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| <h3 class="sectionHead"><span class="titlemark">2.6 </span> <a |
| id="x18-790002.6"></a>Interpolators</h3> |
| <!--l. 4--><p class="noindent" >VIPS has a general system for representing pixel interpolators. |
| You can select an interpolator to pass to other VIPS |
| operations, such as <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">im_affinei()</span></span></span>, you can add new |
| interpolators, and you can write operations which take a |
| general interpolator as a parameter. |
| <!--l. 9--><p class="indent" > An interpolator is a function of the form: |
| <div class="verbatim" id="verbatim-119"> |
| typedef void (⋆VipsInterpolateMethod)( VipsInterpolate ⋆, |
|  <br />  PEL ⋆out, REGION ⋆in, double x, double y ); |
| </div> |
| <!--l. 14--><p class="nopar" > |
| <!--l. 16--><p class="noindent" >given the set of input pixels <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">in</span></span></span>, it has to calculate a value |
| for the fractional position <span |
| class="cmr-10">(</span><span |
| class="cmmi-10">x,y</span><span |
| class="cmr-10">) </span>and write this value to the |
| memory pointed to by <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">out</span></span></span>. |
| <!--l. 21--><p class="indent" > VIPS uses corner convention, so the value of pixel <span |
| class="cmr-10">(0</span><span |
| class="cmmi-10">,</span><span |
| class="cmr-10">0)</span> |
| is the value of the surface the interpolator fits at the |
| fractional position <span |
| class="cmr-10">(0</span><span |
| class="cmmi-10">.</span><span |
| class="cmr-10">0</span><span |
| class="cmmi-10">,</span><span |
| class="cmr-10">0</span><span |
| class="cmmi-10">.</span><span |
| class="cmr-10">0)</span>. |
| <!--l. 24--><p class="noindent" > |
| <h4 class="subsectionHead"><span class="titlemark">2.6.1 </span> <a |
| id="x18-800002.6.1"></a>How an interpolator is represented</h4> |
| <!--l. 26--><p class="noindent" >See the man page for <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">VipsInterpolate</span></span></span> for full |
| details, but briefly, an interpolator is a subclass of |
| <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">VipsInterpolate</span></span></span> implementing the following |
| items: |
| <ul class="itemize1"> |
| <li class="itemize">An interpolation method, with the type signature |
| above. |
| </li> |
| <li class="itemize">A function <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">get_window_size()</span></span></span> |
| 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. |
| </li> |
| <li class="itemize">Or if the window size is constant, you can leave |
| <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">get_window_size()</span></span></span> NULL and just set the |
| int value <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">window_size</span></span></span>. |
| </li></ul> |
| <!--l. 46--><p class="noindent" > |
| <h4 class="subsectionHead"><span class="titlemark">2.6.2 </span> <a |
| id="x18-810002.6.2"></a>A sample interpolator</h4> |
| <!--l. 48--><p class="noindent" >As an example, Figure <a |
| href="#x18-8100110">2.10<!--tex4ht:ref: fg:newinterpolator --></a> shows how to register a new |
| interpolator in a plugin. |
| |
| |
| |
| <!--l. 52--><p class="indent" > <a |
| id="x18-8100110"></a><hr class="float"><div class="float" |
| > |
| |
| |
| |
| |
| <div class="verbatim" id="verbatim-120"> |
| // This interpolator adds no new members. |
|  <br />typedef VipsInterpolate Myinterpolator; |
|  <br />typedef VipsInterpolateClass MyinterpolatorClass; |
|  <br /> |
|  <br />G_DEFINE_TYPE( Myinterpolator, myinterpolator, VIPS_TYPE_INTERPOLATE ); |
|  <br /> |
|  <br />static void |
|  <br />myinterpolator_interpolate( VipsInterpolate ⋆interpolate, |
|  <br />PEL ⋆out, REGION ⋆in, double x, double y ) |
|  <br />{ |
|  <br />  MyinterpolatorClass ⋆class = |
|  <br />    MYINTERPOLATOR_GET_CLASS( interpolate ); |
|  <br /> |
|  <br />  /⋆ Nearest-neighbor. |
|  <br />   ⋆/ |
|  <br />  memcpy( out, |
|  <br />    IM_REGION_ADDR( in, floor( x ), floor( y ) ), |
|  <br />    IM_IMAGE_SIZEOF_PEL( in->im ) ); |
|  <br />} |
|  <br /> |
|  <br />static void |
|  <br />myinterpolator_class_init( MyinterpolatorClass ⋆class ) |
|  <br />{ |
|  <br />  VipsObjectClass ⋆object_class = (VipsObjectClass ⋆) class; |
