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### 2.6 Interpolators

VIPS has a general system for representing pixel interpolators. You can select an interpolator to pass to other VIPS operations, such as im_affinei(), you can add new interpolators, and you can write operations which take a general interpolator as a parameter.

An interpolator is a function of the form:

typedef void (⋆VipsInterpolateMethod)( VipsInterpolate ⋆,
PEL ⋆out, REGION ⋆in, double x, double y );

given the set of input pixels in, it has to calculate a value for the fractional position (x,y) and write this value to the memory pointed to by out.

VIPS uses corner convention, so the value of pixel (0,0) is the value of the surface the interpolator fits at the fractional position (0.0,0.0).

#### 2.6.1 How an interpolator is represented

See the man page for VipsInterpolate for full details, but briefly, an interpolator is a subclass of VipsInterpolate implementing the following items:

- An interpolation method, with the type signature above.
- A function get_window_size() which returns the size of the area of pixels that the interpolator needs in order to calculate a value. For example, a bilinear interpolator needs the four pixels surrounding the point to be calculated, or a 2 by 2 window, so window size should be 2.
- Or if the window size is constant, you can leave get_window_size() NULL and just set the int value window_size.

#### 2.6.2 A sample interpolator

As an example, Figure 2.10 shows how to register a new interpolator in a plugin.

// This interpolator adds no new members.
typedef VipsInterpolate Myinterpolator;
typedef VipsInterpolateClass MyinterpolatorClass;
G_DEFINE_TYPE( Myinterpolator, myinterpolator, VIPS_TYPE_INTERPOLATE );
static void
myinterpolator_interpolate( VipsInterpolate ⋆interpolate,
PEL ⋆out, REGION ⋆in, double x, double y )
{
MyinterpolatorClass ⋆class =
MYINTERPOLATOR_GET_CLASS( interpolate );
/⋆ Nearest-neighbor.
⋆/
memcpy( out,
IM_REGION_ADDR( in, floor( x ), floor( y ) ),
IM_IMAGE_SIZEOF_PEL( in->im ) );
}
static void
myinterpolator_class_init( MyinterpolatorClass ⋆class )
{
VipsObjectClass ⋆object_class = (VipsObjectClass ⋆) class;
VipsInterpolateClass ⋆interpolate_class = (VipsInterpolateClass ⋆) class;
object_class->nickname = "myinterpolator";
object_class->description = _( "My interpolator" );
interpolate_class->interpolate = myinterpolator_interpolate;
}
static void
myinterpolate_init( Myinterpolate ⋆object )
{
}
char ⋆
g_module_check_init( GModule ⋆self )
{
// register the type
myinterpolator_get_type();
}

Figure 2.10: Registering an interpolator in a plugin

#### 2.6.3 Writing a VIPS operation that takes an interpolator as an argument

Operations just take a VipsInterpolate as an argument, for example:

int im_affinei_all( IMAGE ⋆in, IMAGE ⋆out,
VipsInterpolate ⋆interpolate,
double a, double b, double c, double d,
double dx, double dy );

To use the interpolator, use vips_interpolate():

void vips_interpolate( VipsInterpolate ⋆interpolate,
PEL ⋆out, REGION ⋆in, double x, double y );

This looks up the interpolate method for the object and calls it for you.

You can save the cost of the lookup in an inner loop with vips_interpolate_get_method():

VipsInterpolateMethod
vips_interpolate_get_method(
VipsInterpolate ⋆interpolate );

#### 2.6.4 Passing an interpolator to a VIPS operation

You can build an instance of a VipsInterpolator with the vips_object_⋆() family of functions, see §2.4.

Convenience functions return a static instance of one of the standard interpolators:

VipsInterpolate ⋆vips_interpolate_nearest_static( void );
VipsInterpolate ⋆vips_interpolate_bilinear_static( void );

Don’t free the result.

Finally, vips_interpolate_new() makes a VipsInterpolate from a nickname:

VipsInterpolate ⋆vips_interpolate_new( const char ⋆nickname );

For example:

VipsInterpolate ⋆interpolate = vips_interpolate_new( "nohalo" );

You must drop your ref after you’re done with the object with g_object_unref().

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