ST_ConvexHull
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## Name

ST_ConvexHull — Return the convex hull geometry of the raster including pixel values equal to BandNoDataValue. For regular shaped and non-skewed rasters, this gives the same result as ST_Envelope so only useful for irregularly shaped or skewed rasters.

## Synopsis

`geometry ST_ConvexHull(`raster rast`)`;

## Description

Return the convex hull geometry of the raster including the NoDataBandValue band pixels. For regular shaped and non-skewed rasters, this gives more or less the same result as ST_Envelope so only useful for irregularly shaped or skewed rasters.

ST_Envelope floors the coordinates and hence add a little buffer around the raster so the answer is subtly different from ST_ConvexHull which does not floor.

## Examples

Refer to PostGIS Raster Specification for a diagram of this.

    -- Note envelope and convexhull are more or less the same
    SELECT ST_AsText(ST_ConvexHull(rast)) As convhull,
        ST_AsText(ST_Envelope(rast)) As env
    FROM dummy_rast WHERE rid=1;

                            convhull                        |                env
    --------------------------------------------------------+------------------------------------
     POLYGON((0.5 0.5,20.5 0.5,20.5 60.5,0.5 60.5,0.5 0.5)) | POLYGON((0 0,20 0,20 60,0 60,0 0))

    -- now we skew the raster
    -- note how the convex hull and envelope are now different
    SELECT ST_AsText(ST_ConvexHull(rast)) As convhull,
        ST_AsText(ST_Envelope(rast)) As env
    FROM (SELECT ST_SetRotation(rast, 0.1, 0.1) As rast
        FROM dummy_rast WHERE rid=1) As foo;

                            convhull                        |                env
    --------------------------------------------------------+------------------------------------
     POLYGON((0.5 0.5,20.5 1.5,22.5 61.5,2.5 60.5,0.5 0.5)) | POLYGON((0 0,22 0,22 61,0 61,0 0))

## See Also

ST_Envelope, ST_MinConvexHull, ST_ConvexHull, ST_AsText

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