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<html><head><title>Anti-Grain Geometry - Working with Gradients</title>
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<table width="640px"><tbody><tr><td><h1>Working with Gradients<span
class="subtitle"><br>A Simple Step-by-Step Tutorial</span></h1></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>This
article will explain to you how to set up gradients and render them.
We will use a simple command-line example that produces the result in
the
<code>agg_test.ppm</code> file. You can use, for example
<a href="http://www.irfanview.com/"><img
src="gradients_tutorial_files/link.gif" border="0">IrfanView
(www.irfanview.com)</a> to see the
results.</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>You
will need to tell the compiler the <b>AGG</b> include directory and add
three source files to the project or to the command line:
<code>agg_rasterizer_scanline_aa.cpp</code>,
<code><a
href="http://www.antigrain.com/__code/src/agg_trans_affine.cpp.html">agg_trans_affine.cpp</a></code>,
and <code><a
href="http://www.antigrain.com/__code/src/agg_sqrt_tables.cpp.html">agg_sqrt_tables.cpp</a></code>.
You
can find the source file here: <a
href="http://www.antigrain.com/tips/gradients_tutorial/gradients.cpp"><img
src="gradients_tutorial_files/download.gif" border="0"> (gradients.cpp)</a>.</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre><span class="kw2">#include</span> <span class="op"><</span>stdio<span class="op">.</span>h<span class="op">></span>
<span class="kw2">#include</span> <span class="op"><</span>string<span class="op">.</span>h<span class="op">></span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_pixfmt_rgb.h.html">agg_pixfmt_rgb.h</a>"</span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_renderer_base.h.html">agg_renderer_base.h</a>"</span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_renderer_scanline.h.html">agg_renderer_scanline.h</a>"</span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_scanline_u.h.html">agg_scanline_u.h</a>"</span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_rasterizer_scanline_aa.h.html">agg_rasterizer_scanline_aa.h</a>"</span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_ellipse.h.html">agg_ellipse.h</a>"</span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html">agg_span_gradient.h</a>"</span>
<span class="kw2">#include</span> <span class="str">"<a href="http://www.antigrain.com/__code/include/agg_span_interpolator_linear.h.html">agg_span_interpolator_linear.h</a>"</span>
<span class="kw1">enum</span>
<span class="op">{</span>
frame_width <span class="op">=</span> <span class="num">320</span><span class="op">,</span>
frame_height <span class="op">=</span> <span class="num">200</span>
<span class="op">}</span><span class="op">;</span>
<span class="rem">// Writing the buffer to a .PPM file, assuming it has </span>
<span class="rem">// RGB-structure, one byte per color component</span>
<span class="rem">//--------------------------------------------------</span>
<span class="kw1">bool</span> write_ppm<span class="op">(</span><span class="kw1">const</span> <span class="kw1">unsigned</span> <span class="kw1">char</span><span class="op">*</span> buf<span class="op">,</span>
<span class="kw1">unsigned</span> width<span class="op">,</span>
<span class="kw1">unsigned</span> height<span class="op">,</span>
<span class="kw1">const</span> <span class="kw1">char</span><span class="op">*</span> file_name<span class="op">)</span>
<span class="op">{</span>
FILE<span class="op">*</span> fd <span class="op">=</span> fopen<span class="op">(</span>file_name<span class="op">,</span> <span class="str">"wb"</span><span class="op">)</span><span class="op">;</span>
<span class="kw1">if</span><span class="op">(</span>fd<span class="op">)</span>
<span class="op">{</span>
fprintf<span class="op">(</span>fd<span class="op">,</span> <span class="str">"P6 %d %d 255 "</span><span class="op">,</span> width<span class="op">,</span> height<span class="op">)</span><span class="op">;</span>
fwrite<span class="op">(</span>buf<span class="op">,</span> <span class="num">1</span><span class="op">,</span> width <span class="op">*</span> height <span class="op">*</span> <span class="num">3</span><span class="op">,</span> fd<span class="op">)</span><span class="op">;</span>
fclose<span class="op">(</span>fd<span class="op">)</span><span class="op">;</span>
