                                   Tux Paint
                                versión 0.9.33
                           Advanced Stamps 'How-To'

     Copyright © 2006-2024 by Albert Cahalan and others; see AUTHORS.txt.
                             https://tuxpaint.org/

                                 junio 2, 2024

+----------------------------------------+
|           Table of Contents            |
|----------------------------------------|
|   * About this 'How-To'                |
|   * Image choice is crucial            |
|   * Prepare the mask                   |
|   * Replace the fringe and junk pixels |
|   * Save the image for Tux Paint       |
+----------------------------------------+
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About this 'How-To'

    This 'How-To' assumes that you want to make an excellent Tux Paint stamp,
    in PNG bitmapped format, from a JPEG image (e.g., a digital photograph).
    There are easier and faster methods that produce lower quality.

    This 'How-To' assumes you are dealing with normal opaque objects. Dealing
    with semi-transparent objects (fire, moving fan blade, kid's balloon) or
    light-giving objects (fire, lightbulb, sun) is best done with custom
    software. Images with perfect solid-color backgrounds are also best done
    with custom software, but are not troublesome to do as follows.

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Image choice is crucial

    License

        If you wish to submit artwork to the Tux Paint developers for
        consideration for inclusion in the official project, or if you wish to
        release your own copy of Tux Paint, bundled with your own graphics, you
        need an image that is compatible with the GNU General Public License
        used by Tux Paint.

        Images produced by the US government are Public Domain, but be aware
        that the US government sometimes uses other images on the web. Google
        image queries including either site:gov or site:mil will supply many
        suitable images. (Note: the *.mil sites include non-military content,
        too!)

        Your own images can be placed in the Public Domain or a suitable
        license, such as the Creative Commons CC0 by declaring it so. (Hire a
        lawyer if you feel the need for legal advice.)

        For personal use, any image you can legitimately modify and use for
        your own personal use should be fine.

    Image Size and Orientation

        You need an image that has a useful orientation. Perspective is an
        enemy. Images that show an object from the corner are difficult to fit
        into a nice drawing. As a general rule, telephoto side views are the
        best. The impossible ideal is that, for example, two wheels of a car
        are perfectly hidden behind the other two.

        Rotating an image can make it blurry, especially if you only rotate by
        a few degrees. Images that don't need rotation are best, images that
        need lots of rotation (30 to 60 degrees) are next best, and images that
        need just a few degrees are worst. Rotation will also make an image
        darker because most image editing software is very bad about gamma
        handling. (Rotation is only legitimate for gamma=1.0 images.)

        Very large images are more forgiving of mistakes, and thus easier to
        work with. Choose an image with an object that is over 1000 pixels
        across if you can. You can shrink this later to hide your mistakes.

        Be sure that the image is not too grainy, dim, or washed out.

        Pay attention to feet and wheels. If they are buried in something, you
        will need to draw new ones. If only one is buried, you might be able to
        copy the other one as a replacement.

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Prepare the image

    First of all, be sure to avoid re-saving the image as a JPEG. This causes
    quality loss. There is a special tool called jpegtran that lets you crop an
    image without the normal quality loss.

        jpegtran -trim -copy none -crop 512x1728+160+128 < src.jpg >
        cropped.jpg

    Bring that image up in your image editor. If you didn't crop it yet, you
    may find that your image editor is very slow. Rotate and crop the image as
    needed. Save the image — choose whatever native format supports layers,
    masks, alpha, etc. GIMP users should choose "XCF", and Adobe Photoshop
    users should choose "PSD", for example.

    If you have rotated or cropped the image in your image editor, flatten it
    now. You need to have just one RGB layer without mask or alpha.

    Open the layers dialog box. Replicate the one layer several times. From top
    to bottom you will need something like this:

     1. unmodified image (write-protect this if you can)
     2. an image you will modify — the "work in progress" layer
     3. solid green (write-protect this if you can)
     4. solid magenta (write-protect this if you can)
     5. unmodified image (write-protect this if you can)

    Give the work in progress (WIP) layer a rough initial mask. You might start
    with a selection, or by using the grayscale value of the WIP layer. You
    might invert the mask.

