Working with images/ Image menu

Most items in the Image menu can be animated. If you have an animation initialized, Howler will ask you if you want to apply the given action to the entire animation.

The swap image
Storing images
The clipboard

Make an image seamless

Expanding dynamic range

Clearing and filling
Swap mixing
The image menu and animation

An image in Howler is always associated with a swap image (often used for a scratch pad) and an alpha channel. The alpha channel is an 8 bit image that can contain a selection or mask that can be used like a template or artistic friskette.

The Swap image. It's like the back side of your paper!

The swap image can be useful in many ways. You can use it to store a spare copy of an image, or to store a checkpoint for your work. You can rub one side through to the other, or you can use it with some filters to displace, merge, composite, or generally modify your main image.

To swap images, just press the j key. “j” is for “jump.”

Pressing the J (capital j) key will copy the current image to the swap image.

The current swap status is displayed in the main title bar. It appears as either (Main) or (Swap) just before the zoom indicator.

There are menu items under the Image menu for managing your image, including functions for copying the image to and from the alpha channel.

Swap images swaps between the Main and Swap images
Copy to swap copies the current image to the swap image
Copy swap image to alpha copies the swap image to the Alpha channel, turning it into a selection mask.
Copy alpha to swap image copies the image in the alpha channel to the swap image.

These functions are helpful when you need to work on a selection that is comprised of a bitmap image instead of a primitive shape, such as a rectangle or ellipse. Another option is to paint directly on the image using “Paint on alpha” under the selection menu.

*When you copy the swap image to the alpha channel, it is converted to grayscale.

Storing images for later use.

*Storing an image does not save it to disc...

Under the Image menu, you will see the option to Store an image.

Generally since Howler is a dedicated painting application, you work on one image at a time, however should you wish, you can store an image or a number of images to access at a later time.

Storing an image puts it into it's own window that can be minimized for later use. To restore it later, all you have to do is click on the thumbnail.

Storing an image has many other options in addition to just restoring a previous copy. You could also use several combine modes to combine your images in interesting ways. You could just restore the alpha channel (selection) that was associated with the stored image. You can convert the stored image into a brush, or copy it to the swap image, or you can use one of several different color channels from the stored image.

By clicking on the Functions button, you get a menu of options.

Replace image replaces the current image with the one that was stored. If your image has changed size since the stored image was stored, it will be cropped into the new size.

The combine menu lets you combine the stored image with the current image in a number of different ways.

Replace alpha replaces the current alpha with the one that was stored with the image.

Use image as alpha converts the stored image to a grayscale image and uses it as the alpha channel.

Replace existing image replaces the current image with the stored image, obliterating whatever you are working on. If needed, the image will be resized.

RGB Channels copies any of the Red green or blue channels of the stored image into the main image. Say you only wanted to work with one channel, you could store your image, clear your main image and get just the one channel you wanted to work on. When you are done, you can add in the other two channels.

HSV Channels converts the stored image to HSV space and copies any of the Hue, saturation, or Value channel into the main image as a grayscale image.

YUV Channels converts the stored image to YUV space and copies any of the YUV channels into the main image as a grayscale image.

Working with Red, Green, and Blue channels independently, using a stored image.

As an example, this image was cleaned up extensively by modifying only the blue channel, which contained most of the film grain in the image.

The process was to store the original image and clear the image to black. The blue channel was then added back, and a median filter was applied to it several times to reduce the grain. Then the red and green channels were added back to the image.

Copying images to / from the clipboard

You can copy your entire image to the system clipboard with the Copy to clipboard menu item under the Image menu.

New from clipboard creates a new image from an image contained on the system clipboard, destroying any previous image.

Overlay from clipboard places the contents of the system clipboard into the current image. If the image on the clipboard does not fit the current resolution, it will be cropped.

If you want to place an image from the clipboard into your image manually, one way to do it would be to load it from the clipboard into a custom brush, then stamp it down manually. See the section on custom brushes for more information.

Transforming images

There are several transformation options available from the Image menu. (It's worth noting that all of them also work on entire animations)

Flipping an imagae creates a mirror image.
In the examples, the image is flipped on the horizontal axis, and on the vertical axis.

Rotating an image tilts the image, like turning a photograph sideways by 90 degrees.