<span class="kw1">return</span> <span class="kw1">true</span><span class="op">;</span>
<span class="op">}</span>
<span class="kw1">return</span> <span class="kw1">false</span><span class="op">;</span>
<span class="op">}</span>
<span class="rem">// A simple function to form the gradient color array </span>
<span class="rem">// consisting of 3 colors, "begin", "middle", "end"</span>
<span class="rem">//---------------------------------------------------</span>
<span class="kw1">template</span><span class="op"><</span><span class="kw1">class</span> Array<span class="op">></span>
<span class="kw1">void</span> fill_color_array<span class="op">(</span>Array<span class="op">&</span> array<span class="op">,</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a> begin<span class="op">,</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a> middle<span class="op">,</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a> end<span class="op">)</span>
<span class="op">{</span>
<span class="kw1">unsigned</span> i<span class="op">;</span>
<span class="kw1">unsigned</span> half_size <span class="op">=</span> array<span class="op">.</span>size<span class="op">(</span><span class="op">)</span> <span class="op">/</span> <span class="num">2</span><span class="op">;</span>
<span class="kw1">for</span><span class="op">(</span>i <span class="op">=</span> <span class="num">0</span><span class="op">;</span> i <span class="op"><</span> half_size<span class="op">;</span> <span class="op">++</span>i<span class="op">)</span>
<span class="op">{</span>
array<span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> begin<span class="op">.</span>gradient<span class="op">(</span>middle<span class="op">,</span> i <span class="op">/</span> <span class="kw1">double</span><span class="op">(</span>half_size<span class="op">)</span><span class="op">)</span><span class="op">;</span>
<span class="op">}</span>
<span class="kw1">for</span><span class="op">(</span><span class="op">;</span> i <span class="op"><</span> array<span class="op">.</span>size<span class="op">(</span><span class="op">)</span><span class="op">;</span> <span class="op">++</span>i<span class="op">)</span>
<span class="op">{</span>
array<span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> middle<span class="op">.</span>gradient<span class="op">(</span>end<span class="op">,</span> <span class="op">(</span>i <span class="op">-</span> half_size<span class="op">)</span> <span class="op">/</span> <span class="kw1">double</span><span class="op">(</span>half_size<span class="op">)</span><span class="op">)</span><span class="op">;</span>
<span class="op">}</span>
<span class="op">}</span>
<span class="kw1">int</span> <span class="kw1">main</span><span class="op">(</span><span class="op">)</span>
<span class="op">{</span>
<span class="kw1">unsigned</span> <span class="kw1">char</span><span class="op">*</span> buffer <span class="op">=</span> <span class="kw1">new</span> <span class="kw1">unsigned</span> <span class="kw1">char</span><span class="op">[</span>frame_width <span class="op">*</span> frame_height <span class="op">*</span> <span class="num">3</span><span class="op">]</span><span class="op">;</span>
agg::<a href="http://www.antigrain.com/doc/basic_renderers/basic_renderers.agdoc.html#rendering_buffer">rendering_buffer</a> rbuf<span class="op">(</span>buffer<span class="op">,</span>
frame_width<span class="op">,</span>
frame_height<span class="op">,</span>
<span class="op">-</span>frame_width <span class="op">*</span> <span class="num">3</span><span class="op">)</span><span class="op">;</span>
<span class="rem">// Pixel format and basic renderers.</span>
<span class="rem">//-----------------</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_pixfmt_rgb.h.html#pixfmt_rgb24">pixfmt_rgb24</a> pixfmt_type<span class="op">;</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/doc/basic_renderers/basic_renderers.agdoc.html#renderer_base">renderer_base</a><span class="op"><</span>pixfmt_type<span class="op">></span> renderer_base_type<span class="op">;</span>
<span class="rem">// The gradient color array</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_array.h.html#pod_auto_array">pod_auto_array</a><span class="op"><</span>agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a><span class="op">,</span> <span class="num">256</span><span class="op">></span> color_array_type<span class="op">;</span>
<span class="rem">// Gradient shape function (linear, radial, custom, etc)</span>
