Adobe Photoshop has become the most valuable auxiliary software application used by video professionals and video editors of all types. Whether you use it to doctor client-supplied graphics and photos or simply as the ersatz type tool for your nonlinear edit system, Photoshop has been a veritable Swiss Army knife to solve design issues for video. In this installment I’ll pass along a few pointers that might make your use of Photoshop more productive. Generally these tips apply to versions 6, 7 or higher (Mac and PC), so some may not work the same way with earlier versions.
The NTSC video frame is sized at 720 x 486 (480 for DV) non-square, i.e. rectangular, pixels. Computers work with a square pixel aspect ratio, so new graphics created in Photoshop for video should start out at 720 x 540 pixels (72 dpi). Depending on which video editing application you use, you might have to resize the file as your last step. For example, Avid software can automatically resize the frame from the 540 height to 486 (or 480), whereas Final Cut Pro does not. In the case of FCP, you would have to resize the graphic in Photoshop, before importing it into Final Cut. The reason for these corrections is so that the aspect ratio of graphics, such as the roundness of a circular logo, will look correct once you get it into the video realm. In short, create in 720 x 540 and then resize to 720 x 486 for all NTSC video formats except DV/DVCPRO/DVCAM, which use a 720 x 480 frame size. Turn “constrain proportions” off when altering image sizes in Photoshop.
Computers and digital video use an 8-bit color depth, meaning that colors, brightness, etc. are each divided into 256 increments. There is also 10-bit color for some video systems, but generally even on these systems, rendering is still based on 8-bit math. Computers work with RGB images. Each component color element – red, blue and green – has a value of from 0 to 255. Black would be 0, 0, 0 and white would be 255, 255, 255. Digital video, known as YUV (conforming to the ITUR-601 spec), splits video into a luminance and two color-difference signals. You will often see this expressed as a ratio, such as 4:2:2 or 4:1:1, representing the relative (not actual) values of these components. In most digital video systems, black is actually placed at the value of 16 and white at 235. This gives the system enough headroom at both ends to deal with things like dark video dipping below the NTSC analog “set-up” signal (analog black is at 7.5 IRE not 0) and peak white levels that exceed 100 IRE in the camera (up to 110 IRE).
Like the issue of sizing, various NLEs deal with video levels in different manners. For instance, Avid software allows you to import graphics with either RGB or 601 video levels. This choice will affect the brightness and contrast of your image. Final Cut Pro gives you no such option and assumes your graphic to be at RGB values. It is best to know where your graphic will be used and adjust values accordingly. This can be done using Photoshop’s “levels” adjustment. Here you can alter input and output values, as well as the midpoint or gamma of the image. Changing the gamma value allows you to adjust the midrange portion of an image between brighter and more visible to darker and less visible. Since computer screens have different gamma values than monitors, you will frequently get a totally different amount of contrast between the appearance of a photo or graphic in Photoshop (viewed on a computer screen) versus your NLE playing on a video monitor. If your NLE expects a graphic with an RGB range, output settings at 0 and 255 are fine, but if it expects a 601 range, then you need to adjust things to 16 and 235.
It is ideal to be able to check the results of your adjustments by viewing the Photoshop output on an actual waveform monitor or video monitor. There are some applications and plug-ins that approximate this with software scopes, but the real thing is better. The Echo Fire plug-in works with the AJA Io to allow you to see Photoshop images on a video monitor. Some video graphics cards, like the Matrox Parhelia card, also give you this functionality.
Layers and Layer Effects
One of the biggest features that Photoshop offers is the ability to work in layers and many NLEs now allow you to import layered files. These usually come in as sequences with each Photoshop layer becoming a separate video layer. After importing, you can use your NLE’s DVE effects to move or animate any of these elements. Adobe’s blend modes and layer effects cause trouble for most NLEs. The blend modes determine how each layer interacts with the rest of the composite. These can create quite unique looks, but most NLEs really only understand the “normal” blend mode and use this transparency information to correctly key or superimpose one layer over another. The layer effects are used to add drop shadows, glows and embossing to an object. If these are left separate, most NLEs will not derive the correct transparency and/or edge information of a graphic. In order to prevent problems, layer effects must first be merged to their layers before importing into an NLE.
