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Postsdbo
PO
 

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  1. PO
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    <p>First, let me explain what an "edge" is.</p> <p>An edge is the virtual line between two contiguous pixels.</p> <pre><code>+---+---+ | A | B | // The middle line is the edge between pixel A and pixel B +---+---+ </code></pre> <p>Edges have a starting point and an end point, and so are oriented upward, downward, "left going" or "right going".</p> <p>In order to handle polygons crossing image boundaries, we gonna add a 1 pixel white border around our image.</p> <p>Here is the algorithm:</p> <pre><code>For each row of our image { For each pixel of the row except the last one { If current pixel is white and next pixel is black { Create a new upward edge between the two pixels and add it to the global edge list. } Else if current pixel is black and next pixel is white { Create a new downward edge between the two pixels and add it to the global edge list. } } } For each column of our image { For each pixel of the column except the last one { If current pixel is white and next pixel is black { Create a new "left going" edge between the two pixels and add it to the global edge list. } Else if current pixel is black and next pixel is white { Create a new "right going" edge between the two pixels and add it to the global edge list. } } } For each edge of the global edge list { Find the edge starting from the point where your current edge ends (for now on, we gonna call it next_edge). Connect the two edges to form a linked list (edge-&gt;next = next_edge; next_edge-&gt;previous = edge;) } While there is edges in the global edge list { Create a new polygon and add it to the polygon list Take the first edge of the list (for now on, we gonna call it first_edge) For each edge in the linked list starting at first_edge { Remove edge from global edge list Add edge to polygon's edge list } } </code></pre> <p>Done, you have a list of polygons.</p> <p><strong>EDIT</strong></p> <p>Of course, you have to optimize it a little before using it, but it's really easy : consecutive edges with the same orientation can be replaced with a single, longer edge.</p>
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    1. POHow can I convert a solid-black and white image into a collection of polygons?
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    1. USNicolas Repiquet
    1. USNicolas Repiquet
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    1. COWhat about diagonal edges or lines going from (0,0)->(10,3) ?
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      1. USAki Suihkonen
    2. CO@AkiSuihkonen Wouldn't all edges be orthonormal, since pixels are square?
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      1. USAsad Saeeduddin
    3. CO@Asad: In some context yes, but IMO the meaningful interpretation of OP's question deals e.g. with vectorizing a triangle into three vertices.
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      1. USAki Suihkonen
 

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