You may think this is a simple operation and it is. So simple, that IM provides a huge number of ways and methods of actually doing this task. So many that I needed to give it its own page of examples just to demonstrate them all. Crop cutting images down to size Crop and Canvas Page The " -crop " image operator will simply cut out the part of all the images in the current sequence at the size and position you specify by its geometry argument.
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You may think this is a simple operation and it is. So simple, that IM provides a huge number of ways and methods of actually doing this task. So many that I needed to give it its own page of examples just to demonstrate them all. Crop cutting images down to size Crop and Canvas Page The " -crop " image operator will simply cut out the part of all the images in the current sequence at the size and position you specify by its geometry argument. The actual image itself has been cropped, and may be smaller, but the canvas on which the GIF image is displayed is still the same size as the original canvas.
You will also notice that the size of the actual image produces may not be the actual size you requested from the crop. It could be very much smaller that what you expected, as the crop itself was either partially or fully outside the actual image area being cropped. That is the image contents itself does not move, even though the actual image itself is smaller.
This means if you now modify the smaller image, then overlay the image using image layering operators back over the original, it will fit back exactly where the sub-image originally came from.
This is especially important for the correct working for GIF animation handling. For more information on this see. Deconstruct GIF Animations. JPEG is a typical example of a format that removes this info. IM will also add a small amount of meta-data to preserve the virtual canvas size for later use by other IM commands.
The GIF disposal method however may be modified to ensure animations remain correct, after cropping. This tells IM to not output informational warning messages, only real errors. However such a method has been proposed for a future release of IM. Mail me if you find you need such a method. Crop an image with existing Virtual Canvas If a image already has an existing Virtual Canvas , for example a frame of a GIF animation , then the " -crop " operation will be applied relative to the virtual canvas, and NOT to the actual image.
That is it will try to preserve the offset of the actual pixel data of the cropped image on the canvas. That is a specific pixel before the crop should still be located at the same offset relative to the virtual canvas afterward. Here we create an image centered on a page canvas, and we crop it in various ways. As before the canvas size itself is not modified by the operation.
Note that I suppressed the normal warning message from IM using a " -quiet " setting. Just so you can see just what is going on, lets have a closer look at the paged crop of the lower right corner of the image.
Here I have drawn a semi-transparent square over the area that was cropped. Even though the crop is contained completely in the page canvas, the crop did not cover the actual image completely. The result is that the actual image is smaller than the user may have intended, but still positioned on a larger canvas or page. It is actually such a common thing to do that you could call it a rule of thumb. Unless you actually need to preserve that info.
The last image in the above is of course the special crop missed image , but I supressed the warning message using the " -quiet " operational setting. But that was only usable when either reading or writing the image to a file, preventing its use between multiple image processing operations. As such separate " -repage " and " -set page" operators were added to allow users to set or change the virtual canvas information.
In other words, regardless of the resulting size of the actual image cropped, the canvas and offset of the image returned will be adjusted to match the area you requested cropped. For example Notice how the canvas size of the image returned now matches the area in which the image was cropped. However any partial or missed cropping of the image, the result will a larger canvas and a possible offset to the resulting image. For example when Padding out an Image.
Without this option cropping a GIF animation is very difficult requiring external correction of the image canvas size and offsets. For and example of this, see Animation Crop, with the canvas too.
See the appropriate sections below. Crop relative to Gravity The offset position of the " -crop " by default is relative to the top-left corner of the image. However by setting the " -gravity " setting, you can tell " -crop " to cut the image relative to either the center, corner, or an edge of the image. This is done with all gravity effected operations. Crop a Percentage of an Image The " -crop " operator also understands how to crop an image to just a percentage of its original size.
For example this will half the size of the image. The offset is not optional. Note that while the size can be a percentage the offset will always be in pixels. You can not provide an offset as a percentage of the image size. This is especially important when only a single number is provided for both width and height of the crop area. More commonly a percentage crop is done from the center of an image.
Offsets are always given in pixels. Caution however is still recommended, for input images that may also have virtual canvas size and offsets. That is you give a size, and not a position within the image to crop. In this case instead of generating just one image, crop generates a whole series of images.. This is because in one special case centered percentage crop the gravity setting can turn off tile cropping.
