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ImageMagick Frequently Asked Questions. Jun 1997

ImageMagick Frequently Asked Questions. Jun 1997


Original of this document is at http://www.wizards.dupont.com/cristy/www/Magick.html
  1. I received the following message, "?????? translation failed ...". What does it mean?
  2. How can I improve the appearance of 24-bit images displayed on my colormapped X server?
  3. How do I set the transparency index in a GIF image so it displays properly within Mosaic?
  4. How can I stop the filenames from changing in the title bar of the animate(1) image window?
  5. The image grabbed by import(1) does not look like the image on my X server. What's wrong
  6. How do I animate a digital YUV image sequence?
  7. How do I view only the red channel of an RGB image?
  8. How do I change the default PostScript page size?
  9. When I display or convert an image, I get Memory allocation error. What can I do?
  10. How do I concatenate three images left-to-right with no borders, frames, or text?
  11. How do I create a GIF animation sequence to display within Netscape?
  12. When I display a PostScript image it appears to be trimmed. I would like to display the entire page, including any white space.
  13. What are visual image directories? How do I use them?
  14. I have an HP. How do I take advantage of the Color Recovery option?
  15. I use the window ID reported by xwininfo(1) with import and it does include the window manager frame as expected. How can I save the window with its frame?
  16. I displayed an image and it appears as one solid color. What did I do wrong?
  17. I received the following message, "???? library is not available ...". What does it mean?
  18. I want to inspect the values of the matte channel within my image. How can I do that?
  19. Why was LZW compression removed from the ImageMagick distribution (or why are my GIF images so huge)?
  20. How can I add one of those cool bevels to my image that I see used on the Web?
  21. I try to launch display from my window manager and it fails. What's up?
  22. When I convert Postscript to another format, how can I improve the appearance of the text?
  23. How can I annotate an image with text that is 2 to 3 inches tall?
  24. How can I convert my GIF animation sequence to individual image files?
  25. How can I remove the background that prints around my image when I display it with Netscape?


1. I received the following message, "?????? translation failed ...". What does it mean?

ImageMagick uses several freely available packages to perform the translation of certain image formats (PostScript, PICT, etc.). Make sure these packages are available as described in the README file. Also verify that you have plenty of temporary disk space. If not, set the TMPDIR environment variable to an area where space is available. Finally, for PostScript, verify that Ghostscript supports the pnmraw or ppmraw device (gs -h) and that the document contains valid PostScript statements (gs image.ps).



2. How can I improve the appearance of 24-bit images displayed on my colormapped X server?

If you have a colormapped X server, the number of unique colors in an image must first be reduced to match the requirements of your server before it can be displayed. Display tries to make the color reduced image closely resemble the original. However, there are some options that may improve the appearance.

You can use a private colormap. By default, your image must share colors with existing applications on your X server. The colors in your image may be very different than the shared colors and the results may be visually unsatisfactory. Try

    display -colormap private image.miff
The disadvantage is the annoying colormap loading and unloading as you move your pointer in and out of the image window.

The YCbCr colorspace may give better results when color reducing images of people faces. Try

    display -colorspace YCbCr image.miff

If you intend to view an image frequently, reduce the number of colors to match the resolution of your X server once and write to a new image. Then display this new image. For example,

    convert -colors 256 -dither image.jpeg image.miff
    display image.miff

This forces the time-consuming process of color reducing the image to one-time, rather than each time the image is displayed.



3. How do I set the transparency index in a GIF image so it displays properly within Mosaic?

Display your GIF image with display. Choose Matte from the Image Edit command menu and identify a pixel that has the transparency color. Press Method and select a matte edit method from a pop-up menu. Choose from these methods:

    point
    replace
    floodfill

The point method changes the matte value of any pixel selected with the pointer until the button is is released. The replace method changes the matte value of any pixel that matches the color of the pixel you select with a button press. Floodfill changes the matte value of any pixel that matches the color of the pixel you select with a button press and is a neighbor.

Select your transparent pixel with the pointer and press a button. The image is redisplayed with any transparent pixels recolored to the background color. You can select other pixels or areas to force to transparent. When you are satisfied, press Return.