<span class="rem">//-----------------</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_x">gradient_x</a> gradient_func_type<span class="op">;</span>
<span class="rem">// Span interpolator. This object is used in all span generators </span>
<span class="rem">// that operate with transformations during iterating of the spans,</span>
<span class="rem">// for example, image transformers use the interpolator too.</span>
<span class="rem">//-----------------</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_span_interpolator_linear.h.html#span_interpolator_linear">span_interpolator_linear</a><span class="op"><</span><span class="op">></span> interpolator_type<span class="op">;</span>
<span class="rem">// Span allocator is an object that allocates memory for </span>
<span class="rem">// the array of colors that will be used to render the </span>
<span class="rem">// color spans. One object can be shared between different </span>
<span class="rem">// span generators.</span>
<span class="rem">//-----------------</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_span_allocator.h.html#span_allocator">span_allocator</a><span class="op"><</span>agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a><span class="op">></span> span_allocator_type<span class="op">;</span>
<span class="rem">// Finally, the gradient span generator working with the agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a> </span>
<span class="rem">// color type. </span>
<span class="rem">// The 4-th argument is the color function that should have </span>
<span class="rem">// the [] operator returning the color in range of [0...255].</span>
<span class="rem">// In our case it will be a simple look-up table of 256 colors.</span>
<span class="rem">//-----------------</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#span_gradient">span_gradient</a><span class="op"><</span>agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a><span class="op">,</span>
interpolator_type<span class="op">,</span>
gradient_func_type<span class="op">,</span>
color_array_type<span class="op">,</span>
span_allocator_type<span class="op">></span> span_gradient_type<span class="op">;</span>
<span class="rem">// The gradient scanline renderer type</span>
<span class="rem">//-----------------</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_renderer_scanline.h.html#renderer_scanline_aa">renderer_scanline_aa</a><span class="op"><</span>renderer_base_type<span class="op">,</span>
span_gradient_type<span class="op">></span> renderer_gradient_type<span class="op">;</span>
<span class="rem">// Common declarations (pixel format and basic renderer).</span>
<span class="rem">//----------------</span>
pixfmt_type pixf<span class="op">(</span>rbuf<span class="op">)</span><span class="op">;</span>
renderer_base_type rbase<span class="op">(</span>pixf<span class="op">)</span><span class="op">;</span>
<span class="rem">// The gradient objects declarations</span>
<span class="rem">//----------------</span>
gradient_func_type gradient_func<span class="op">;</span> <span class="rem">// The gradient function</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine">trans_affine</a> gradient_mtx<span class="op">;</span> <span class="rem">// Affine transformer</span>
interpolator_type span_interpolator<span class="op">(</span>gradient_mtx<span class="op">)</span><span class="op">;</span> <span class="rem">// Span interpolator</span>
span_allocator_type <a href="http://www.antigrain.com/__code/include/agg_span_allocator.h.html#span_allocator">span_allocator</a><span class="op">;</span> <span class="rem">// Span Allocator</span>
color_array_type color_array<span class="op">;</span> <span class="rem">// Gradient colors</span>
<span class="rem">// Declare the gradient span itself. </span>
<span class="rem">// The last two arguments are so called "d1" and "d2" </span>
<span class="rem">// defining two distances in pixels, where the gradient starts</span>
<span class="rem">// and where it ends. The actual meaning of "d1" and "d2" depands</span>
<span class="rem">// on the gradient function.</span>
<span class="rem">//----------------</span>
span_gradient_type <a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#span_gradient">span_gradient</a><span class="op">(</span><a href="http://www.antigrain.com/__code/include/agg_span_allocator.h.html#span_allocator">span_allocator</a><span class="op">,</span>
span_interpolator<span class="op">,</span>
