Resizing vs. Re-Sampling. Photo Images

Transcription

1 Resizing vs. Re-Sampling Photo Images In this tutorial, I will explore resizing (often called rescaling) and re-sampling photo images. The fundamental difference between the two terms is that in rescaling, the number of pixels making up the image is unchanged. When an image is re-sampled the number of pixels in the image is either increased or decreased. To see the differences in the following, a picture with some detail works best. The screen shots are from PSE 15, but are similar for earlier versions. 1. With the image opened in the Editor, on the Menu bar click on Image > Resize > Image Size. The Image Size dialog box opens. Note, do not press Ok. Image Size Dialog Box 2. Ensure that there is no checkmark next to Resample Image as shown in the above figure. Set the Resolution to 300 ppi. a. For this image, the width changes to inches b. The height changes to 8.16 inches. c. The pixel dimensions remain at 3264x2448, and the file size is 22.9 Mb. PSE 15 Resizing vs Resampling.docx 1

2 If you were to send this image to the printer with the resolution as shown, the resulting print would be of high quality and measure inches wide and 8.16 inches high. 3. To see the impact of rescaling the picture, change the Width to 19 inches. Note what happens to the Height, Resolution, and the Pixel Dimensions. a. The pixel dimensions remain the same, 3264x2448. b. The height is now inches c. But the resolution has been reduced to about 171 ppi. If you were to send this file to a printer, it would be printed at the size shown, but the quality of the print would be reduced somewhat. PSE 15 Resizing vs Resampling.docx 2

3 4. Change the Resolution back to 300 pixels/inch. 5. Place a checkmark in the Resample Image box and select Bicubic Smoother (best for enlargement) by clicking on the down arrow. 6. Change the Width to 19 inches again and note the Height, Pixel Dimensions and the size of image in megabytes. We have just resampled the image. More precisely, we have upsampled it. See the figure below. a. Now the pixel dimensions will be 5700x4275, and the size of the file has increased to 69.7 Mb. The added pixels have been mathematical generated. b. The image will be printed at 19 inches x inches from a high resolution file off 300 ppi, but the vast majority of the pixels have been artificially produced, which reduces the print quality. 7. Click Cancel to exit the Image Size dialog without making any changes. 8. Use the Crop Tool to crop the image somewhere in the center where there are details with the following entries on the Tool Option bar: a. Width = 5 in. b. Height = 3 in. c. Resolution = 300 pixels/inch PSE 15 Resizing vs Resampling.docx 3

4 9. On the Menu bar click File > Duplicate and name it 75 ppi 10. Using the technique discussed earlier, Resample the image to 75 ppi using the Bicubic Sharper option, since we are down-sampling the image. Click Ok to exit the Image Size dialog. 11. Now reenter the Image Size dialog and change the Resolution back to 300 pixels per inch using the technique described earlier using the Bicubic Smoother (best for enlargement) option. Click OK. 12. To see the differences in sharpness between the two images, view both of them in the main editing window and zoomed in to Actual Pixels. a. Alternately, save both versions of the image and compare them side by side in the Organizer Full Screen mode as done in the figure below. PSE 15 Resizing vs Resampling.docx 4

5 For this example, the image that has been down-sampled and then resampled up to 300 ppi (image on the left) is not as sharp or clear. What is not clear is whether it is better to send a file to the printer that has a resolution of less than 300 ppi (240 ppi for most Epson models) or to upsample an image to 300 ppi in PSE 15 prior to sending the file to the printer. It is a general rule of thumb that the loss of quality of the print begins to become visible at about 200 ppi. To really emphasize the loss of quality we used only 75 ppi to make the comparison in this tutorial. PSE 15 Resizing vs Resampling.docx 5