Demystifying the "S" Curve

Making an "S" curve is ideal for adjusting the shadows and highlights regions without adversely altering the midtones. Applying a simple "S" curve allows adjustment of highlights, midtones and shadows separately thus allowing you to adjust only what needs adjusting as opposed to other methods that use a sledge hammer approach. The illustration at the left shows a typical "S" curve with the red diagonal line representing the initial starting point after dropping the Color Curves filter on the timeline with the black and gray lines the result of you manipulating it.

There are two basic methods for creating a simple "S" curve with Vegas. You can either drag the Tangent handles at either end of the initial diagonal line forming a curve in the line that way or right click on the line near the center then select 'Create a Point.' If the image you're working on is either very light or dark, shifting the position of the control point higher or lower on the diagonal line will effect shadows or highlights more than placing it close to center as will dragging the Tangent handles further away from the initial settings.

All images can be divided into roughly three broad areas. The top third of the graph adjusts highlights, the middle midtones, and the lower third shadows. To make a simple "S" Curve, left click your mouse on the control point you just created and while holding your left mouse button down slowly drag in the direction the arrow points. You'll notice that the midtones in the image start to lighten proportionally depending on how far you drag up and to the left. This is because the curve in the lower left is raising pixels in the shadows region more into the midtone range. How much and which pixels are effected the most is determined by the slope of the curve which is effected by the position of the set point or points you added.

You'll also notice, since at this point the curve acts as a unit, pixels in the highlight region are decreased due the top half of the curve sloping down. This combination works in a similar way to applying the brightness and contrast filter, only you have much more control WHERE the brightness and contrast gets applied. Because pixels are also shifted you effect hue as well. With a little practice you'll learn to only effect the regions that need correction and not disturb those areas that don't as applying the brightness and contrast would.

Best results are generally obtained by moving on a diagonal. Dragging the center control point down and to the right has the effect of darkening. As you drag the control point you added far enough off its default position you'll see the tangent lines represented as lighter gray lines start to move away. By right clicking on the tangent boxes (the smaller light gray rectangles on either side of the larger control point you added) you can uncheck lock tangents which allows you to more precisely control highlights and shadows separately by sweeping the respective lines in an arc. The lower tangent box control regions below the set point more. The upper controls highlights more. As you rotate either tangent note the effect it has on bending the curve as you watch the image in the preview window, preferably on an external monitor to get truer colors if your project is going to be displayed on a TV, either played from a video camera or later burned to a VCD or DVD.