Other effects of gravity on tiled cropping is also undefined. By using " -mosaic " or " -flatten " image layering operators, see Layer Flatten you can layer these images all on top of each other, restoring the original image. An alternative is to use " -layers merge" see Layer Merge , which merges the multiple layer images into a new layer image just large enough to hold all the given images. That is the virtual canvas is not filled in, as " -mosaic " or " -flatten " layering methods would.
You will need to tell montage how many rows or columns of images were extracted from the original image. This is not very nice as the filenames give no easy indication of the actual position each tile belongs to, or the total number of tiles per row and column. As of IM v6. Tricky but it does work. Unfortunately you can not format the number generated by a Percent Escapes , to include padding with zeros or specify an exact number of floating point digits.
At least not at this time. By doing this we maximize the effect of the complete tiles by placing them in the center of the image. Of course you also end up with more incomplete edge tiles.
The solution was to center the input image on a virtual canvas, that was some multiple of the tile size wanted.
For example to tile crop the "rose:" image 70x46 pixels in size into the maximum number of full 30x20 tiles, in the center of the image and surrounded by reminder tiles you would do the following. First calculate the number of full tiles you can get from the image by dividing the image sizes And here we use the above calculations perform a Centered Tile Crop If you did not add two columns and rows to the number of tiles and thus generate a negative offset you can effectively ignore the remainder edge tiles, and only output full and complete tiles that exist in the image.
An alternative for a centered tile crop and ignoring reminders is to simply replace the virtual canvas setting with an appropriate crop. However less calculations are needed. But if you want to recover the original tile offsets, you will still need to calculate the relative offset you needed, in which case you may as well do the task by the former faster method using the images virtual canvas.
Strip Cropping, cropping out rows and columns With IM version 6. In most cases this is large enough to cover the image located on the canvas, if the related offset is also set to zero. This is why the first and last actual image generated in the above example is only 10 pixels wide. The warning that IM would have normally produced was suppressed using a " -quiet " setting. This is not recommended unless you are expecting such an event, and are prepared for it, such as in a script.
It is possible for an image to be positioned such that it does not even appear on its own page or virtual canvas, or be so large that the page canvas can only contain a small window or part of the image. In such rare cases, strip cropping without any size arguments will get the image sub-division wrong, and produce respectively, missed images , or smaller tiles of only the parts within the virtual canvas bounds.
The " -crop " operator however will not be fixed to handle these rare special cases, as doing so will prevent its use in other cases, such as those exampled below. A simple option lets you remove those gaps.
Quadrants, cutting around a single point As any of the crop size numbers are missing then they are replaced with the size of the image canvas of the image you are cropping. This should in most cases result in the whole of the image in that dimension becoming part of the crop result. This allows, with cautious use of the arguments, is the ability to crop and image into quarters around a specific point with that specific pixel placed as the top-right pixel of the bottom-left quadrant image.
You do not need to know how big the image is to do this. Using Negative Offsets, remove bottom or left edge Their is no reason that you can not use a negative offset with " -crop ". In fact at times it can have very definite benefits. For example lets take our paged rose image and progressively crop it with larger negative offsets.
We will not supply a image size to " -crop " argument, so it will default to the images canvas size. If we took this one step further a Missed Image will be generated. See Chop, Removing Edges. Cropping into roughly Equally Sized Divisions The biggest problem with Tile Cropping is that you can only define the final size of each tile. For example lets again divide the built in rose image, but try to divide it into 3x3 tiles. The original image is 70x46 pixels so divided by 3 make each tile 23x15 pixels This situation gets worse as the number of tiles wanted gets larger.
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Virtual pixel support convenient access to pixels outside the image boundaries. Examples of ImageMagick Usage shows how to use ImageMagick from the command-line to accomplish any of these tasks and much more. With Magick. News ImageMagick best practices strongly encourages you to configure a security policy that suits your local environment. We are utilizing a new community discussion service. We will continue to support the previous discourse server for a minimum of one year. We discovered a bug in the pseudo-random generator prior to ImageMagick 7.
Lossless compression of alpha is achieved using a value of , while the lower values result in a lossy compression. The default is A value of 0 turns off any filtering. Higher values increase the strength of the filtering process applied after decoding the image.