    Warning: once you have the mask, you may not rotate or scale the image
    normally. This would cause data loss. You will be given special scaling
    instructions later.

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Prepare the mask

    Get used to doing [Control]-click and [Alt]-click on the thumbnail images
    in the layers dialog. You will need this to control what you are looking at
    and what you are editing. Sometimes you will be editing things you can't
    see. For example, you might edit the mask of the WIP layer while looking at
    the unmodified image. Pay attention so you don't screw up. Always verify
    that you are editing the right thing.

    Set an unmodified image as what you will view (the top one is easiest). Set
    the WIP mask as what you will edit. At some point, perhaps not immediately,
    you should magnify the image to about 400% (each pixel of the image is seen
    and edited as a 4x4 block of pixels on your screen).

    Select parts of the image that need to be 100% opaque or 0% opaque. If you
    can select the object or background somewhat accurately by color, do so. As
    needed to avoid selecting any pixels that should be partially opaque
    (generally at the edge of the object) you should grow, shrink, and invert
    the selection.

    Fill the 100% opaque areas with white, and the 0% opaque areas with black.
    This is most easily done by drag-and-drop from the foreground/background
    color indicator. You should not see anything happen, because you are
    viewing the unmodified image layer while editing the mask of the WIP layer.
    Large changes might be noticable in the thumbnail.

    Now you must be zoomed in.

    Check your work. Hide the top unmodified image layer. Display just the
    mask, which should be a white object on a black background (probably with
    unedited grey at the edge). Now display the WIP layer normally, so that the
    mask is active. This should show your object over top of the next highest
    enabled layer, which should be green or magenta as needed for maximum
    contrast. You might wish to flip back and forth between those backgrounds
    by repeatedly clicking to enable/disable the green layer. Fix any obvious
    and easy problems by editing the mask while viewing the mask.

    Go back to viewing the top unmodified layer while editing the WIP mask. Set
    your drawing tool the paintbrush. For the brush, choose a small fuzzy
    circle. The 5x5 size is good for most uses.

    With a steady hand, trace around the image. Use black around the outside,
    and white around the inside. Avoid making more than one pass without
    switching colors (and thus sides).

    Flip views a bit, checking to see that the mask is working well. When the
    WIP layer is composited over the green or magenta, you should see a tiny
    bit of the original background as an ugly fringe around the edge. If this
    fringe is missing, then you made the object mask too small. The fringe
    consists of pixels that are neither 100% object nor 0% object. For them,
    the mask should be neither 100% nor 0%. The fringe gets removed soon.

    View and edit the mask. Select by color, choosing either black or white.
    Most likely you will see unselected specks that are not quite the expected
    color. Invert the selection, then paint these away using the pencil tool.
    Do this operation for both white and black.

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Replace the fringe and junk pixels

    Still viewing the mask, select by color. Choose black. Shrink the selection
    by several pixels, being sure to NOT shrink from the edges of the mask (the
    shrink helps you avoid and recover from mistakes).

    Now disable the mask. View and edit the unmasked WIP layer. Using the color
    picker tool, choose a color that is average for the object. Drag-and-drop
    this color into the selection, thus removing most of the non-object pixels.

    This solid color will compress well and will help prevent ugly color
    fringes when Tux Paint scales the image down. If the edge of the object has
    multiple colors that are very different, you should split up your selection
    so that you can color the nearby background to be similar.