gradient_func<span class="op">,</span>
color_array<span class="op">,</span>
<span class="num">0</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
<span class="rem">// The gradient renderer</span>
<span class="rem">//----------------</span>
renderer_gradient_type ren_gradient<span class="op">(</span>rbase<span class="op">,</span> <a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#span_gradient">span_gradient</a><span class="op">)</span><span class="op">;</span>
<span class="rem">// The rasterizing/scanline stuff</span>
<span class="rem">//----------------</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_rasterizer_scanline_aa.h.html#rasterizer_scanline_aa">rasterizer_scanline_aa</a><span class="op"><</span><span class="op">></span> ras<span class="op">;</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_scanline_u.h.html#scanline_u8">scanline_u8</a> sl<span class="op">;</span>
<span class="rem">// Finally we can draw a circle.</span>
<span class="rem">//----------------</span>
rbase<span class="op">.</span>clear<span class="op">(</span>agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a><span class="op">(</span><span class="num">255</span><span class="op">,</span> <span class="num">255</span><span class="op">,</span> <span class="num">255</span><span class="op">)</span><span class="op">)</span><span class="op">;</span>
fill_color_array<span class="op">(</span>color_array<span class="op">,</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a><span class="op">(</span><span class="num">0</span><span class="op">,</span><span class="num">50</span><span class="op">,</span><span class="num">50</span><span class="op">)</span><span class="op">,</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a><span class="op">(</span><span class="num">240</span><span class="op">,</span> <span class="num">255</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">,</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_color_rgba.h.html#rgba8">rgba8</a><span class="op">(</span><span class="num">80</span><span class="op">,</span> <span class="num">0</span><span class="op">,</span> <span class="num">0</span><span class="op">)</span><span class="op">)</span><span class="op">;</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_ellipse.h.html#ellipse">ellipse</a> ell<span class="op">(</span><span class="num">50</span><span class="op">,</span> <span class="num">50</span><span class="op">,</span> <span class="num">50</span><span class="op">,</span> <span class="num">50</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
ras<span class="op">.</span>add_path<span class="op">(</span>ell<span class="op">)</span><span class="op">;</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_renderer_scanline.h.html#render_scanlines">render_scanlines</a><span class="op">(</span>ras<span class="op">,</span> sl<span class="op">,</span> ren_gradient<span class="op">)</span><span class="op">;</span>
write_ppm<span class="op">(</span>buffer<span class="op">,</span> frame_width<span class="op">,</span> frame_height<span class="op">,</span> <span class="str">"agg_test.ppm"</span><span class="op">)</span><span class="op">;</span>
<span class="kw1">delete</span> <span class="op">[</span><span class="op">]</span> buffer<span class="op">;</span>
<span class="kw1">return</span> <span class="num">0</span><span class="op">;</span>
<span class="op">}</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>It
looks rather complex, especially the necessity to declare a lot
of types and objects. But the “complexity” gives you freedom, for
example, you can define your own gradient functions or even arbitrary
distortions.</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>The
example renders a circle with linear gradient from (0,0) to (100,0).
In <b>AGG</b> you can define an arbitrary color function, in our case
it's a
simple look-up table generated from three colors, <code>start</code>, <code>middle</code>,
and
<code>end</code>.</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Here
is the result (the axes and text were added in <a
href="http://www.xara.com/"><img src="gradients_tutorial_files/link.gif"
border="0"><b><nobr>Xara X</nobr></b></a>):</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td><center><img
src="gradients_tutorial_files/gradients1.png" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>It
also can seem like an overkill for this simple task, but later
you will see that it's not so.
