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<div>
<h1>Liquid Rescale GIMP Plug-In version 0.7 help page</h1>
</div>
<p>
<div>
The Liquid Rescale GIMP Plug-In is an implementation of the content-aware resizing by
seam carving algorithm by Shai Avidan and Ariel Shamir.
</div>
</p>
<p>
<div>
It aims at resizing pictures non uniformly while preserving the features of the picture,
i.e. avoiding distortion of the important parts of the picture.
It can also be used to remove portions of the picture in a consistent way.
</div>
</p>
<p>
<div>
It works both ways, shrinking and enlaging (enlarging is performed in successive steps), and
it can use extra layers as masks to select which features of the image should be
preserved and which should be discarded.
</div>
</p>
<p>
<div>
See also <a href="#seam_carving">Appendix: The seam carving technique in brief</a> at the end
of this page for an introduction to seam carving concepts.
</div>
</p>
<p>
<div>
The plugin works on the active layer or floating selection. It can also operate on multiple layers
through the <i>"Filter all layers"</i> plugin and using the GIMP Animation Package facilities for
video editing (see <a href="#GAP">this section</a> for details).
</div>
</p>
<p>
<div>
The plugin only works on whole layers, if a selection is present it is saved to a channel and removed.
If the layer has a transparency mask, an option is given to select the behaviour (apply/discard).
</div>
</p>
<p>
<div>
The plugin can also be operated in batch mode from the command line, see
<a href="#batch">this section</a> for details.
</div>
</p>
<h2>User interface description (non-interactive mode)</h2>
<p>
<img title="Dialog example" alt="Dialog example" src="images/dialog1.png">
</p>
<h3>Select new width and height</h3>
<p>
<div>
In this section it is possible to choose the final size.
It is advisable to rescale always in one direction at a time. If both
the width and the height are changed, rescaling is performed by default on the
width first, then on the height. This can be changed in the <i>"Advanced"</i> tab.
</div>
<div>
The two small buttons on the right can be used to reset the size parameters
to the image size (top) or set them to the last used values (bottom), while the
bigger button to the right switches to the interactive mode (see the
<a href="#interactive">corresponding section</a> for more details).
</div>
</p>
<p>
<div>
Note that the final size can be set automatically when removing objects, see below.
</div>
</p>
<h3>Feature masks</h3>
<h4>Quick guide</h4>
<p>
<div>
The easiest way to manually select the features of the image that you want to protect
or discard is the following:
<ul>
<li>
Select the layer you want to resize (not a floating selection) and invoke the plugin
</li>
<li>
Press the <i>"New"</i> button in the <b>"Feature preservation mask"</b> section
(this will create a transparent layer, with 50% opacity, and pick an appropriate
colour; also, the plugin main dialog will be substituted by a small dialog).
</li>
<li>
Paint on the areas of the image you want to preserve, e.g. with the paintbrush
</li>
<li>
When you're done, press OK in the small window: the main dialog will appear again.
</li>
<li>
Repeat everything for the features you want to discard
</li>
</ul>
</div>
</p>
<p>
<div>
If you want to remove portions of the image, you can press one of the two
buttons besides the <i>"Auto size"</i> label to determine
the final size automatically (this is only reliable when dealing with very simple
masks).
</div>
</p>
<p>
<div>
<b>Note #1:</b> object removal is only possible when shrinking. By default,
feature discard masks are ignored when enlarging, because in that case the masked
areas would be inflated rather then removed. If you actually want to get this effect,
you need to unset the corresponding option in the <i>"Advanced"</i> tab (or use
interactive mode).
</div>
</p>
<p>
<div>
<b>Note #2:</b> preservation of features is not possible if enlarging too much,
because the inflation process is the exact reverse of the shrinking process,
so the maximum amount of pixels you can add to a layer corresponds to the
amount of pixels which are not protected. For example, if you have a 1000 pixel
wide image and you have marked a 800 pixel wide area for protection, the final
width should be less than 1200 pixels. If you want to enlarge more, do it in
steps, or set the <i>"Max enlargment per step"</i> option in the advanced settings
to an appropriate value.
</div>
</p>
<h4>Full description</h4>
<p>
<div>
In the <b>"Feature preservation mask"</b> section
it is possible to specify if there are regions of the image which
should be preserved from modification, by checking the <i>"Activate feature preservation"</i>
box, and by choosing a layer among the ones which belong to the current image (excluding the active one)
in the <i>"Available layers"</i> menu.