I usually follow this procedure for a layer with effects. Duplicate the layer and create a new blank layer above it. “Rasterize Layer” on the duplicate if this is a type layer. Link the duplicate and the new blank layer and then chose “merge linked”. You now have a single layer, complete with the composited layer effects, as well as the original layer with its individual elements. Keeping the originals intact permits later changes. Then I usually save a copy of the complete file and delete the original component layers in that copy, leaving only the various final layers. This is the file I will import into the NLE.
Normally you will import a flattened file (no layers), if you don’t intend to create any further animation with it. Most NLEs will deal with all the popular formats: PSD, PICT, TIFF, Targa, JPEG, BMP or JPEG. Make sure the files are in the RGB and not CMYK mode. To flatten a file, select that option on the layers menu or simply save the file in one of these formats, but do not enable layers in the saving dialogue. Make sure you do this with a copy so you can go back and make changes to the original.
When creating a graphic to be used as a key, such as a logo or super, you will need to have a companion alpha channel to “cut the hole” for this key. Most NLEs can only deal with a single alpha channel. You will need to merge all layers except the solid-color background layer into a single composite layer. “Load Selection” on the merged layer and you’ll see the crawling dotted outline of Photoshop’s selection tool. In the channels menu, add a new alpha channel and then fill the selection with white. You should now see a white-on-black version of the graphic, complete with soft shadows and gray values for semi-transparent objects or edges. If your NLE expects to see an alpha channel with the key-cutter object as black on a white background, then invert the video for the alpha channel. This is the case with Avids. Flatten the file and save as a 32-bit file (with alpha) in one of the above formats.
I use Photoshop a lot as the character generator for my NLE. I like the layer effects and it is easy to do a lot of similar supers simply by creating one layer with the right fonts, sizes and attributes, then duplicate and enter the revised text on the new layer. Repeat the process for as many supers as you need. I did this recently with 200 direct response phone number supers. When I was done, I had a single Photoshop file with 200 layers, each with separate layer effects for drop shadows and a subtle glow effect. I still had to get these into individual graphic files for my NLE. Along the way, I discovered yet another way that was far easier than the process I just finished describing. The PNG format is another Adobe image file that preserves transparency information and embeds it into the file. I took my 200-layer Photoshop file and turned off the layer visibility of all layers except the one I was exporting. With one layer active and visible, I saved a copy of the file in the PNG format. This new file had discarded all invisible layers except the one, rasterized the type, merged the layer effects and embedded alpha information. I was able to import this into Avid (reverse alpha selected) and had my real-time title super. Then I repeated this process for each of the different phone numbers.
Processing Moving Video Clips
Adobe After Effects is frequently used to add effects and filtering to video clips, but you can also do this with Photoshop. Most NLEs will permit you to export a clip or a sequence as a series of sequential image files. In other words, your :10 video clip can be exported as a series of still frame graphics in one of the standard files, numbered as such – NAME001.tga, NAME002.tga, NAME003.tga and so on. Any of the Photoshop level and sizing functions or filter effects can be applied to these files. Photoshop offers a way to automate such procedures through its Batch Automation and Actions menus. For instance, if you’d like a Watercolor Artistic effect filter applied to :10 of video, it’s as simple as creating the right actions and batch commands to do this. It’s a good idea to set up the batch to save the processed files as new images to a separate folder, in order to leave your original exported files unaffected.
On the opposite end, most NLEs will also import sequential image files and put them back together as a single video clip or animation. As long as no size values where changed, you shouldn’t need to worry about resizing the images or any video interlace issues during the exporting and importing of these files.
There’s a lot more to Photoshop than these few tips, but hopefully one of these ideas has added a couple of new tricks to your NLE expertise. If you want to do a bit more in-depth research, take a look at Photoshop for Nonlinear Editors (Richard Harrington, CMP Books).
© 2004 Oliver Peters