https://akeil.de/enSat, 02 Jan 2021 21:07:04 GMTNikola (getnikola.com)http://blogs.law.harvard.edu/tech/rsshttps://akeil.de/posts/grayscale-images-with-imagemagick/Alexander Keil<div class="section" id="grayscale-images-with-imagemagick"> <h2>Grayscale Images with ImageMagick</h2> <dl class="field-list simple"> <dt>author</dt> <dd><p>akeil</p> </dd> <dt>date</dt> <dd><p>2014-05-10</p> </dd> <dt>version</dt> <dd><p>2</p> </dd> </dl> <p>Converting a photo to a grayscale version is simple enough with the <a class="reference external" href="http://www.imagemagick.org">ImageMagick</a> <a class="footnote-reference brackets" href="https://akeil.de/posts/grayscale-images-with-imagemagick/#id1" id="id2">1</a> <code class="docutils literal">convert</code> command:</p> <pre class="code shell-session"><a name="rest_code_d46d89b41526444991162a5a31a9a6ad-1"></a><span class="gp">$</span> convert photo.jpg -colorspace Gray out.jpg </pre><p>However, the result leaves something to be desired. It looks "flat" and boring. Better results can be achieved with this command:</p> <pre class="code shell-session"><a name="rest_code_bb9cbcee42fb4b5e9a5e8645f323615d-1"></a><span class="gp">$</span> convert photo.jpg -type Grayscale out.jpg </pre><!-- TEASER_END --> <p>If you want more control, use parameters to increase contrast. To make the best use of the available range from pure black to pure white, one can <em>stretch</em> contrast. That means the lightest pixels are converted to white and the darkest pixels to black. The other colors are distributed among the available range.</p> <p>The <code class="docutils literal"><span class="pre">-linear-stretch</span></code> option allows a threshold parameter which defines the number of pixels that are allowed to be converted to pure white an d pure black. A higher threshold means that have a greater range of values to depict the midtones at the expense of details in the highlights and/or shadows.</p> <pre class="code shell-session"><a name="rest_code_acc7bea3d75b45ee8455a0e401352a94-1"></a><span class="gp">$</span> convert photo.jpg -linear-stretch 1x1 -colorspace Rec709Luma out.jpg </pre><div class="section" id="examples"> <h3>Examples</h3> <div class="section" id="lock"> <h4>Lock</h4> <div class="figure"> <img alt="Original photo" src="https://akeil.de/images/grayscale/01.jpg"> <p class="caption">The original photo.</p> </div> <div class="figure"> <img alt="Photo converted to grayscale with default settings." src="https://akeil.de/images/grayscale/01-csgray.jpg"> <p class="caption">Simple conversion to grayscale colorspace loses most of the detail in the dark areas.</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_9b245e717fd94522b1a7416c5c64a07d-1"></a><span class="gp">$</span> convert photo.jpg -colorspace Gray out.jpg </pre></div> </div> <div class="figure"> <img alt="Photo with conversion after increased contrast" src="https://akeil.de/images/grayscale/01-csgray+contrast.jpg"> <p class="caption">Increasing contrast before conversion makes some of the details and textures stand out more. The effect of pure black and pure white pixels is clearly visible.</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_e2f59f57b34c4c9ab82e737e1f8f1dd6-1"></a><span class="gp">$</span> convert photo.jpg -contrast-stretch 4x4 colorspace Gray out.jpg </pre></div> </div> <div class="figure"> <img alt="Using a non-linear colorspace and linear stretch" src="https://akeil.de/images/grayscale/01-csluma+stretch.jpg"> <p class="caption">Linear-stretch and non-linear colorspace gives a balanced result with good textures.</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_d931a24505204bf881541a869a37e596-1"></a><span class="gp">$</span> convert photo.jpg -linear-stretch 1x1 -colorspace Rec709Luma out.jpg </pre></div> </div> </div> <div class="section" id="window"> <h4>Window</h4> <div class="figure"> <img alt="Original photo" src="https://akeil.de/images/grayscale/02.jpg"> <p class="caption">The original photo.</p> </div> <div class="figure"> <img alt="Photo converted to grayscale with default settings." src="https://akeil.de/images/grayscale/02-csgray.jpg"> <p class="caption">Simple conversion to grayscale colorspace clearly looks less interesting than the color version.</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_7674be51403b417fb7db6827e023479c-1"></a><span class="gp">$</span> convert photo.jpg -colorspace Gray out.jpg </pre></div> </div> <div class="figure"> <img alt="Photo with conversion after increased contrast" src="https://akeil.de/images/grayscale/02-csgray+contrast.jpg"> <p class="caption">Increase contrast before conversion:</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_814c906727ea459e9b5b71491f5e3938-1"></a><span class="gp">$</span> convert photo.jpg -contrast-stretch 4x4 colorspace Gray out.jpg </pre></div> </div> <div class="figure"> <img alt="Using a non-linear colorspace and linear stretch" src="https://akeil.de/images/grayscale/02-csluma+stretch.jpg"> <p class="caption">linear-stretch and non-linear colorspace:</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_5fb315602cbb40799eb88d1e32f4bf9c-1"></a><span class="gp">$</span> convert photo.jpg -linear-stretch 1x1 -colorspace Rec709Luma out.jpg </pre></div> </div> </div> <div class="section" id="doors"> <h4>Doors</h4> <div class="figure"> <img alt="Original photo" src="https://akeil.de/images/grayscale/03.jpg"> <p class="caption">The original photo.</p> </div> <div class="figure"> <img alt="Photo converted to grayscale with default settings." src="https://akeil.de/images/grayscale/03-csgray.jpg"> <p class="caption">Simple conversion to grayscale colorspace loss of details is visible at the top areas of the doors.</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_46a29de6d4604fa19c9fbec911979ad5-1"></a><span class="gp">$</span> convert photo.jpg -colorspace Gray out.jpg </pre></div> </div> <div class="figure"> <img alt="Photo with conversion after increased contrast" src="https://akeil.de/images/grayscale/03-csgray+contrast.jpg"> <p class="caption">Increase contrast before conversion reveals more details on the doors. The wall looks "whiter".</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_25d6ee3ac5d14d61ac37ffcb3568f3dc-1"></a><span class="gp">$</span> convert photo.jpg -contrast-stretch 4x4 colorspace Gray out.jpg </pre></div> </div> <div class="figure"> <img alt="Using a non-linear colorspace and linear stretch" src="https://akeil.de/images/grayscale/03-csluma+stretch.jpg"> <p class="caption">Linear-stretch and non-linear colorspace shows almost no difference to the simpler contrast stretch.</p> <div class="legend"> <pre class="code shell-session"><a name="rest_code_1ed319b52eed48b19c9153090f6a6be3-1"></a><span class="gp">$</span> convert photo.jpg -linear-stretch 1x1 -colorspace Rec709Luma out.jpg </pre></div> </div> <hr class="docutils"> <dl class="footnote brackets"> <dt class="label" id="id1"><span class="brackets"><a class="fn-backref" href="https://akeil.de/posts/grayscale-images-with-imagemagick/#id2">1</a></span></dt> <dd><p><a class="reference external" href="http://www.imagemagick.org">http://www.imagemagick.org</a></p> </dd> </dl> </div> </div> </div>commandlineimagemagickphotohttps://akeil.de/posts/grayscale-images-with-imagemagick/Sat, 10 May 2014 09:45:00 GMT