The <i>"New"</i> button creates a transparent layer with 50% opacity and an appropriate name,
sets it as the selected layer mask and switches to a "mask editor" mode, in which there only is a reduced,
always-on-top dialog, and in which the foregroud colour is set to green. When you exit the "mask editor" mode,
the previous foreground colour is restored and the plugin main dialog is presented again with the updated
information.
The <i>"Edit"</i> button invokes the "mask editor" mode over the currently selected preservation mask.
Note that the "mask editor" mode is useless if the plugin is invoked on a floating selection unless a new layer
is created from it, because GIMP does not allow to work on other layers in such case.
</div>
<div>
The intensity of the effect of the mask on each pixel is obtained as
the average on the pixel's colour channels, multiplied by the alpha channel. Thus, for an RGB layer
with an alpha channel, the maximum value is achieved on white pixels, while black ones
and transparent ones are ignored.
In general, it is advisable to use a single colour for each mask, and, if needed, fine-tune the strenght
of the mask for different areas by using the transparency value, instead of lightness or saturation.
</div>
<div>
The size and position of the selected layer can be arbitrary, the plugin will only use
those portions which overlap with the active layer.
The <i>"Strength"</i> scale can be used as a global parameter to modulate the feature preservation.
Note that thinner selection areas (in the sense of the resizing) need higher strength values, and
vice-versa.
</div>
<div>
When a preservation mask is selected, the small green icon under the layer thumbnail on the left will
become active, and its tooltip will give you the name of the selected preservation layer. The remaining two
icons serve the same purpose for the discard mask (red) and the rigidity mask (blue).
</div>
</p>
<p>
<div>
The <b>Feature discard selection</b> section is nearly identical to the previous one,
but the effect is reversed, and the default colour for the <i>"New"</i> and <i>"Edit"</i> buttons is red instead of green.
</div>
<div>
Furthermore, there are two additional buttons besides the <i>"Auto size:"</i> label, which can be used for
"quick & dirty" object removals. When one of these buttons is pressed, the plug-in tries to compute the exact
amount of rescaling which is needed in order to remove the areas of the image marked by the current discard mask,
and sets the final size accordingly (in the <b>"Select new width and height"</b> section).
These buttons will always produce a single-direction rescale: the <i>"Width"</i> button changes just the width
and sets the height to its original value, the <i>"Height"</i> button does the opposite.
</div>
<div>
Note that this option is only suitable for naïve object removals, and it may fail if the masks are
non-trivial.
</div>
<div>
<i>[Technical information: the final size is determined by simply finding out the maximum number of
pixels in a line for which the mask value is above a given threshold, which is set to the 50% of a full
RGB colour (e.g. red in the default setting).]</i>
</div>
</p>
<h3>Output</h3>
<p>
<img title="Output" alt="Output" src="images/dialog2.png">
</p>
<p>
<div>
This tab has a number of options related to the output of the plugin.
</div>
</p>
<p>
<div>
<dl>
<dt>Output target</dt>
<dd>
Specifies if the resized image should be written on the currently selected layer, or on
a new one, or on a new image. In the latter case, the newly created image will contain all the
output from the plugin, including the rescaled auxiliary layers and the seams map (see below).
By default, it is set to the selected layer.
</dd>
<dt>Resize image canvas</dt>
<dd>
Specifies whether the image canvas should be resized to meet the new size of the active
layer. By default, it is on.
</dd>
<dt>Resize auxiliary layers</dt>
<dd>
Specifies whether the layers used to manually select the features of the image
or to specify a rigidity mask will be resized along with the active layer.
If this option is checked, those layers will be first cropped to the active layer's size,
then they will undergo the same seam carving and inserting process as the active layer.
By default, it is on, but it is only active when some layers are selected in
the <i>"Feature masks"</i> tab or if a rigidity mask is being used (see below).
</dd>
<dt>Output the seams on a new layer</dt>
<dd>
Specifies whether to create a new layer with the seams map in interactive mode.
The two buttons on the right of it let the user choose the seams colour range.
The seams are computed on the original layer as far as they are needed for the new size
(e.g. there will be 100 seams if reducing or enlarging by 100 pixels), the remaining
areas are transparent. In order to read the result, use this option with the
<i>"Output target"</i> set to <i>"new layer"</i>, and overlay the seams map to the original layer.
If you rescale in two directions at once, you will obtain two maps, but the second
one will be computed over an invisible intermediate image, so this is not very useful.
The same applies in the case you choose to rescale back to the original size with LqR,
or when the resizing goes beyond the limit set with <i>"Max enlargment per step"</i>.
