = "../images/objects.png" > < img src = "../images/objects.png" width = "256" height = "171" class = "image-slices" alt = "Objects" / > < / a >
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< p > Object statistics is useful to review. To display them, use this command:< / p >
< ul > < pre class = "highlight" > < code > convert objects.gif -define connected-components:verbose=true -connected-components 4 objects.png< / code > < / pre > < / ul >
< p > Five objects were detected in the source image with these statistics:< / p >
< ul > < pre class = "highlight" > < code > Objects (id: bounding-box centroid area mean-color):
0: 256x171+0+0 119.2,80.8 33117 srgb(0,0,0)
2: 120x135+104+18 159.5,106.5 8690 srgb(255,255,255)
3: 50x36+129+44 154.2,63.4 1529 srgb(0,0,0)
4: 21x23+0+45 8.8,55.9 409 srgb(255,255,255)
1: 4x10+252+0 253.9,4.1 31 srgb(255,255,255)
< / code > < / pre > < / ul >
< p > Add < code > -define connected-components:exclude-header=true< / code > to show the objects without the header-line.< / p >
< p > Use < code > -connected-components 8< / code > to visit 8 neighbors rather than 4. By default, neighbor colors must be exact to be part of a unique object. Use the < a href = "command-line-options.html#fuzz" > -fuzz< / a > option to include pixels as part of an object that are < var > close< / var > in color.< / p >
< p > You might want to eliminate small objects by merging them with their larger neighbors. If so, use this command:< / p >
< ul > < pre class = "highlight" > < code > convert objects.gif -define connected-components:area-threshold=410 -connected-components 4 \
-auto-level objects.jpg< / code > < / pre > < / ul >
< p > Here are the expected results. Notice, how the small objects are now merged with the background.< / p >
< ul >
< a href = "../images/objects.jpg" > < img src = "../images/objects.jpg" width = "256" height = "171" class = "image-slices" alt = "Objects" / > < / a >
< / ul >
< p > Notice how two of the objects were merged leaving three remaining objects:< / p >
< ul > < pre class = "highlight" > < code > Objects (id: bounding-box centroid area mean-color):
0: 256x171+0+0 118.0,80.4 33557 srgb(0,0,0)
2: 120x135+104+18 159.5,106.5 8690 srgb(255,255,255)
3: 50x36+129+44 154.2,63.4 1529 srgb(0,0,0)< / code > < / pre > < / ul >
< p > Area thresholding does not always work if objects are stacked on top of one-another. We're seeking a robust algorithm to tackle this use case.< / p >
< p > By default, the labeled image is grayscale. You can instead replace the object color in the labeled image with the mean-color from the source image. Simply add this setting, < code > -define connected-components:mean-color=true< / code > , to your command line.< / p >
< p > Thresholds can optionally include ranges, e.g. < code > -define connected-components:area-threshold=410-1600< / code > . To keep the background object, identify it with < code > -define connected-components:background-id=< var > object-id< / var > < / code > . The default background object is the object with the largest area.< / p >
< p > You may want to remove certain objects. Use < code > -define connected-components:remove-ids=< em > list-of-ids< / em > < / code > (e.g. -define connected-components:remove-ids=2,4-5). Or use < code > -define connected-components:keep-ids=< em > list-of-ids< / em > < / code > to keep these objects and merge all others. For convenience, you can keep the top objects with this option: < code > -define connected-components:keep-top=< em > number-of-objects< / em > < / code > < / p >
< p > Objects in your image may look homogeneous but have slightly different color values. By default, only pixels that match exactly are considered as part of a particular object. For slight variations of color in an object, use < code > -fuzz< / code > . For example,< / p >
< ul > < pre class = "highlight" > < code > convert star-map.png -fuzz 5% -define connected-components:verbose=true \
-define connected-components:mean-color=true -connected-components 4 stars.gif< / code > < / pre > < / ul >
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