Raise Image Resolution Directly in InDesign
[Editor's note: This is an article submitted for your enjoyment by InDesign expert Branislav Milic, who runs a wonderful French-language InDesign site. It's an interesting technique that reveals a fascinating and rarely explored aspect of the transparency flattener.]
It is sometimes difficult for users to foresee image resolution and dimensions they will need while they are laying out an InDesign document, and it is particularly difficult to set the images' output resolution to 300 ppi during the layout process. Very often, some images, due to numerous scalings up and down during the layout, have an effective resolution of less than 300 ppi, and that resolution is passed through to the print-ready PDF--a quality that you usually accept. However, when the PDF is received by the prepress operator of a newspaper or a magazine, it is typically checked in a preflight tool such as Enfocus PitStop or Adobe Acrobat Pro -- which will warn that the resolution of some images is not 300 ppi! The result: the operator too-often sends the PDF back to its creator.
So, how to fix the resolution without opening each image in Photoshop, especially when there are dozens or even hundreds of images involved? Very simple... just force InDesign to raise the resolution to 300 ppi! Here's how to do it.
1. In Photoshop, create a document with the following specifications:
- The dimensions must be the same as the InDesign document. Also pay respect to the orientation of the InDesign document. If it is oriented in Portrait mode, do the same in the Photoshop file.
- Set the Resolution to 300 ppi.
- Set the Color Mode to CMYK.
- In Background Contents, fill the Photoshop file with a white background. In fact the type of file, the fact that it is flattened or layered does not matter at all.
2. Save the file. It can be a JPEG, but do not include any ICC profile. Name the document something like "White Background.jpg".
3. In your InDesign document, it is important that the images and the texts are wisely distributed among layers. All the images should have been placed on the "Images" layer, and all the texts should have been placed on the "Text" layer. I recommend that the "Text" layer is placed above the images, and more specifically above all the other objects using transparency options (opacity of less than 100%, blending mode, effects,etc) otherwise the flattening engine will sandwich the texts and raster/outline them.
4. Jump to a master page that has an influence on all the other document or master pages.
5. Create a layer called something like "Resolution" that you will position above the "Images" layer but beneath the "Texts" layer!
6. On that master page and on that layer, import the "White Background.jpg" file so it covers the whole InDesign document. If the document has been created with a master page having an influence on all the other master pages, all the images of the document will now suddenly be hidden. If some images are still visible, this means that some master pages are independent. Thus place the "White Background.jpg" on each master page so none images are visible.
7. Using the Selection Tool (Black Arrow), select the graphic frame that contains the "White Background.jpg" and change its blending mode to "Multiply" in the Effects panel. Now all the document's pages display properly again. What you have done here is placed a 300 ppi transparent object which covers the whole document. When the transparency flattener will blend the images, it will use the resolution of the highest image file in the pile and will blend all the images at its resolution -- thus 300 ppi.
8. For example, in the image below, I have placed a 72 ppi screenshot just under the "White Background.jpg" graphic frame. This screenshot is slightly scaled down so its resolution is increased up to 90 ppi.
9. Export the InDesign document as a PDF (or create a PostScript file that you will place into the Distiller) and choose the PDF-X/1a:2001 preset or any other method that invokes the transparency flattener (the Acrobat 4, PDF 1.3 format).
10. Open the resulting PDF in Adobe Acrobat Pro. Choose Advanced > Print Production > Preflight. Among the three little squares on top of the palette, select the one in the middle "Select Single Checks". In the list of presets, select the one in the Images section named Resolution of color and grayscale images is greater than 250 ppi.
11. Select and double-click this preset or click the "Analyze" button in the lower right corner: you'll notice that Acrobat displays a warning.
12. Click on the little gray arrow next to the red cross to reveal the list of images and you will notice that they all have a resolution of 300 ppi.
There appear to be a lot of images because the transparency flattener has created a jigsaw-puzzle of flattened and contiguous images that have no influence on the texts and vector shapes above.
Note #1: if you want to increase the resolution from 300 ppi up to 450 ppi, just open the "White Background.jpg" file in Photoshop and increase its resolution to 450 ppi (Image menu > Image Size). Save the file, go back to InDesign and update the link. Re-export and enjoy this trick!
Note #2: To scale images, InDesign uses the legendary Bicubic Interpolation Algorithm that has been present in Adobe software for years. We can thus expect a very good resampling quality. Remember: this trick is only really intended to fool the preflight tools when you're too lazy to adjust the effective/output resolution of each image in Photoshop.
Note #3: If you are working in a unflattened workflow (Acrobat 5 - PDF 1.4 or higher), the flattening of transparency will be executed by an Adobe Certified RIP or manually via the "Control Flattener Setting" on the Preflight panel (in Acrobat).
Note #4: Zevrix's Link Optimizer for InDesign is a fantastic utility to control the output resolution of images in InDesign document (and other image formatting changes).
Brussels based Branislav Milic is an Adobe Certified Expert and Trainer. He travels all over Europe and the United States to host seminars, trainings, and to implement complex but efficient publishing workflows. His website has been covering all-things-InDesign in the French language since the release of InDesign 1.0 in September 1999.