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Tuesday, November 29, 2011

The Right Resolution

The conventional wisdom says set 300 dpi and forget it, but it turns out that there’s more to optimizing image resolution for fine-art printing

This Article Features Photo Zoom

Figure 2 I suggest checking the Dimension option in the Guides panel to see a readout of the dimensions and resolution of the image at the final print size.

The people at Epson say that the print driver doesn't do the re-sampling, and since the application sending the print doesn't do the resampling unless asked to do so, the resampling must be happening in the print pipeline. I've tried, unsuccessfully, to get confirmation from Apple and Microsoft about their respective print engine activities regarding the resampling of the image data. So I really don't know where or how the resampling is being done. But I'm convinced some sort of resampling is being done. Is it an optimal resampling algorithm, or is it something done for speed? I don't know, but I suspect, at best, it's a compromise in favor of speed. I'm pretty sure there are better, optimized resampling algorithms that could do a superior job. In fact, Adobe Photoshop Lightroom resampling is a hybrid Bicubic algorithm that interpolates between Bicubic and Bicubic Smoother for upsampling and Bicubic and Bicubic Sharper for downsampling.

Printing From Lightroom
First off, let me disclose the fact that I'm partial to and perhaps a bit biased toward the Lightroom Print functionality. I had a little something to do with the development of printing from Lightroom. I've worked with the engineers to optimize the printing workflow and incorporate output sharpening for printing directly in Lightroom. As a founding member of a company called PixelGenius, LLC, I was involved with former PG member Bruce Fraser when he developed a sharpening plug-in called PhotoKit Sharpener. PixelGenius worked with Adobe to incorporate the PhotoKit output sharpening for inkjet and screen display directly into Lightroom and Camera Raw.

Figure 3 This is the readout of the native resolution of 236 ppi of the image when printed at the final print size.

The output sharpening is processed at the end of the printing command just before the image data is sent to the printer. This offers a variety of benefits—not the least of which is the ability to upsample to a specific output resolution directly in the Print module and then apply the final output sharpening. If you want optimal inkjet output, you should be upsampling (never downsampling, because why waste pixels) before sending the image to the printer.

Native Resolution Vs. Upsampled Resolution
Before I get into the details, let me outline exactly how I created the following print output tests. The original raw images were printed from Lightroom after setting the optimal capture sharpening in the Develop Module of Lightroom. As shown in the following figure, the image had the Amount, Radius and Detail sliders set for optimal sharpening. I also applied a bit of Luminance Noise Reduction because sharpening will often increase the perception of the noise (Fig. 1).

Figure 4 Here, the Print Resolution option has been checked in the Print Job panel, and the resolution has been set to 360 ppi.

After preparing the image in Develop, I went into the Print Module of Lightroom 3.5. At the native size of the capture (a cropped portion of an image shot with a Canon EOS Digital Rebel XTi), the image would print at 6.2x9.5 at 236 ppi. Based on the normal viewing distance of about 22 inches, 236 ppi would be plenty of resolution. But there's a question of how the details in the image would reproduce after the print pipeline did the resampling to 360 ppi. High-frequency texture and high-contrast diagonal lines often can produce a jaggedness to edges that can be seen in the final print. Whether this is due to the error-diffusion dither process of the printer or the pipeline's resampling, I don't know (it's probably both). But I do know that upsampling to the reported resolution of the print can eliminate the problem (Figs. 2-4).

I printed the image at both the native resolution and the upsampled resolution from Lightroom. I used the Standard setting on the Print Sharpening option and chose Glossy as the Media Type. In general, you'll want to select the glossy settings for papers considered photo papers and the matte settings for matte and watercolor papers. The sharpening requirements are different due to factors such as ink absorption, dot gain and the texture and reflectance of the paper. Lightroom automatically applies the proper output sharpening, based upon the resolution being sent to the print.

For the purposes of the test prints, I used two printers; an Epson Stylus Pro 4900 and a Canon imagePROGRAF iPF6300. Both printers represent the state-of-the-art, 12-color, high-end fine-art printers. The respective print dialogs were set up as shown in Figures 5 and 6. For the Epson printer, I used 8.5x11-inch Epson Ultra Premium Photo Paper Glossy. For the Canon printer, I used Canon's Photo Paper Pro II.

You'll notice that in both drivers' settings, I've chosen to turn off bidirectional printing. The odds are, printing normal output at these resolutions using bidirectional printing wouldn't negatively impact the final output. However, unidirectional printing eliminates any potential issues.