Note that this option is ignored in interactive mode, except for the colours you choose,
which are used when dumping the seam maps manually.
</dd>
<dt>Scale back to the original size</dt>
<dd>
Specifies whether to perform a scale back operation after the liquid rescale operation
has been performed.
This can be useful for example for object removal and content enhancement.
If this option is checked, a menu appears below it to let the user
choose the details of the operations: it is possible to revert to the opriginal width
and height using either liquid rescale or the standard scaling algorithm, or it is
possible to perform a uniform scaling with the standard algorithm and reach back only
the original width, or only the original height.
Note that this option is ignored in interactive mode.
</dd>
</dl>
</div>
</p>
<h3>Advanced</h3>
<p>
<div>
This tab has some options which can be used to tweak the plugin behavior.
</div>
</p>
<h4>Seams control</h4>
<p>
<img title="Seams control" alt="Seams control" src="images/dialog3.png">
</p>
<p>
<div>
<dl>
<dt>Max transversal step</dt>
<dd>
<div>
This option lets you choose the maximum transversal step that the pixels in the seams can
take. In the standard algorithm, corresponding to the default value step = 1, each pixel
in a seam can be shifted by at most one pixel with respect to its neighbors.
This implies that the seams can form an angle of at most 45 degrees with respect
to their base line.
</div>
<div>
Increasing the step value lets you overcome this limit, but may lead to the
introduction of artifacts. In order to balance the situation, you can use
the rigidity setting.
</div>
</dd>
<dt>Overall rigidity</dt>
<dd>
<div>
Use this value to give a negative bias to the seams which are not straight. May be useful
to prevent distortions in some situations, or to avoid artifacts from pixel skipping
(it is better to use low values in such case). This setting applies to the whole
selected layer if no rigidity mask is used.
</div>
<div>
<i>[Technical information: the bias is proportional to the difference in the transversal
coordinate between each two successive points, elevated to the power of 1.5, and summed up
for the whole seam.]</i>
</div>
</dd>
<dt>Use a rigidity mask</dt>
<dd>
<div>
It is possible to specify different rigidity values for different areas of the image,
using an auxiliary layer, by following the same procedure used to specify the feature masks
(the default colour for the "Mask editor" mode is blue in this case).
If a rigidity mask is specified, its value at each pixel (computed in the same way as
for the feature masks) will be used as a rigidity coefficient, to be multiplied with the
overall one.
</div>
<div>
Note #1: transparent (or black) areas in the rigidity mask always correspond to
0 rigidity, no matter what the overall value is.
</div>
<div>
Note #2: rigid areas have a slight repulsive effect on seams, similar to that
of a preservation mask. In order to compensate for this, you may use a discard mask.
</div>
</dd>
<dt>Gradient function</dt>
<dd>
It is possible to choose which function
is to be used to calculate the energy map of the image. For each pixel in the image,
the x,y components of the gradient are computed from the pixel's four nearest neighbors.
The exact form of the energy map depends on the choice of the gradient function, and
this in turn determines which elements of the image are more important, and should
thus be preserved, and which are not, and can be removed (if shrinking) or inflated
(if enlarging). Using the <i>Null</i> function in conjunction with the feature preservation
mask can be used to override completely the automatic feature detection, thus
allowing the user to generate his own energy map (e.g. by another plugin).
</dd>
</dl>
</div>
</p>
<h4>Operations control</h4>
<p>
<img title="Operations control" alt="Operations control" src="images/dialog4.png">
</p>
<p>
<div>
<dl>
<dt>Max enlargement per step</dt>
<dd>
This option sets the maximum enlargement (in percent) which will be performed in a
single rescale step. When the final size is greater than this, the plugin will automatically
stop and restart the rescaling as many times as needed. You might need to reduce this
value if you have large preservation masks (but then the same areas will be affected
over and over by the rescaling).
</dd>
<dt>Rescale order</dt>
<dd>
When rescaling in both directions at the same time, it can be chosen whether to resize
first in the horizontal and then in the vertical direction, or vice versa.
</dd>
<dt>Ignore dicard mask when enlarging</dt>
<dd>
The effect of the discard mask is reversed when the plugin is used for enlarging an image;
as this is normally undesirable, they are ignored by default when the
first scaling direction is to be enlarged (in such cases, a small warning icon will
appear in the <i>"Feature discard mask"</i> section).
Unset this option if you want to override this behaviour.
Note that this option is ignored in interactive mode.