Original image

Flip x

Rotate Clockwise

Flip y


Re-sampling, or scaling an image changes the size of the image by stretching it to fit a new size, reducing or adding new pixels as needed. The algorithm used to re-sample the image determines its final quality. While an image with more pixels is generally of higher quality, often large images need to be reduced in size for various reasons, such as memory requirements.

The list on the left of the panel lets you select from a number of preset sizes.  These are image formats often used in the field of video production.  It is possible to edit this list. See the section on preferences for more information.

If you do not want to use one of the preset sizes, you can enter your own image size in pixels into the text boxes on the right.

The Constrain check box forces your new size to match the same aspect ratio as the original image.

The drop-down control lets you select the type of interpolation used to re-sample the image. Since images are composed of a series of pixels, it is necessary to interpolate them to get a smoother result.

The interpolation types are nearest neighbor, bi-linear, bi-linear+, and bi-cubic+.
The "+" in bi-linear+ and bi-cubic+ means that the image is also super-sampled, or multiple samples are taken of every pixel when they are scaled, for a better result. Bi-cubic often produces the best result, and is the default setting. If working on a very large megapixel image, it may be required to use of the other methods, as Bi-cubic is sensitive to overruns on very long lines of pixels.

The flip button controls the orientation of the image by flipping the width and height.

The Other units button allows you to enter units other than pixels, such as inches or metric units.


You can change the image size without scaling the image. This results either in the image being cropped, or in new areas being revealed.

The area at the top shows you the size of your original image, and the middle section lets you set the new size, in pixels.

The Anchor lets you specify where in the new image that the old image should be placed.

Say you wanted to make some room to add some text to the top of a picture. You could add 100 pixels to the height of the image, and change the anchor so the image would keep to the bottom.

The new area will be filled with the secondary color

Crop to Selection

You can crop an image to keep only the part that interests you.

This is like chopping off parts of a photograph with scissors.

To do so, 'select' the area you want to keep with an alpha channel tool, like the Rectangle to alpha tool on the tool panel.

Use it to create a marquis (marching ants) around the area. Select 'Crop to selection' from the Image menu, and your image will be cropped.

Result of cropping.

See the section on the Alpha channel for more information on selections.

The Crop tool

You can also use the crop tool to interactively select the area you want to crop, including removing rotation and skew from the image before cropping.

The corners of the crop tool allow you to scale the area to be cropped.

The circular control on the crop tool allows the tool to rotate to correct for rotation in your image.

The parallelogram control on the crop tool allows you to correct for shear (for example, an inexpensive digital camera may capture a moving image slowly enough that motion is captured as a top-down shearing effect.

Make an image seamless

Sometimes you want to make an image or texture seamless. This can be useful for game textures or image maps for a 3d object. It can also be useful for creating terrain images for the Puppy Ray ray-tracer. No matter what the use, it can be very easy to accomplish with the “Make seamless” command under the image menu.

Here, the Seamless filter is used to create a “tileable” noise pattern (generated with the plasma noise filter.)

The filter trims part of the left and bottom of the image, and blends them with the top and right part of the image. The trim amount can be set for both horizontal and vertical directions separately.

An option button allows you to apply the filter either to the current image, or the current brush. The seamless feature on the Image menu is, in fact, the same one found on the brush menu, but with this option checked, depending on if it was started from the Image menu or the Brush menu.

The “Keep original size” checkbox, if checked, causes the image to be enlarged to fit the original size of the image, to account for the amount that has been trimmed off to blend the edges.

The next few features act like filters to change the appearance of the image . The difference between the features on the image menu, and the ones under the Filter menu is that here, the alpha channel is ignored, and the way the feature handles animation is somewhat different.


The Grayscale menu item converts a color image into a greyscale image. Two modes are Average, and Video weighted, while a third allows exposing the grayscale image as if through a colored lens to create unique effects.

In the first case, the red, green, and blue channels are mathematically averaged with the formula (R+G+B)/3

With Video weighting averaging, the channels are averaged in a way that is more consistent with human vision and video systems. The formula is 0.299 * red + 0.587 * green + 0.114 * blue.

Exposing a grayscale image through a lens

Below, the same color image is converted to 2 different grayscale images, each with a very different look and feel.

Here, a grayscale image is created by “exposing” the color image through a blue lens, greatly deepening the blue sky in the image.