Finally, choose Save from the command menu and write your GIF image to a file. Note that setting transparency works best on a TrueColor or DirectColor visual. If your server only exports colormapped visuals you will need to use a Standard Colormap to set transparency.

    xstdcmap -best
    display -map list image.gif
      choose Matte Edit
      select your transparent pixel then press Return
      choose Save
If you do not have the xstdcmap(1) program, try

    display -visual TrueColor image.gif


4. How can I stop the filenames from changing in the title bar of the animate(1) image window?

Animate updates the image file name in the title bar of the image window as each image is displayed from the image sequence. To display just a single name that will not change, use -title:

    animate -title "My Image Sequence" images.


5. The image grabbed by import(1) does not look like the image on my X server. What's wrong?

Use the -descend option:

    import -descend image.miff
or set this X resource:

    import.descend:  True

By default, import quickly grabs the image from the X server. However, it may not always have the correct colors in some areas. This can happen when a subwindow has a different colormap than its parent. With -descend, import descends the window hierarchy. Descending involves grabbing the image and colormap of each window or subwindow associated with the window you select and compositing it on a blank canvas. This can be significantly slower than just grabbing the top-level window but ensures the correct image.



6. How do I animate a digital YUV image sequence?

Suppose your sequence is 72 352x240 frames titled frame0.Y, frame0.U, frame0.V, frame1.Y, frame1.U, etc. Use this command:

    animate -geometry 352x240 -scene 0-71 yuv3:frame%d



7. How do I view only the red channel of an RGB image?

Use the -gamma option, for example,

    display -gamma 1.0,0.0,0.0 image.miff



8. How do I change the default PostScript page size?

The default dimensions of a PostScript page is 612x792. If you prefer another default, change the page geometries in magick/image.h and recompile.



9. When I display or convert an image, I get Memory allocation error. What can I do?

The simplest thing to do is to increase your real memory or virtual memory by expanding your swap partition. For ImageMagick to work effectively, you should have at least 24mb of real memory and 48mb of virtual on your computer. And keep in mind processor speed as well. An image operation that takes 5 seconds on my 200mhz Sun Ultra may take 2 minutes on a 486pc.

ImageMagick is designed to be general purpose. It can display many image storage formats (Monochrome, PseudoColor, or TrueColor) on many different types of X visuals (StaticGray, StaticColor, PseudoColor, GrayScale, DirectColor, or TrueColor). To support all these combinations of image storage formats and X visuals, extra memory is required. In fact, each image requires up to 6 bytes per pixel to store in memory. Additionally, animate and montage store an entire image sequence in memory. ImageMagick does reduce memory requirements by compressing pixel information with run-length encoding. However, this is only effective for images with large runs of like pixels such as Postscript or computer generated images. As an example, a 1728x2500 fax image may require only 0.5mb of memory. In contrast a JPEG image this size could require 20mb of memory.

For animation, consider using the MPEG programs from Berkeley if you encounter memory limitations with animate. It only stores a few frames at a time in memory. Convert has an option to create the proper CCIR 601 2:1:1 files as input to the MPEG program. However, the CCIR 601 2:1:1 image format is lossy. Computer generated images may not look very good.



10. How do I concatenate three images left-to-right with no borders, frames, or text?

Assume your three images are called image1.ppm, image2.ppm, and image3.ppm. Type

    montage -mode concatenate -tile 3x1 image1.ppm image2.ppm \
      image3.ppm concatenated.miff

To concatenate the images top-to-bottom, use -tile 1x3.

For more control over the placement of an image, use combine. First create a matte image and position your images onto the matte. For example,

    convert -size 350x500 xc:black composite.miff
    combine -geometry +0+0   composite.miff image1.gif composite.miff
    combine -geometry +100+0 composite.miff image2.gif composite.miff
    combine -geometry +0+300 composite.miff image3.gif composite.miff
    combine -geometry +0+375 composite.miff image4.gif composite.miff



11. How do I create a GIF animation sequence to display within Netscape?

Use convert with the -delay and -page options. The -delay option is used to specify the delay in 1/100ths of a second between the display of each frame of the animation. For example,
    convert -delay 20 frame*.gif animation.gif
You can also declare specific delays for each frame of the image sequence. For example, if the delay was 20, 10, and 5, use
    convert -delay 20 frame1.gif -delay 10 frame2.gif \
	-delay 5 frame3.gif animation.gif
Use -page to specify the left and top locations of the image frame:
    convert frame1.gif -page +50+100 frame2.gif -page +0+100 \
	frame3.gif animation.gif
Finally, if you want the image to loop within Netscape, use -loop:
    convert -loop 50 frame*.gif animation.gif

For further information about GIF animation, see GIF Animation on the WWW.

