270 lines
14 KiB
Text
270 lines
14 KiB
Text
Tux Paint
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version 0.9.23
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Advanced Stamps HOWTO
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Copyright 2006-2008 by Albert Cahalan for the Tux Paint project
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New Breed Software
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albert@users.sf.net
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http://www.tuxpaint.org/
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About this HOWTO
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This HOWTO assumes that you want to make an excellent Tux Paint stamp,
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in PNG bitmapped format, from a JPEG image (e.g., a digital photograph).
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There are easier and faster methods that produce lower quality.
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This HOWTO assumes you are dealing with normal opaque objects. Dealing
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with semi-transparent objects (fire, moving fan blade, kid's baloon) or
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light-giving objects (fire, lightbulb, sun) is best done with custom
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software. Images with perfect solid-color backgrounds are also best done
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with custom software, but are not troublesome to do as follows.
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Image choice is crucial
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License
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If you wish to submit artwork to the Tux Paint developers for
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consideration for inclusion in the official project, or if you wish to
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release your own copy of Tux Paint, bundled with your own graphics,
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you need an image that is compatible with the GNU General Public
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License used by Tux Paint.
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Images produced by the US government are Public Domain, but be aware
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that the US government sometimes uses other images on the web. Google
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image queries including either site:gov or site:mil will supply many
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suitable images. (Note: the *.mil sites include non-military content,
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too!)
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Your own images can be placed in the Public Domain by declaring it so.
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(Hire a lawyer if you feel the need for legal advice.)
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For personal use, any image you can legitimately modify and use for
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your own personal use should be fine.
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Image Size and Orientation:
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You need an image that has a useful orientation. Perspective is an
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enemy. Images that show an object from the corner are difficult to fit
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into a nice drawing. As a general rule, telephoto side views are the
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best. The impossible ideal is that, for example, two wheels of a car
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are perfectly hidden behind the other two.
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Rotating an image can make it blurry, especially if you only rotate by
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a few degrees. Images that don't need rotation are best, images that
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need lots of rotation (30 to 60 degrees) are next best, and images
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that need just a few degrees are worst. Rotation will also make an
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image darker because most image editing software is very bad about
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gamma handling. (Rotation is only legitimate for gamma=1.0 images.)
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Very large images are more forgiving of mistakes, and thus easier to
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work with. Choose an image with an object that is over 1000 pixels
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across if you can. You can shrink this later to hide your mistakes.
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Be sure that the image is not too grainy, dim, or washed out.
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Pay attention to feet and wheels. If they are buried in something, you
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will need to draw new ones. If only one is buried, you might be able
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to copy the other one as a replacement.
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Prepare the image:
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First of all, be sure to avoid re-saving the image as a JPEG. This
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causes quality loss. There is a special tool called jpegtran that lets
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you crop an image without the normal quality loss. If you want a GUI for
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it, use ljcrop. Otherwise, use it like this:
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jpegtran -trim -copy none -crop 512x1728+160+128 < src.jpg >
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cropped.jpg
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Bring that image up in your image editor. If you didn't crop it yet, you
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may find that your image editor is very slow. Rotate and crop the image
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as needed. Save the image - choose whatever native format supports
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layers, masks, alpha, etc. GIMP users should choose "XCF", and Adobe
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Photoshop users should choose "PSD", for example.
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If you have rotated or cropped the image in your image editor, flatten
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it now. You need to have just one RGB layer without mask or alpha.
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Open the layers dialog box. Replicate the one layer several times. From
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top to bottom you will need something like this:
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1. unmodified image (write-protect this if you can)
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2. an image you will modify - the "WIP" layer
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3. solid green (write-protect this if you can)
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4. solid magenta (write-protect this if you can)
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5. unmodified image (write-protect this if you can)
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Give the WIP layer a rough initial mask. You might start with a
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selection, or by using the grayscale value of the WIP layer. You might
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invert the mask.
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Warning: once you have the mask, you may not rotate or scale the image
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normally. This would cause data loss. You will be given special scaling
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instructions later.
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Prepare the mask:
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Get used to doing Ctrl-click and Alt-click on the thumbnail images in
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the layers dialog. You will need this to control what you are looking at
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and what you are editing. Sometimes you will be editing things you can't
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see. For example, you might edit the mask of the WIP layer while looking
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at the unmodified image. Pay attention so you don't screw up. Always
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verify that you are editing the right thing.
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Set an unmodified image as what you will view (the top one is easiest).
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Set the WIP mask as what you will edit. At some point, perhaps not
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immediately, you should magnify the image to about 400% (each pixel of
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the image is seen and edited as a 4x4 block of pixels on your screen).
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Select parts of the image that need to be 100% opaque or 0% opaque. If
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you can select the object or background somewhat accurately by color, do
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so. As needed to avoid selecting any pixels that should be partially
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opaque (generally at the edge of the object) you should grow, shrink,
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and invert the selection.
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Fill the 100% opaque areas with white, and the 0% opaque areas with
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black. This is most easily done by drag-and-drop from the
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foreground/background color indicator. You should not see anything
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happen, because you are viewing the unmodified image layer while editing
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the mask of the WIP layer. Large changes might be noticable in the
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thumbnail.
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Now you must be zoomed in.
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Check your work. Hide the top unmodified image layer. Display just the
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mask, which should be a white object on a black background (probably
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with unedited grey at the edge). Now display the WIP layer normally, so
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that the mask is active. This should show your object over top of the
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next highest enabled layer, which should be green or magenta as needed
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for maximum contrast. You might wish to flip back and forth between
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those backgrounds by repeatedly clicking to enable/disable the green
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layer. Fix any obvious and easy problems by editing the mask while
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viewing the mask.
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Go back to viewing the top unmodified layer while editing the WIP mask.