Here, a grayscale image is created by “exposing” the color image through a yellow lens, greatly brightening the blue sky in the image.

Inverting an image

Inverting an image creates a negative image, like you may or may not remember from the old days of film cameras.

Expanding dynamic range

Expanding dynamic range is a quick way to automatically adjust the contrast of an image.

Dynamic range refers to the ratio of the highest (lightest) ,to the lowest (darkest) intensity or value in an image.

In a 24 bit image, with 8 bit per channel, the highest value for any channel is 255, and the lowest value is 0. If the value for a pixel in all three channels is 0, then the color is black. If all the value for all channels is 255, then the color is white.

However, not all imaging devices capture perfect blacks or whites, and may give you an image that looks muddy. Expanding the dynamic range can often make these images look nice.

A histogram shows the effects of expanding dynamic range.  The closer the range of values is to the edges of the histogram, the more contrast your image will have.

Clearing and filling

Clearing an image from the Image menu clears it to the secondary color.

Aside from this, there are several more options if you right click the clear button on the tool panel. You can clear the whole image, or you can clear a 'selection' made with the alpha channel (Selection) tools. You can also clear to the primary color, the secondary color, black, or white from here.

There is also the option to clear all the frames of an animation. They will be cleared to the secondary color.

If you want to fill your image to a specific value, or if you want to fill using any of the Fill tools, then you can use the Fill item under the Image menu. It fills the image, or your current selection using the current fill mode. This mode can be a plain color, a gradient, a pattern from a custom brush, or a number of other effect.  See the section on Fill tools for more information.

Paint fill is a special tool that fills the image or selection with a paint like effect. The values in the alpha channel have an impact on the result of the paint fill.

Adjustable paint fill goes a step further and lets you adjust the effects orientation with a rubber band control.

You can make your paint fill fade out like a brush stroke. The effect is also heavily effected by your current opacity.

Mixing images (Swap mixing)

Very similar to the idea of Layer Mixing, is Swap Mixing. In fact, they are part of the same sub-system, but swap mixing has its own uses if you only want to work with your main and swap image.

See the section on Layer Mixing to learn more.

Swap mixing options can be accessed by right clicking the icon on the top, right corner of the tool panel, under the previews tab. (They are also available on the Layer mixing panel.

*Swap mixing is very similar to layer mixing

Swap mixing activates swap mixing. Your main image will be mixed with the swap image visibly in real-time as you work. The images remain separate, but you can see the results of them being combined on screen.

Swap images (buffers) swaps the main and swap images. It’s the same as pressing the ‘j’ key.

Merge with swap causes the two images to be merged using the current mixing mode. See below for an explanation of the mixing modes

Open layers panel opens the layers panel should you need it.

Set current layers mode lets you change the current layer's mode, or in this case, the mode for the swap mixing. If you where working with layers, you could change the current layers mode.

Set tracing paper opacity lets you change the opacity of the tracing paper mixing mode.

Show alpha causes the image thumbnail to represent the alpha channel.

Note Layers are not saved with a file, so you will have to merge your layers before saving your image.

With this sub-menu, you can set the current mode for swap mixing.

Default mode is Multiply mode.

Additive Adds the RGB values of the main and swap images

Subtractive subtracts the main image from the swap image.

Multiply mode multiplies the pixels of the swap and main images (and divides them by 255.) The result is similar to painting with ink on ink. The result is always darker.

Divide is the complement of multiply. Use it as you see fit.

Screen is similar to additive but keeps an image from ‘blowing out’ or becoming overly bright. It’s often useful for effects like stars and fire.

Around gray combines two colors around their middle brightness level.

Tracing paper lets the swap image show through, a lot like tracing paper.

One way that the swap image is useful; you can use the swap image to displace your main image. See the section on Displacement filters for more information

The Image menu, and working with Animation

Basically, almost anything you do with the image menu can just as easily be done to an entire animation, if you are working with one. If an animation is in use, the program will ask if you want to apply the changes to the whole animation.

Have your recorded an animation, but you want to crop off the edges? Need to scale your animation down to size? Basically anything you can do to a image from the Image menu, you can also do to an animation.

Here, the animation has been inverted.

See the section on the time-line if you want to apply FX filters to an animation.