</dd>
</dl>
</div>
</p>
<a name="GAP">
</a>
<h3>Using the plugin for video and animation editing</h3>
<h4>Using the plugin with GAP</h4>
<p>
<div>
The plugin can take advantage of the GIMP Animation Package (GAP) for video editing,
making it possible to apply liquid rescaling to all the frames of a video (please refer
to the GAP documentation for instructions about video editing in GIMP).
</div>
</p>
<p>
<div>
Note however that the GAP facilities only allow to select the working layer, but the
liquid rescale plugin needs auxiliary layers as well to work at its full potential.
In order to work around this problem, auxiliary layers are set according to their names:
therefore, if you want to use a preservation mask in each frame, for example, you should
take care that all the layers which you intend to use as such have the same name in all
of the frames. The same applies to discard masks and rigidity masks.
</div>
</p>
<p>
<div>
In order to use liquid rescale on all frames, you need select <i>"Frame Modify"</i> in
the <i>"Video"</i> menu; this will open a dialog: in the <i>"Function"</i> section select <i>"Apply filter
on layer(s)"</i>, then set appropriate rules for layer selection and frame range and
click OK. Another dialog will appear, asking you to choose a filter: select
<i>plug-in-lqr</i>, then choose either <i>"Apply constant"</i> or <i>"Apply varying"</i>.
</div>
<div>
In the latter case, you will be presented the liquidrescale plugin dialog twice, once
for the first frame and once for the last one: these will set the extreme values for
all the graded parameters (such as sizes, strengths, rigidity etc.), and
intermediate values will be used in the intermediate frames (for example, if you set
the width to 100% for the first frame and 110% for the last one, and there are 11 frames
overall, intermediate frames will have widths of 101%, 102%, 103% and so on). However, all the
on/off options (such as the resize order, the auxiliary layer names etc.)
will be taken from the <u>last</u> dialog for all the intermediate frames.
</div>
</p>
<h4>Applying the plugin to all layers (e.g. for animations)</h4>
<p>
<div>
It is also possible to apply the plugin to all the layers in a given image (for example
when editing an animation), using the option <i>"Filter all layers..."</i> from the
<i>"Filters"</i> menu and choosing <i>"plug-in-lqr"</i> in the dialog which appears.
Operations follow the rules explained in the GAP section; however, you will run into
problems if using auxiliary layers as masks, since the plugin will be applied
to those as well.
</div>
</p>
<a name="interactive">
</a>
<h2>User interface description (interactive mode)</h2>
<p>
<img title="Interactive interface" alt="Interactive interface" src="images/dialog_interactive.png">
</p>
<p>
<div>
Pressing the interactive button in the main plugin dialog switches the plugin to interactive mode, and
applies the current settings, including the current size settings. In interactive mode only the size
can be modified (and modifications will be applied immidiately), and some options are ignored:
<ul>
<li>Output the seams (you have to do it manually, see below)</li>
<li>Scale back to the original size</li>
<li>Ignore discard mask when enlarging</li>
</ul>
Use the <i>"Back"</i> button to get back to the main dialog.
</div>
</p>
<h3>Select new width and height</h3>
<p>
<div>
This section includes the controls for the layer size. The application of the changes is almost immediate, but
only after the buttons are released; this means that, if you have activated the chain button to preserve the
aspect ratio, the results will be different if you do many small steps or you do the rescaling in one sweep.
The reset button on the right brings the size back to the initial value: this recovers the original image if
the map was never reset, but it will produce a different image otherwise (see also the Map section below).
</div>
</p>
<h3>Map</h3>
<p>
<div>
The additional <i>"Map"</i> section in the dialog is about the internal seam map which the plugin uses
for its computations. Any map has an orientation (depending on the scaling direction), a reference
value (the one it was initialized with) and a range. The map allows to rescale almost in real-time within
its given range and along its direction; going beyond the current range will produce the updating of the map.
The maps store all the scaling information about the specified range: therefore, going back to the reference
value will reproduce the initial image. However, a new map is generated automatically any time the direction
of the scaling is changed, or when the size exceeds the max enlargment per step; when this happens, all
previous information is lost.
There are three buttons in this section:
<dl>
<dt>Info button</dt>
<dd>Displays or hides information about the internal seam map</dd>
<dt>Reset button</dt>
<dd>
Force the map reset. This is like restarting the plugin from the current situation.
For example, if you change the size, then press this
button, then press the reset button in the size section, you will have the same result as using
the <i>"Scale back to the original size"</i> option (with the <i>"liquid rescale"</i> setting) in
non-interactive mode.
