Wednesday, June 10, 2015

Photoshop's Photomerge: Pt. 2

In my previous post, I demonstrated using Photomerge to create a panorama. Nowadays, if I want a panorama, I'll probably just use my iPhone, but I still use Photomerge to piece together parts of an illustration that I scanned in sections. I used to accomplish this the old-fashioned way: Using the Difference Layer Blending Mode and painstakingly rotating and tapping the image over pixel-by-pixel, but now I let Photoshop's Photomerge do all the work.

Click here to see my previous post, Photomerge Pt. 1.

There are a few things I do to prepare to use Photomerge in this way. First, I am careful that my illustration's edges are at right angles. To do this, when I trim the edges of my illustration, I use a transparent gridded rectangle against a cutting board with a grid on it. Then, in my scanner, I make sure to push an edge flush against the frame of the scanner glass. Although Photoshop can rotate the image a few degrees in Auto Blend, you'll get better image quality by avoiding this (Photoshop uses "interpolation" on images that aren't rotated at 90 degree increments, which will cause slight image degradation).

It's also important to give your image a couple inches of overlap. This gives Photoshop a margin to use in computing how to align the sections and gives room to create the Layer Mask.

I make sure that my paper edges are square and I include a
generous overlap. Click any image to enlarge.

Now, File > Automate > Photomerge. I use the default settings: Layout > Auto and I make sure the box is checked for Blend Images Together.

These are the default settings, and I use them to piece
together scans.

This screen shot shows the Layer Masks. 
I turned off one of the Layers to show the effect of the Layer Mask that
Photoshop generated.

I always check to make sure that Photomerge blended the section seamlessly. To do that, I hit cmd-1 (View > Actual Pixels) and then scroll around the seam. Once I'm satisfied with the merged sections, I'll flatten the image and start correcting the scan for color and tone, and that will be the topic of my next Photoshop Tips blog post.

Tuesday, June 2, 2015

Photoshop's Photomerge, Pt. 1

Photoshop has a great feature, called Photomerge, that was originally designed to stitch together photos to make panoramic images.

In this first part, I'll show how to use this feature to create a panorama from three photos. In a following post, I'll demonstrate how I usually use Photomerge: To piece together an illustration that I had to scan in sections.

To get started, I took three photos of the Museum of Science and Industry in Chicago, IL (this building was part of the World's Columbian Exposition of 1893). I didn't use a tripod and I left the camera on "auto exposure," so the exposure shifted a little bit for each shot. I left some overlap between the shots.

I opened the photos in Photoshop and went to File > Automate > Photomerge.

I'm going to show the effects of three different settings. For the first one, I'm not going to use any of the blending features to contrast with the next two in which I let Photomerge work its magic. See A below for the settings and B for the result.

A I'm going to choose Reposition Only, uncheck Blend Images Together,
and Add Open Files.

B This shows the result of merging the three photos, using Reposition Only and
none of the blending features. Click to enlarge.

Now, let's use Layout > Auto and check the Blend Images Together option:

The blending looks good, but Photoshop has distorted the image to
correct for the perspective. Click to enlarge.

This Layer Panel screen shot shows how Photoshop is skewing
the individual photos and using Layer Masks to help in the
merging of the photos.

This last one shows the Blending, but I've also checked the box for Geometric Distortion Correction (found under the Blend Images Together option):

I used Layout > Auto, checked Blend Images Together and
Geometric Distortion Correction.

So, there you have it. After some cropping you'll have a blended panoramic image. Next up: Using Photomerge for piecing together scans.