Quote:
Originally Posted by HarryT
Yes. As I said, if you simply want to double the pixel density in an image that's the same physical size, double the linear size of the image (in pixels) while leaving it the same physical size (in cm).
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... or keep the same pixels and simply reduce the physical size.
Digital images, in and of themselves, do not have PPI (Pixels Per Inch; DPI or Dots Per Inch is
always a incorrect term to use when discussing digital images). They only have pixels. It is not until they are placed in a page layout application with a concept of a virtual inch, or when printed on a physical page, that they "acquire" a physical dimension that can be measured in inches and thus you can calculate a PPI value.
Many digital image formats have a metadata field where the creator application can insert a PPI value. This value has no real impact on the number of pixels and thus no effect on the quality of the image itself. It is merely a instruction to a page layout application which, if it chooses to, can set its placement size so that the image is scaled to that PPI by default. If you scale the image in the layout application the PPI value is ignored and the true effective on paper PPI changes.
When a book is printed, the final effective PPI of an image is critical to getting good reproduction. Exactly what PPI is the best depends on the printing process. When the printing process uses halftone screens, the PPI needs to be a certain factor of the halftone screen frequency, 1.7-2.5x the screen frequency when using elliptical dot screens. Excessively high PPI actually lowers quality.
In the OP's situation, you need to first examine the final reproduction size of the images
in inches. Only then can you begin to deal with how you resample the image to yield an appropriate PPI at that final size. You also need to be certain that the PDF generation tool doesn't further resample the images. If you are not using a true page layout application (Adobe InDesign, ...) but are, instead, merely converting an existing ebook, you need to review the settings in you conversion tool. You need to be certain that it resamples the images to the desired 300ppi value during the conversion, up-sampling or down-sampling as needed.
The final quality of the images will depend largely on the performance of the application doing the resampling. Screen captures can generally be upsampled a good bit and still look good if the resampling is done properly. The best results will generally be from using a good image processor (Photoshop, ...) to process the images to size and then building the master document in a good page layout application (InDesign, ...) without scaling the image in the layout program.