Amazing Computing Sync Tips Column

by Oran Sands III

Video Color Correction with your Amiga

One of the least understood features of any video camera is the White Balance control. This is probably because the manual never explains what is is, telling you only how to set it. Some cameras now have automatic white balancing so you never even touch the white balance controls. In the case of white balancing ignorance is NOT bliss, in fact it's downright dangerous!

A SHORT LESSON IN WHITE BALANCING
Despite the wonderful advances in video over the years your camera/camcorder still hasn't a clue as to what color "white" is. It just knows what it ought to be. Theoretically "white" is  a mixture of red, green and blue. Unfortunately this describes a perfect white which you'll never find in nature. The actual percentages of red, green and blue vary depending upon the source of the light i.e. sunlight, incandescent (tungsten), florescent lighting or a mixture of light sources. Although all of these sources look "white" to us our cameras see these light sources for what they really are, colored sources of light. Our eyes see the colors but our brain interprets the overall combination as "white". Occasionally we see the light for what it is; sodium lights look very yellow, particularly when seen at a distance. But spend any time in a parking lot lit with sodium lights and you'll eventually see it as white light. Our cameras don't have that adaptability so we have to let them know what combination of colors make up "white" at any particular moment. And that's the reason for the white balance controls on your camera.   

When you flick that switch to Sunlight you're telling the camera that there is more blue in the light than usual (or more red in the case of tungsten light). This allows the camera to rebalance the r,g,b outputs of the pickups to accurately reflect the colors in the scene you're shooting. Of course, the setting s built into the camera are those designed for an "average" sunlight (or tungsten-lit) scene and aren't really perfect for the  your lighting. That's why I always suggest that you manually white balance if your camera allows it. By focusing on a white card and manually balancing for white you make sure that the camera knows what "white" is at that very moment, in that particular light. And since the color of sunlight changes thruout the day you need to rebalance the camera about once an hour to make sure your "on target". 

This brings us to our topic of the month. What do you do if you're incorrectly balanced? Say you've shot inside all day with your camera set on "daylight". Or the most likely of circumstances, shooting indoors, set on "Indoors" but most of the light is actually sunlight coming in thru the windows. The resulting video can be overly reddish or bluish (see Picture 1) depending on the situation and you'll find that the "tint" or "hue" control on your tv doesn't change the balance but merely gives you a whole new set of "bad" colors. The problem is that there is literally too much blue or red in the signal and somehow we must reduce it.

There are two approaches to color reduction using your Amiga. One is a bit more expensive than the other but both use equipment that you probably already have or should have. I'll describe the first technique since it'll help you better understand the second. 

DPS PERSONAL TBC AND V-SCOPE
Do you have one of those nifty internal TBC's? I do, it's a DPS Personal TBC and it's very useful in stabilizing video signals. You can also use it as a processing amplifier giving you control over the chroma, hue, video level and other parameters (see Figure 1). There is also another feature that allows you to shift the encoding of the red, green , blue signals. Normally I'd say LEAVE THESE ALONE! since you can really screw up an image by tweaking these settings but for this month's purposes this is just what the  doctor ordered. I also have a DPS V-Scope which is very handy for monitoring my video signals allowing for the checking of video and sync levels, hue (tint)  and chroma levels as well. Without some method of measuring your signals any correction you make are guesswork.

Looking at the axis of Figure 2 you'll see the various primary and secondary colors located around the screen (think of the axis as being on a clock face). At the center of the two axis is the location of the color "white" as the electronics define it. If your camera is correctly balanced a white subject would cause there to be a blob of activity about the center of a vectorscope display (see Figure 3).  Note that the vectorscope face looks the same as the color balance screen. If there is too much blue in the picture then the blob would be off center, moving towards the blue location on the vectorscope display (see Figure 4). What we want to do is create a situation that subtracts the blue causing that blob to pull back to the center of the vectorscope display   

We could literally subtract the blue but electronically that's not that easy. So what we'll do is to add the complement color, the color that is directly across the center of the vectorscope display from your blue. That color (see Figure 2) is yellow. Of course the blue we have may not be pure blue and somewhat magenta-ish. In that case the color we want will be not pure yellow but a greenish yellow.. Think of the vectorscope as a clock face. If your offending color is at 5 o'clock on the clock dial then you'll find your "opposite" color at 10 'clock. 

By adding in our complement color we change the balance of colors bringing the overall balance back closer to the center of the vectorscope. The examples I've provided here (see Figure 4 & 5 and Picture One) are exaggerated and show a great deal of blue being present and a large amount of yellow being added to compensate. In real life the amounts are much smaller. With the DPS TBC the way to "add" the yellow  (or required color) is to change the actual balance of colors that the TBC processes. Using the color balance screen we simply move the balance point from th e normal position at the center and offset it towards the color we wish to add. Without a vectorscope of some sort this will have to be done visually. If that's your only choice then look for a coloration of the whites in the pic, shoot for a non-tinted white and pay particular attention to the flesh tones. Studies show that most people consider a picture  "normal" flesh tones appear correct, regardless of the actual color balance. 

Offsetting the color encoding balance will bring the picture back to where it should have been had you been correctly white balanced to begin with. Just one reminder though, don't forget to return the settings back to unity or "normal" after you've finished processing your video, otherwise you'll start finding yourself with a lot of yellow-looking video. Changing the color balance is a very powerful tool in processing video to correct for many problems. It can also be used to "tint" the image to create effects or to make one scene to another. If you've ever had any film transferred to video it probably has a strange color balance that can be corrected using this technique. Although I used the DPS Personal TBC  other tbc's may have the same capability. The DPS software allows me to save my settings to a file so I can build up preset corrections, calling them up later as needed.

The Other Method for Video Color Correction
Another method for adding the color we need for color correction is to simply use your Amiga, a paint program and a genlock that permits a cross-fade (dissolve ) between  the genlock's reference video and the Amiga's image. 

Using a paint program change the background color to a yellow (or required color). ( Again I stress that you need a vectorscope to really do this right. Otherwise you're going to have to do it visually. So go right out a buy a DPS V-Scope if you're serious about your video) Now remove the menu bars and tools so you have a blank screen. Using the faders on your genlock "mix" the incoming video (the tape you need corrected) with the Amiga's screen until you achieve the balance you need. It won't take much! Just a little of the Amiga's color  is all that's required. This is a sensitive adjustment so don't overdo it. You'd just end up with a yellowish picture instead of a bluish one. Again this technique can be used to create special effects. 

Unlike the tbc solution we discussed earlier this technique allows us to mix in less than a full screen of color correction. If the background color was all yellow except for a black square in the upper corner then the video would have an area of uncorrected color where the corresponding to the black area in the Amiga's image. (Black, when added to another color, changes nothing) If you put together an animation of  black shapes then you'd see the animation only as areas of differently tinted video. In fact forget that color correction was our topic today. Just dwell on the possibilities of using your genlock with the faders positioned somewhere in the middle of their range. Just what could we do? We'll find out next month as we put the new SuperGenSX external genlock through its paces. But before we sign off can anyone name the movie from which i pulled the images for this month's column? This is tricky. It only ran in the New York area but is for sale directly on tape. It was co-produced by a famous Amiga animator and that's all the clues you get. The first reader to correctly guess the animator's name and the title of the movie wins a copy of CLUE for his Amiga. See you next month!

Mr. Sands can be reached by EMAIL at ojsands@portal.cup.com or on Portal as OJSANDS. He is also often  available during th elive conferences on Portal on Wednesday nights at 9pm Central time. He can also be contacted via  this magazine.
