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ianp69641526
ianp69641526
Participant
September 13, 2018
Answered

Indexed colors output

  • September 13, 2018
  • 4 replies
  • 1981 views

Dear all,

I'm exporting an ID project to PDF. I want b/w bitmaps to have a resolution of 1200 dpi and RGB images with 300 dpi. So far no problem. However, I also use images with 3 or 4 colors and I want those to maintain their format with a target resolution of 1200 dpi (especially because of the superior quality/file size ratio with lossless compression). InDesign only offers me color management for bitmap, gray scale and RGB images, but not for indexed colors. Is there a way to have indexed colors in the exported PDF? Thanks a lot.

(For those who wonder: I intend to distribute HQ graphics principally as a PDF file, which is why I'm concerned about both ! file size and image quality. I see no need for 3-color images to be converted to full RGB; photos, however, are just fine with a standard JPG compression. The 3-color images are mainly line drawings with a red line here or an orange dot there. Unfortunately, I don't have them as vector graphics and don't want to redraw.)

    This topic has been closed for replies.
    Correct answer hammer0909

    When exporting to a PDF file, the PDF presets only offer the ability to control resolution for RGB, Grayscale, and Bitmap images. InDesign offers no way to control individual image resolution when exported to PDF. Are your line drawings pixel-based or vector-based? If vector-based, resolution doesn't come into play during PDF export. If pixel-based, you can convert those line drawings to bitmap format in Photoshop and control those independently.

    You might be able to get creative though as the PDF settings will downsample images only if they are currently above a specific resolution. So images that don't fall under that parameter will not have their resolution changed. I'd say the surest way to get what you want is to resize the images and adjust their resolution in Photoshop before you even drop them into InDesign.

    4 replies

    ianp69641526
    ianp69641526Author
    Participant
    September 18, 2018

    Thank you all so much for your help (and sorry for the late reply). I will probably opt for downsampling all color images to 300 dpi during export and then replace some of the downsampled files in the PDF file with Acrobat with higher quality ones with zip compression (since these will only be a few).

    @rob day: I might also look into your solution that involves multiple bitmap files with different color overlays. Thanks!

    ianp69641526
    ianp69641526Author
    Participant
    September 14, 2018

    Thank you both so much for your explanations. I now understand that my problem is defining the correct compression and downsampling values in the export options. When I chose downsampling to 300 dpi for color Images, this will include any 3 or 4 color images. Disabling downsampling, I get solid colors.

    Now, I want to define that certain images (photographs) are downsampled to 300 dpi with JPG compression, while color line drawings must be downsampled to 1200 dpi with ZIP compression, strictly maintaining the defined colors. Is there a way to do this in InDesign?

    Again, thank you so much for your help!

    hammer0909
    Community Expert
    hammer0909Community ExpertCorrect answer
    Community Expert
    September 14, 2018

    When exporting to a PDF file, the PDF presets only offer the ability to control resolution for RGB, Grayscale, and Bitmap images. InDesign offers no way to control individual image resolution when exported to PDF. Are your line drawings pixel-based or vector-based? If vector-based, resolution doesn't come into play during PDF export. If pixel-based, you can convert those line drawings to bitmap format in Photoshop and control those independently.

    You might be able to get creative though as the PDF settings will downsample images only if they are currently above a specific resolution. So images that don't fall under that parameter will not have their resolution changed. I'd say the surest way to get what you want is to resize the images and adjust their resolution in Photoshop before you even drop them into InDesign.

    Mike Witherell
    Community Expert
    Mike WitherellCommunity Expert
    Community Expert
    September 13, 2018

    If your 3 or 4 colors are defined as spot (solid) colors, a CMYK-flavored PDF will preserve the colors as separate.

    Mike Witherell
    Dov Isaacs
    Dov Isaacs
    Legend
    September 13, 2018

    I think you might be misunderstanding exactly what indexed color is. It is not a color space separate from RGB or CMYK, but rather, a it is part of a lossless compression technique used when there are less than 256 distinct colors in an image. Color management applies to RGB images regardless of whether they are JPEG, JPEG2000, or Flate (ZIP) compressed.

    If you have a raster image with less than 256 distinct colors and you specify Automatic (JPEG) as the compression method when exporting PDF, each image is checked as to number of colors and suitability for use of indexed color output in conjunction with ZIP compression in the resultant PDF file.

              - Dov

    - Dov Isaacs, former Adobe Principal Scientist (April 30, 1990 - May 30, 2021)
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