|  <br />  VipsInterpolateClass ⋆interpolate_class = (VipsInterpolateClass ⋆) class; |
|  <br /> |
|  <br />  object_class->nickname = "myinterpolator"; |
|  <br />  object_class->description = _( "My interpolator" ); |
|  <br /> |
|  <br />  interpolate_class->interpolate = myinterpolator_interpolate; |
|  <br />} |
|  <br /> |
|  <br />static void |
|  <br />myinterpolate_init( Myinterpolate ⋆object ) |
|  <br />{ |
|  <br />} |
|  <br /> |
|  <br />char ⋆ |
|  <br />g_module_check_init( GModule ⋆self ) |
|  <br />{ |
|  <br />  // register the type |
|  <br />  myinterpolator_get_type(); |
|  <br />} |
| </div> |
| <!--l. 96--><p class="nopar" > |
| <br /><div class="caption" |
| ><span class="id">Figure 2.10: </span><span |
| class="content">Registering an interpolator in a plugin</span></div><!--tex4ht:label?: x18-8100110 --> |
| |
| |
| |
| </div><hr class="endfloat" /> |
| <h4 class="subsectionHead"><span class="titlemark">2.6.3 </span> <a |
| id="x18-820002.6.3"></a>Writing a VIPS operation that takes an interpolator |
| as an argument</h4> |
| <!--l. 103--><p class="noindent" >Operations just take a <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">VipsInterpolate</span></span></span> as an |
| argument, for example: |
| <div class="verbatim" id="verbatim-121"> |
| int im_affinei_all( IMAGE ⋆in, IMAGE ⋆out, |
|  <br />  VipsInterpolate ⋆interpolate, |
|  <br />  double a, double b, double c, double d, |
|  <br />  double dx, double dy ); |
| </div> |
| <!--l. 110--><p class="nopar" > |
| <!--l. 112--><p class="indent" > To use the interpolator, use <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">vips_interpolate()</span></span></span>: |
| |
| |
| |
| <div class="verbatim" id="verbatim-122"> |
| void vips_interpolate( VipsInterpolate ⋆interpolate, |
|  <br />  PEL ⋆out, REGION ⋆in, double x, double y ); |
| </div> |
| <!--l. 117--><p class="nopar" > |
| <!--l. 119--><p class="noindent" >This looks up the interpolate method for the object and calls |
| it for you. |
| <!--l. 122--><p class="indent" > You can save the cost of the lookup in an inner loop with |
| <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">vips_interpolate_get_method()</span></span></span>: |
| <div class="verbatim" id="verbatim-123"> |
| VipsInterpolateMethod |
|  <br />  vips_interpolate_get_method( |
|  <br />    VipsInterpolate ⋆interpolate ); |
| </div> |
| <!--l. 129--><p class="nopar" > |
| <!--l. 131--><p class="noindent" > |
| <h4 class="subsectionHead"><span class="titlemark">2.6.4 </span> <a |
| id="x18-830002.6.4"></a>Passing an interpolator to a VIPS operation</h4> |
| <!--l. 133--><p class="noindent" >You can build an instance of a <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">VipsInterpolator</span></span></span> |
| with the <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">vips_object_⋆()</span></span></span> family of functions, see |
| <span |
| class="cmsy-10">ยง</span><a |
| href="vipsmanualse10.html#x16-670002.4">2.4<!--tex4ht:ref: sec:object --></a>. |
| <!--l. 136--><p class="indent" > Convenience functions return a static instance of one of |
| the standard interpolators: |
| |
| |
| |
| <div class="verbatim" id="verbatim-124"> |
| VipsInterpolate ⋆vips_interpolate_nearest_static( void ); |
|  <br />VipsInterpolate ⋆vips_interpolate_bilinear_static( void ); |
|  <br />VipsInterpolate ⋆vips_interpolate_bicubic_static( void ); |
| </div> |
| <!--l. 143--><p class="nopar" > |
| <!--l. 145--><p class="noindent" >Don’t free the result. |
| <!--l. 148--><p class="indent" > Finally, <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">vips_interpolate_new()</span></span></span> makes a |
| <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">VipsInterpolate</span></span></span> from a nickname: |
| <div class="verbatim" id="verbatim-125"> |
| VipsInterpolate ⋆vips_interpolate_new( const char ⋆nickname ); |
| </div> |
| <!--l. 153--><p class="nopar" > |
| <!--l. 155--><p class="indent" > For example: |
| |
| |
| |
| <div class="verbatim" id="verbatim-126"> |
| VipsInterpolate ⋆interpolate = vips_interpolate_new( "nohalo" ); |
| </div> |
| <!--l. 159--><p class="nopar" > |
| <!--l. 161--><p class="noindent" >You must drop your ref after you’re done with the object |
| with <span class="obeylines-h"><span class="verb"><span |
| class="pcrr7t-">g_object_unref()</span></span></span>. |
| |
| |
| |
| |
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