Note, that all the images are combined into a single multi-image GIF animation. If you want a single image produced for each frame, use +adjoin:

    convert +adjoin images.* frames%d.gif



12. When I display a PostScript image it appears to be trimmed. I would like to display the entire page, including any white space.

ImageMagick automatically trims any PostScript image as defined by the bounding box. To preempt this behavior, remove the bounding box statement from the Postscrpt or explicitly set the page size. For example,

    display -page letter image.ps



13. What are visual image directories? How do I use them?

A visual image directory (VID) is an image that contains thumbnails of one or more images in a file directory. Rather than displaying each individual image at its full resolution, you can browse the visual image directory and choose an image to display. You can create a VID with either of these commands:

    montage *.jpg directory.vid
    convert 'vid:*.jpg' directory.vid

Of course you can substitute any filenames you desire. Montage has many relevant command line options. You can exercise more control over the appearance of the VID than with convert.

Next display the directory:

    display directory.vid

Finally browse and select an image to display. Move the pointer to the image and press button 3.

You can create the VID directory with this command:

    display 'vid:*.jpg'

You can also select Visual Image... from the File menu of the command widget.

Note, that creating a VID is time consuming. Creating them on-the-fly within display may be less convenient than using montage or convert . Also, if you create them with montage. or convert, you can reuse them as often as necessary.

Note that a visual image directory is useful for looking at individual frames of an image sequence:

    display vid:movie.mpg



14. I have an HP. How do I take advantage of the Color Recovery option?

Type

    display -map list image.miff
If you want to use Color Recovery all the time, put this in your X defaults file:

    display.map: list

Now if you need to turn off Color Recovery temporarily, type

    display +dither image.miff



15. I use the window ID reported by xwininfo(1) with import and it does include the window manager frame as expected. How can I save the window with its frame?

By default, xwininfo(1) returns the ID of the window you click on. Use the -frame option to get the reparented window ID:

    xwininfo -frame
You can then use the returned window ID with import:

    import -frame -window ID window.miff



16. I displayed an image and it appears as one solid color. What did I do wrong?

A blank image generally means that the image is either corrupt or it has a matte channel and the matte values are all zero. ImageMagick treats a matte value of zero as completely transparent. To determine if this is the problem, try

    display +matte image.miff



17. I received the following message, "???? library is not available ...". What does it mean?

ImageMagick requires source libraries not included with the distribution to view or convert certain image formats such as JPEG or TIFF. The above message means you did not compile the required library and link with the ImageMagick utilities. See README for the location of these libraries and compiling instructions. Also edit Magick.tmpl if you use xmkmf or Makefile.in if you use configure and uncomment the appropriate plug-in defines as instructed. Finally, type

    cd ImageMagick
    rm display
    rm magick/decode.o magick/encode.o
    make



18. I want to inspect the values of the matte channel within my image. How can I do that?

View the matte image as a gray scale image. Suppose you have a TIFF image that has a matte channel and is 640 pixels in width and 480 in height. Type:

    convert image.tiff image.matte
    display -size 640x480 gray:image.matte


19. Why was LZW compression removed from the ImageMagick distribution (or why are my GIF images so huge)?

LZW compression is no longer available in the ImageMagick distribution (GIF pixel data is saved uncompressed). I am in agreement with L. Peter Deutsch's arguments regarding LZW within Ghostscript:

  • Unisys claims that they have the right to demand licenses and/or fees from free software incorporating the LZW algorithms, even though they are currently not doing this. This will hang over the head of the developer of any free software that creates GIF files until the Welch patent expires on December 10, 2002 (17 years after its award date).

  • I wish to support the industry move from GIF to PNG. The next Aladdin Ghostscript release (and possibly the next GNU Ghostscript release as well) will include PNG drivers; experimental PNG drivers for Ghostscript are already available. More generally, I wish to support any industry move away from reliance on patented software, which my studies have led me to consider a bad deal for everyone except a few lawyers and a few very large companies.

