Issues with PDF image downsampling in export from InDesign or display in Acrobat

Question

I'm having issues rasterizing images for inclusion in PDFs. I currently rasterize in Illustrator, import to InDesign, and downsample from roughly 300 ppi to 150 ppi, or thereabouts. I would prefer to rasterize to 150 ppi in InDesign instead but that doesn't seem to be possible. The resulting images are always "crunchy" regardless of the Illustrator output resolution. I've tried going as high as 600 with PNGs. The images look okay coming out of Illustrator.

Also, when I then view these PDFs in Acrobat, there are often small image errors that only appear at certain zoom levels.

Images below:

Crunchy

Error

Answer 1

To the best of my knowledge, the errors are caused by exporting to the PDF/X-1a:2001 standard in InDesign. This standard splits some images into smaller components, an example of which is provided below. The other standards don't appear to do this.

The gray lines are the borders of the panels into which the image was split when exported. You can see this by clicking "Edit PDF" in Acrobat.

The crunchiness appears to be related to rasterization of a relatively low-resolution image. When first exported at 900 ppi instead of 300 ppi, the final, downsampled image (~150 ppi) is a far better approximation of the vector original. At the very least, the edges are better defined and there are fewer to no "steps" on straight lines. Some of the "crunchiness" that I saw earlier was actually related to exporting to PDF/X-1a:2001 and not to rasterization.

Answer 2

The "image errors" appearing at certain zoom levels are essentially rounding errors when Acrobat renders the document. This is pretty much normal behavior, but you can try playing with the display options in the Preferences, to see where you get the best (or least bad) results.

What kind of images are you rasterizing? vector images, raster images?

The former should better be kept as vectors as long as possible (in other words, keep them in the PDF). For the latter, there is a simple tool which can resize raster images much better… this tool is called … "Photoshop". But again, why not keep the initial resolution (with raster images) as long as you need, and let the PDF/Acrobat to its thing?

Answer 3

Place the unedited PDF in indesign sized as appropriate, export to PDF using a custom preset: under compression, set "downsample to 150ppi when image is above 150ppi" for both greyscale (256 greys) and color images, possibly also monochrome (aka one-bit color bitmaps).

This allows you to work with the original document(s), defers downsampling to output time, and allows for rapid creation of multiple quality versions.

If you are targeting monitors (rather than print) check the output section: consider converting the colors to destination and setting the destination to an RGB profile. Since CMYK has four channels instead of 3, each CMYK image has 25% more pixel data given the same dimensions in pixels.

Your samples look like they could be vector, but you might be experiencing quality issues if your originals are raster images. Low pixel density, one color, high contrast is the worst-case for highlighting aliasing artifacts in imagery.