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Set your drawing tool the paintbrush. For the brush, choose a small
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fuzzy circle. The 5x5 size is good for most uses.
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With a steady hand, trace around the image. Use black around the
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outside, and white around the inside. Avoid making more than one pass
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without switching colors (and thus sides).
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Flip views a bit, checking to see that the mask is working well. When
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the WIP layer is composited over the green or magenta, you should see a
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tiny bit of the original background as an ugly fringe around the edge.
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If this fringe is missing, then you made the object mask too small. The
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fringe consists of pixels that are neither 100% object nor 0% object.
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For them, the mask should be neither 100% nor 0%. The fringe gets
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removed soon.
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View and edit the mask. Select by color, choosing either black or white.
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Most likely you will see unselected specks that are not quite the
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expected color. Invert the selection, then paint these away using the
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pencil tool. Do this operation for both white and black.
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Replace the fringe and junk pixels:
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Still viewing the mask, select by color. Choose black. Shrink the
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selection by several pixels, being sure to NOT shrink from the edges of
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the mask (the shrink helps you avoid and recover from mistakes).
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Now disable the mask. View and edit the unmasked WIP layer. Using the
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color picker tool, choose a color that is average for the object.
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Drag-and-drop this color into the selection, thus removing most of the
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non-object pixels.
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This solid color will compress well and will help prevent ugly color
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fringes when Tux Paint scales the image down. If the edge of the object
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has multiple colors that are very different, you should split up your
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selection so that you can color the nearby background to be similar.
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Now you will paint away the existing edge fringe. Be sure that you are
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editing and viewing the WIP image. Frequent layer visibility changes
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will help you to see what you are doing. You are likely to use all of:
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* composited over green (mask enabled)
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* composited over magenta (mask enabled)
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* original (the top or bottom layer)
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* composited over the original (mask enabled)
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* raw WIP layer (mask DISABLED)
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To reduce accidents, you may wish to select only those pixels that are
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not grey in the mask. (Select by color from the mask, choose black, add
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mode, choose white, invert. Alternately: Select all, select by color
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from the mask, subtract mode, choose black, choose white.) If you do
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this, you'll probably want to expand the selection a bit and/or hide the
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"crawling ants" line that marks the selection.
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Use the clone tool and the brush tool. Vary the opacity as needed. Use
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small round brushes mostly, perhaps 3x3 or 5x5, fuzzy or not. (It is
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generally nice to pair up fuzzy brushes with 100% opacity and non-fuzzy
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brushes with about 70% opacity.) Unusual drawing modes can be helpful
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with semi-transparent objects.
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The goal is to remove the edge fringe, both inside and outside of the
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object. The inside fringe, visible when the object is composited over
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magenta or green, must be removed for obvious reasons. The outside
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fringe must also be removed because it will become visible when the
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image is scaled down. As an example, consider a 2x2 region of pixels at
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the edge of a sharp-edged object. The left half is black and 0% opaque.
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The right half is white and 100% opaque. That is, we have a white object
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on a black background. When Tux Paint scales this to 50% (a 1x1 pixel
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area), the result will be a grey 50% opaque pixel. The correct result
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would be a white 50% opaque pixel. To get this result, we would paint
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away the black pixels. They matter, despite being 0% opaque.
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Tux Paint can scale images down by a very large factor, so it is
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important to extend the edge of your object outward by a great deal.
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Right at the edge of your object, you should be very accurate about
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this. As you go outward away from the object, you can get a bit sloppy.
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It is reasonable to paint outward by a dozen pixels or more. The farther
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you go, the more Tux Paint can scale down without creating ugly color
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fringes. For areas that are more than a few pixels away from the object
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edge, you should use the pencil tool (or sloppy select with
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drag-and-drop color) to ensure that the result will compress well.
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Save the image for Tux Paint
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It is very easy to ruin your hard work. Image editors can silently
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destroy pixels in 0% opaque areas. The conditions under which this
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happens may vary from version to version. If you are very trusting, you
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can try saving your image directly as a PNG. Be sure to read it back in
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again to verify that the 0% opaque areas didn't turn black or white,
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which would create fringes when Tux Paint scales the image down. If you
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need to scale your image to save space (and hide your mistakes), you are
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almost certain to destroy all the 0% opaque areas. So here is a better
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way...
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A Safer Way to Save:
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Drag the mask from the layers dialog to the unused portion of the
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toolbar (right after the last drawing tool). This will create a new
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image consisting of one layer that contains the mask data. Scale this
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as desired, remembering the settings you use. Often you should start
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with an image that is about 700 to 1500 pixels across, and end up with
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one that is 300 to 400.
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Save the mask image as a NetPBM portable greymap (".pgm") file. (If
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you are using an old release of The GIMP, you might need to convert
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the image to greyscale before you can save it.) Choose the more
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compact "RAW PGM" format. (The second character of the file should be
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the ASCII digit "5", hex byte 0x35.)
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You may close the mask image.
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Going back to the multi-layer image, now select the WIP layer. As you
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did with the mask, drag this from the layers dialog to the toolbar.
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You should get a single-layer image of your WIP data. If the mask came
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along too, get rid of it. You should be seeing the object and the
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painted-away surroundings, without any mask thumbnail in the layers
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dialog. If you scaled the mask, then scale this image in exactly the
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same way. Save this image as a NetPBM portable pixmap (".ppm") file.
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(Note: ppm, not pgm.) (If you choose the RAW PPM format, the second
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byte of the file should be the ASCII digit "6", hex byte 0x36.)
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Now you need to merge the two files into one. Do that with the
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pnmtopng command, like this:
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pnmtopng -force -compression 9 -alpha mask.pgm fg.ppm >
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final-stamp.png
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