Indexed colors output

[ianp69641526]

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.)

[Dov Isaacs]

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

[Mike Witherell]

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

[ianp69641526]

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!

[Chad Chelius · Accepted solution]

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.

[Laubender]

ianp69641526  wrote

… 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? …

No. At least not with a menu command.

You could downsample all images individually by using PhotoShop before PDF export. A custom script could help with that.

Or you could downsample while exporting and exchange the images that should not be downsampled with high resolution images afterwards in Acrobat Pro DC.

Regards,
Uwe

[rob day]

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.

It is true that black and white line art does want more resolution because in most printing scenarios there's no halftone screen interfering with the high contrast edges.

With colored line art there still could be a resolution problem if there's not much halftone screen interference, for example 100% cyan knocking out of 100% yellow. But if the colors are not primary (100% process), and are going to be heavily screened, I think it is debatable whether going from 300ppi to 1200ppi gets you much quality gain.

I would consider simply turning off downsampling, keeping some form of compression, and living with the extra file size.

Is there a way to do this in InDesign?

1-bit Black & White images (line art) can be colored in InDesign—direct select the image and assign a swatch color, and select the parent frame to assign a background color. Here a 1200ppi:

It's probably not worth the effort, but it would be possible to place multiple 1-bit images in the same position and assign more than 2 colors. Something like this where there are 2, 1-bit Black and White images colored in InDesign:

[ianp69641526]

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!