  • While Unisys' press release says they will not seek licenses or fees from "non-commercial, non-profit" distributors, it is not at all clear to me that they would also exempt people like Walnut Creek CD-ROM, FSF, or Yggdrasil, who distribute free software but charge for the media and documentation. FSF and Yggdrasil, at least, would not redistribute GIF code under those circumstances. This would have a very bad effect on the availability of Ghostscript. I also believe, given that CompuServe gave in to Unisys' demands, that Unisys might demand licenses and fees from any commercial ISP (Netcom, AOL, ...) that distributed free software containing LZW code.

  • Unisys' conditions are not compatible with the GNU License, since they restrict recipients' rights.

A separate distribution is available to enable LZW compression for GIF, Postscript Level II, and the Portable Document Format within ImageMagick. The LZW distribution is available on ftp.wizards.dupont.com.



20. How can I add one of those cool bevels to my image that I see used on the Web?

There are four types of ornamental borders you can add to your image with ImageMagick. Each is listed below with the procedure to use them with your image.

  • Surround the image with a border of color: Use -border followed by the width and height of the border. Set the color of the border with -bordercolor. For example, to surround your image with a red border that is 25 pixels wide on each side, use:

        convert -bordercolor red -border 25x25 image.jpg image.gif
    

  • Lighten or darken image edges to create a 3-D effect: Use -raise followed by the width of the image edge. For example, to create a raised edge effect of 25 pixels, use:

        convert -raise 25 image.jpg image.gif
    

  • Surround the image with an ornamental frame: Use -frame followed by the width and height of the frame. Set the color of the border with -mattecolor. For example, to surround your image with a gray frame that is 25 pixels wide on each side, use:

        convert -mattecolor gray -frame 25x25 image.jpg image.gif
    

  • Surround the image with a raised or sunken bevel: Use -frame followed by the width and height of the bevel. Set the color of the border with -mattecolor. This is just like the description above except you specify a bevel width that matches the frame width. For example, to surround your image with a gray bevel that is 25 pixels wide on each side, use:

        convert -mattecolor gray -frame 25x25+0+25 image.jpg image.gif
        convert -mattecolor gray -frame 25x25+25+0 image.jpg image.gif
    



21. I try to launch display from my window manager and it fails. What's up?

Display determines if it is executing interactively and behaves differently depending on the result. To convince display you are running in an interactive environment when launching from a window manager, use either of

    display logo:
    display < /dev/console



22. When I convert Postscript to another format, how can I improve the appearance of the text?

Simple. Increase the dots-per-inch when converting and sub-sample:

    convert -density 288 -geometry 25% image.ps image.gif
Change the density to 144 and geometry to 50% if the above command fails due to insufficient memory. Alternatively, see the Ghostscript documentation about using high-quality fonts.

The -density option increases the number of pixels (or dots) generated by Ghostscript when processing the input postscript file. However as all other images formats are generally displayed on screens which are typically about 72 to 100 dots per inch, the output image will be larger.

The -geometry option reduces the large image output of ghostscript image back to a normal 72 dpi resolution (25% of 288 dpi gives 72 dpi) but in the process anti-aliases (or smooths) the fonts and lines of the image so as to remove the jaggies you would otherwise get from a normal postscript to image conversion.



23. How can I annotate an image with text that is 2 to 3 inches tall?

If you do not access to a particular named font that is large, try scalable fonts. First see if you have any scalable fonts. Type

    xlsfonts -fn '*-0-0-0-0-*'

Or if you are using display, use the font pattern above within the Font Browser (see Image Edit->Annotate). Next substitute the appropriate resolution. Keep in mind that a scalable font must be fully qualified to work. That is, all 14 fields must be specified. Here is one example where we annotate an image with large Helvetica text:

    convert -font '-*-helvetica-*-*-*--300-300-*-*-*-*-iso8859-1' \
      -pen green -draw 'text 50,300 Magick' image.png annotated.png


24. How can I convert my GIF animation sequence to individual image files?

Use the +adjoin option of the convert command with the scene embedding file format:

    convert +adjoin animation.gif frame%02d.gif

The resulting image files are titled frame01.gif, frame02.gif, frame03.gif, etc.


25. How can I remove the background that prints around my image when I display it with Netscape?

Use the +page option of the convert command:

    convert +page alpha.gif beta.gif

GIF allows for a page offset relative to some background. The page offset information may have been in your GIF image already or it could have been introduced by ImageMagick. Either way, +page removes the unwanted page offset and Netscape should behave as expected.


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