<br><br><br></p></td></tr></tbody></table>
<table style="margin: 0px;" height="1px" width="640px" bgcolor="#583927"
border="0" cellpadding="0" cellspacing="0"><tbody><tr><td></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>The
next step is one little modification. Modify the following:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> <span class="rem">// Declare the gradient span itself. </span>
<span class="rem">// The last two arguments are so called "d1" and "d2" </span>
<span class="rem">// defining two distances in pixels, where the gradient starts</span>
<span class="rem">// and where it ends. The actual meaning of "d1" and "d2" depands</span>
<span class="rem">// on the gradient function.</span>
<span class="rem">//----------------</span>
span_gradient_type <a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#span_gradient">span_gradient</a><span class="op">(</span><a href="http://www.antigrain.com/__code/include/agg_span_allocator.h.html#span_allocator">span_allocator</a><span class="op">,</span>
span_interpolator<span class="op">,</span>
gradient_func<span class="op">,</span>
color_array<span class="op">,</span>
<span class="num">50</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>The
result:</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td><center><img
src="gradients_tutorial_files/gradients2.png" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>It
should explain those freaky <code>d1</code> and <code>d2</code>
arguments. In fact, they
determine the geometrical <code>start</code> and <code>end</code> of the
gradient and their meaning
depends on the gradient function.
<br><br><br>
</p></td></tr></tbody></table><table style="margin: 0px;" height="1px"
width="640px" bgcolor="#583927" border="0" cellpadding="0"
cellspacing="0"><tbody><tr><td></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Now
change the gradient function:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> <span class="rem">// Gradient shape function (linear, radial, custom, etc)</span>
<span class="rem">//-----------------</span>
<span class="kw1">typedef</span> agg::<a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_circle">gradient_circle</a> gradient_func_type<span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Set <code>d1</code>
back to 0:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> <span class="rem">// Declare the gradient span itself. </span>
<span class="rem">// The last two arguments are so called "d1" and "d2" </span>
<span class="rem">// defining two distances in pixels, where the gradient starts</span>
<span class="rem">// and where it ends. The actual meaning of "d1" and "d2" depands</span>
<span class="rem">// on the gradient function.</span>
<span class="rem">//----------------</span>
span_gradient_type <a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#span_gradient">span_gradient</a><span class="op">(</span><a href="http://www.antigrain.com/__code/include/agg_span_allocator.h.html#span_allocator">span_allocator</a><span class="op">,</span>
span_interpolator<span class="op">,</span>
gradient_func<span class="op">,</span>
color_array<span class="op">,</span>
<span class="num">0</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>And
modify the circle:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> agg::<a href="http://www.antigrain.com/__code/include/agg_ellipse.h.html#ellipse">ellipse</a> ell<span class="op">(</span><span class="num">0</span><span class="op">,</span> <span class="num">0</span><span class="op">,</span> <span class="num">120</span><span class="op">,</span> <span class="num">120</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>The
result:</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td><center><img
src="gradients_tutorial_files/gradients3.png" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p><br><br><br>
</p></td></tr></tbody></table><table style="margin: 0px;" height="1px"
width="640px" bgcolor="#583927" border="0" cellpadding="0"
cellspacing="0"><tbody><tr><td></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Modify
<code>d1</code> again:
</p></td></tr></tbody></table><table width="640px" border="0"
cellpadding="0" cellspacing="0"><tbody><tr><td><pre> <span class="rem">// Declare the gradient span itself. </span>
<span class="rem">// The last two arguments are so called "d1" and "d2" </span>
<span class="rem">// defining two distances in pixels, where the gradient starts</span>
<span class="rem">// and where it ends. The actual meaning of "d1" and "d2" depands</span>
<span class="rem">// on the gradient function.</span>
<span class="rem">//----------------</span>
span_gradient_type <a href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#span_gradient">span_gradient</a><span class="op">(</span><a href="http://www.antigrain.com/__code/include/agg_span_allocator.h.html#span_allocator">span_allocator</a><span class="op">,</span>
span_interpolator<span class="op">,</span>
gradient_func<span class="op">,</span>
color_array<span class="op">,</span>
<span class="num">50</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td><center><img
src="gradients_tutorial_files/gradients4.png" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>So
that, in case of a radial gradient, <code>d1</code> and <code>d2</code>
define the starting
and ending radii.
<br><br><br>
</p></td></tr></tbody></table><table style="margin: 0px;" height="1px"
width="640px" bgcolor="#583927" border="0" cellpadding="0"
cellspacing="0"><tbody><tr><td></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>By
default the origin point for the gradients is (0,0). How to draw a
gradient
in some other place? The answer is to use affine transformations.