    Now you will paint away the existing edge fringe. Be sure that you are
    editing and viewing the WIP image. Frequent layer visibility changes will
    help you to see what you are doing. You are likely to use all of:

      + composited over green (mask enabled)
      + composited over magenta (mask enabled)
      + original (the top or bottom layer)
      + composited over the original (mask enabled)
      + raw WIP layer (mask disabled)

    To reduce accidents, you may wish to select only those pixels that are not
    grey in the mask. (Select by color from the mask, choose black, add mode,
    choose white, invert. Alternately: Select all, select by color from the
    mask, subtract mode, choose black, choose white.) If you do this, you'll
    probably want to expand the selection a bit and/or hide the "crawling ants"
    line that marks the selection.

    Use the clone tool and the brush tool. Vary the opacity as needed. Use
    small round brushes mostly, perhaps 3x3 or 5x5, fuzzy or not. (It is
    generally nice to pair up fuzzy brushes with 100% opacity and non-fuzzy
    brushes with about 70% opacity.) Unusual drawing modes can be helpful with
    semi-transparent objects.

    The goal is to remove the edge fringe, both inside and outside of the
    object. The inside fringe, visible when the object is composited over
    magenta or green, must be removed for obvious reasons. The outside fringe
    must also be removed because it will become visible when the image is
    scaled down. As an example, consider a 2x2 region of pixels at the edge of
    a sharp-edged object. The left half is black and 0% opaque. The right half
    is white and 100% opaque. That is, we have a white object on a black
    background. When Tux Paint scales this to 50% (a 1x1 pixel area), the
    result will be a grey 50% opaque pixel. The correct result would be a white
    50% opaque pixel. To get this result, we would paint away the black pixels.
    They matter, despite being 0% opaque.

    Tux Paint can scale images down by a very large factor, so it is important
    to extend the edge of your object outward by a great deal. Right at the
    edge of your object, you should be very accurate about this. As you go
    outward away from the object, you can get a bit sloppy. It is reasonable to
    paint outward by a dozen pixels or more. The farther you go, the more Tux
    Paint can scale down without creating ugly color fringes. For areas that
    are more than a few pixels away from the object edge, you should use the
    pencil tool (or sloppy select with drag-and-drop color) to ensure that the
    result will compress well.

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Save the image for Tux Paint

    It is very easy to ruin your hard work. Image editors can silently destroy
    pixels in 0% opaque areas. The conditions under which this happens may vary
    from version to version. If you are very trusting, you can try saving your
    image directly as a PNG. Be sure to read it back in again to verify that
    the 0% opaque areas didn't turn black or white, which would create fringes
    when Tux Paint scales the image down. If you need to scale your image to
    save space (and hide your mistakes), you are almost certain to destroy all
    the 0% opaque areas. So here is a better way...

    A Safer Way to Save

        Drag the mask from the layers dialog to the unused portion of the
        toolbar (right after the last drawing tool). This will create a new
        image consisting of one layer that contains the mask data. Scale this
        as desired, remembering the settings you use. Often you should start
        with an image that is about 700 to 1500 pixels across, and end up with
        one that is 300 to 400.

        Save the mask image as a NetPBM portable greymap (".pgm") file. (If you
        are using an old release of The GIMP, you might need to convert the
        image to greyscale before you can save it.) Choose the more compact
        "RAW PGM" format. (The second character of the file should be the ASCII
        digit "5", hex byte 0x35.)

        You may close the mask image.

        Going back to the multi-layer image, now select the WIP layer. As you
        did with the mask, drag this from the layers dialog to the toolbar. You
        should get a single-layer image of your WIP data. If the mask came
        along too, get rid of it. You should be seeing the object and the
        painted-away surroundings, without any mask thumbnail in the layers
        dialog. If you scaled the mask, then scale this image in exactly the
        same way. Save this image as a NetPBM portable pixmap (".ppm") file.
        (Note: .ppm, not .pgm.) (If you choose the RAW PPM format, the second
        byte of the file should be the ASCII digit "6", hex byte 0x36.)

        Now you need to merge the two files into one. Do that with the pnmtopng
        command, like this:

            pnmtopng -force -compression 9 -alpha mask.pgm fg.ppm >
            final-stamp.png