</dd>
<dt>Dump seam map button</dt>
<dd>
This button can be used to view the current seam map. Its effect is the same as that of
the <i>"Output the seams"</i> option in noninteractive mode, only you need to do it manually, and
it prints out one map at a time. The first time you press this button in a session, a new layer will
be crated to hold the map, but if you press it again the new map will take the place of the old one.
In order to make sense of the result, you have to bring the layer you're working on to its reference
size after the dump, and overlay the map on it.
This button is only active for RGB images.
</dd>
</dl>
</div>
</p>
<a name="batch">
</a>
<h2>Using the plugin in batch mode</h2>
<p>
<div>
The plugin comes with some convenience scripts which make it easy to operate it the on
files from the command line.
You can find a description of the scrits arguments by using the <i>"Procedure browser"</i> from
the <i>"Help"</i> menu in GIMP and looking for <i>batch-gimp-lqr</i>, <i>batch-gimp-lqr-full</i> and
<i>batch-gimp-lqr-full-use-id</i>; the meaning of the options should be obvious from knowing how the
standard interface works.
</div>
</p>
<h3>Example</h3>
<p>
<div>
This is an example of how you would invoke the simplest of the scripts, <i>batch-gimp-lqr</i>, from the
Linux / BSD / Mac OS X command line:
</div>
</p>
<p>
<div>
<code>
gimp -i -f -b "(batch-gimp-lqr \"my_image.xcf\" 400 300 \"Backgound\" \"\" \"disc. mask\")" -b "(gimp-quit 0)"
</code>
</div>
</p>
<p>
<div>
This will rescale the "Backgound" layer of the file "my_image.xcf" to 400x300 pixels, using the layer named "disc. mask" as
a discard mask and using no preservation masks (notice the empty pair of quotes after "Backgound").
</div>
</p>
<p>
<div>
On Windows, the command would be the same, but you should substitute <code>gimp</code> with something
like <code>%ProgramFiles%/GIMP-2.0/bin/gimp-2.6.exe</code> (the exact command depends on which version of GIMP you have
installed and its installation folder).
</div>
</p>
<a name="seam_carving">
</a>
<h2>Appendix: The seam carving technique in brief</h2>
<p>
<div>
The plugin works by finding so-called "seams" over an image, i.e. continuous, zig-zagged lines transversing
the image from top to bottom ("vertical" seams), or from left to right ("horizontal" seams). When one such seam,
say a vertical one, is removed from an image (the "carving" operation), the width of the image is reduced by one
pixel. Removing horizontal seams reduces the height. Iterating such operations (find a seam and remove it), one
can reduce the image size at will. Collecting together all the seams which were carved from an image, in their
respective order, constitutes what is called a "seams map".
</div>
</p>
<p>
<div>
Mirroring the carving process, by <i>inserting</i> additional seams besides the ones which are found
by the algorithm, instead of removing them, image enlargement can be obtained, too. The meaning of the seam
map is reversed in this case.
</div>
</p>
<p>
<div>
In order to get good results form this technique, the main issue is finding which are the most suitable seams
to carve or to insert. If the aim is simply to change the proportions of the image without affecting too much the
content, for example, "good" seams will be those which don't cross important features of the image, and instead pass
through a background landscape.
</div>
</p>
<p>
<div>
By default, the plugin tries to find the seams which cross the lowest-contrast areas
(how this happens exactly is specified by the gradient function choice in the <i>"Advanced"</i> tab).
Therefore, each pixel of the image is assigned a so-called "energy value": the higher the contrast, the higher
the energy, and seams are less likely to cross high-energy areas.
</div>
</p>
<p>
<div>
Since in many cases this simple contrast-based method is not optimal, the energy function can be "biased" by
the user, who can decide that some areas should have a higher energy (using a preservation masks) or, on the
contrary, that they should have a lower energy (using a discard mask), and therefore effectively "drive" the
seams and the whole process.
</div>
</p>
<p>
<div>
Rigidity masks work in a similar way, but the energy bias that they add does not only depend on the mask value
over each pixel, but also on how much the seams which try to cross that pixel are straight.
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One intuitive picture is as such: the seams try to get from one side to the other of the image keeping on a
path with an energy as low as possible; preservation masks have the effect of putting a barrier on their path,
which, if high enough, deviates the seams away from it; discard mask, on the other hand, are like valleys which
attract seams. Rigidity masks have the effect of putting barriers besides each seam, which tend to make
it straighter.
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