Strictly
speaking, the transformations are fully defined by the span
interpolator. In our case
we use <a
href="http://www.antigrain.com/__code/include/agg_span_interpolator_linear.h.html#span_interpolator_linear">span_interpolator_linear</a>
with an affine matrix. The linear interpolator
allows you to speed up the calculations vastly, because we calculate the
floating
point coordinates only in the begin and end of the horizontal spans and
then
use a fast, integer, Bresenham-like interpolation with <b>Subpixel
Accuracy</b>.</p></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Add
the following code somewhere before calling <code>agg::<a
href="http://www.antigrain.com/__code/include/agg_renderer_scanline.h.html#render_scanlines">render_scanlines</a>(ras,
sl, ren_gradient);</code></p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> gradient_mtx <span class="op">*=</span> agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine_scaling">trans_affine_scaling</a><span class="op">(</span><span class="num">0</span><span class="op">.</span><span class="num">75</span><span class="op">,</span> <span class="num">1</span><span class="op">.</span><span class="num">2</span><span class="op">)</span><span class="op">;</span>
gradient_mtx <span class="op">*=</span> agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine_rotation">trans_affine_rotation</a><span class="op">(</span><span class="op">-</span>agg::<a href="http://www.antigrain.com/__code/include/agg_basics.h.html#pi">pi</a><span class="op">/</span><span class="num">3</span><span class="op">.</span><span class="num">0</span><span class="op">)</span><span class="op">;</span>
gradient_mtx <span class="op">*=</span> agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine_translation">trans_affine_translation</a><span class="op">(</span><span class="num">100</span><span class="op">.</span><span class="num">0</span><span class="op">,</span> <span class="num">100</span><span class="op">.</span><span class="num">0</span><span class="op">)</span><span class="op">;</span>
gradient_mtx<span class="op">.</span>invert<span class="op">(</span><span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>And
modify the circle:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> agg::<a href="http://www.antigrain.com/__code/include/agg_ellipse.h.html#ellipse">ellipse</a> ell<span class="op">(</span><span class="num">100</span><span class="op">,</span> <span class="num">100</span><span class="op">,</span> <span class="num">120</span><span class="op">,</span> <span class="num">120</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td><center><img
src="gradients_tutorial_files/gradients5.png" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>The
code of initializing of the affine matrix should be obvious except
for some strange <code>gradient_mtx.invert()</code>. It's necessary
because
the gradient generator uses <b>reverse</b> transformations instead of
<b>direct</b> ones. In other words it takes the destination point,
applies the transformations and obtains the coordinates in the gradient.
Note that the affine transformations allow you to turn a circular
gradient
into elliptical.
<br><br><br>
</p></td></tr></tbody></table><table style="margin: 0px;" height="1px"
width="640px" bgcolor="#583927" border="0" cellpadding="0"
cellspacing="0"><tbody><tr><td></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Now
it should be obvious how to define a linear gradient from some
<code>Point1</code> to <code>Point2</code>. So, get back to the original
code and
add the following function:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre><span class="rem">// Calculate the affine transformation matrix for the linear gradient </span>
<span class="rem">// from (x1, y1) to (x2, y2). gradient_d2 is the "base" to scale the</span>
<span class="rem">// gradient. Here d1 must be 0.0, and d2 must equal gradient_d2.</span>
<span class="rem">//---------------------------------------------------------------</span>
<span class="kw1">void</span> calc_linear_gradient_transform<span class="op">(</span><span class="kw1">double</span> x1<span class="op">,</span> <span class="kw1">double</span> y1<span class="op">,</span> <span class="kw1">double</span> x2<span class="op">,</span> <span class="kw1">double</span> y2<span class="op">,</span>
agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine">trans_affine</a><span class="op">&</span> mtx<span class="op">,</span>
<span class="kw1">double</span> gradient_d2 <span class="op">=</span> <span class="num">100</span><span class="op">.</span><span class="num">0</span><span class="op">)</span>
<span class="op">{</span>
<span class="kw1">double</span> dx <span class="op">=</span> x2 <span class="op">-</span> x1<span class="op">;</span>
<span class="kw1">double</span> dy <span class="op">=</span> y2 <span class="op">-</span> y1<span class="op">;</span>
mtx<span class="op">.</span>reset<span class="op">(</span><span class="op">)</span><span class="op">;</span>
mtx <span class="op">*=</span> agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine_scaling">trans_affine_scaling</a><span class="op">(</span>sqrt<span class="op">(</span>dx <span class="op">*</span> dx <span class="op">+</span> dy <span class="op">*</span> dy<span class="op">)</span> <span class="op">/</span> gradient_d2<span class="op">)</span><span class="op">;</span>
mtx <span class="op">*=</span> agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine_rotation">trans_affine_rotation</a><span class="op">(</span>atan2<span class="op">(</span>dy<span class="op">,</span> dx<span class="op">)</span><span class="op">)</span><span class="op">;</span>
mtx <span class="op">*=</span> agg::<a href="http://www.antigrain.com/__code/include/agg_trans_affine.h.html#trans_affine_translation">trans_affine_translation</a><span class="op">(</span>x1<span class="op">,</span> y1<span class="op">)</span><span class="op">;</span>
mtx<span class="op">.</span>invert<span class="op">(</span><span class="op">)</span><span class="op">;</span>
<span class="op">}</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Then
modify the circle:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> agg::<a href="http://www.antigrain.com/__code/include/agg_ellipse.h.html#ellipse">ellipse</a> ell<span class="op">(</span><span class="num">100</span><span class="op">,</span> <span class="num">100</span><span class="op">,</span> <span class="num">80</span><span class="op">,</span> <span class="num">80</span><span class="op">,</span> <span class="num">100</span><span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>And
add the transformations:</p></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td><pre> calc_linear_gradient_transform<span class="op">(</span><span class="num">50</span><span class="op">,</span> <span class="num">50</span><span class="op">,</span> <span class="num">150</span><span class="op">,</span> <span class="num">150</span><span class="op">,</span> gradient_mtx<span class="op">)</span><span class="op">;</span>
</pre></td></tr></tbody></table><font style="margin-left: 1em;"><i></i></font>
<table width="640px"><tbody><tr><td><center><img
src="gradients_tutorial_files/gradients6.png" title="" border="0"><br><i></i></center></td></tr></tbody></table>
<table width="640px"><tbody><tr><td style="text-align: justify;"><p>Try
to play with different parameters,
transformations, and gradient functions: <a
href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_circle">gradient_circle</a>,
<a
href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_x">gradient_x</a>,
<a
href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_y">gradient_y</a>,
<a
href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_diamond">gradient_diamond</a>,
<a
href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_xy">gradient_xy</a>,
<a
href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_sqrt_xy">gradient_sqrt_xy</a>,
<a
href="http://www.antigrain.com/__code/include/agg_span_gradient.h.html#gradient_conic">gradient_conic</a>.
Also look at the gradient functions and try to write your own. Actually,
the set of the gradient functions in <b>AGG</b> is rather poor, it just
demonstrates
the possibilities. For example, repeating or reflecting gradients should
be implemented in gradient functions (or you can write adaptors that
will
use the existing functions).
</p></td></tr></tbody></table><table style="margin: 0px;" height="1px"
width="640px" bgcolor="#583927" border="0" cellpadding="0"
cellspacing="0"><tbody><tr><td></td></tr></tbody></table>
<table width="640px" border="0" cellpadding="0" cellspacing="0">
<tbody><tr><td><center><span class="authors">
Copyright <span class="larger">©</span> 2002-2006
<a href="http://www.antigrain.com/mcseem/index.html"><b>Maxim Shemanarev</b></a>
</span></center></td></tr>
<tr><td><center><span class="authors">
Web Design and Programming
<a href="http://www.antigrain.com/mcseem/index.html"><b>Maxim Shemanarev</b></a>
</span></center></td></tr>
</tbody></table>
<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>
<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>
<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>
<br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>
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