Ceci est une ancienne révision du document !
Using Gimp - Partie 2
Color correction (or adjustment in some cases) is probably more associated with photography than anything else – so, let's take a photograph, and correct any color problems it may have. Above is the original photo. First, we will use GIMP to automatically correct the image; then we will change it manually. To open an image file, we go to File > Open, and choose the image we want to manipulate. So, now that we have our photo open in GIMP, we go to the menu and choose Colors > Levels. This window (right) allows us to do many complicated things, but now we'll just click the 'Auto' button. Voilà! One color-corrected photo (right). Satisfactory corrections are not always so easily achieved. If an item in the photo is too bright, for example, GIMP may think it should be white. So, GIMP makes it white, adjusts the rest of the photo accordingly, and presents an awful result. Yet, in most cases the Auto feature gives a satisfactory result. When it doesn't, then manual corrections should be applied. Before you make manual corrections, you need to know how images are colored. In painting, the three primary colors are the red, blue, and yellow pigments. From these, any other color can be created. Digital images, however, are made (essentially) from light, and their three primary colors are the red, green and blue wavelengths, commonly referred to as RGB. Don't believe me? Click (or display) the Channels dialog, and you'll see three items – one red, one green and one blue. These three 'channels' make up the colors in your image. Try clicking the little eye icon beside each layer, and you'll see what the photo looks like without one (or more) basic colors. So, in essence, a badly colored photo will have a combination of too much (or not enough) red, green or blue. Click (or display) the Dialogs > Colors tab, then click the Triangle button: Above is the digital equivalent of the artist's color wheel (circle with a triangle inside) with the three primary colors being at each point of the triangle, blending in to one another around the circle – with dark to light being represented by the triangle. Click a color on the 'wheel', and the triangle will point to it. Now, click inside the triangle to get a lighter or darker version of that color. Play with this for a while to become familiar with where the colors are and how to get the ones you want – we will be using this later when we need to create or pick a color. Let's look through some of the color options in GIMP to see how they can affect our image. Go to the Colors menu, and choose Color Balance (below). This window will let you fine-tune the colors in your photo. It can alter the colors in each of the three main levels: shadow, midtone and highlight. Select Shadows, Midtones or Highlights, and move the sliders to see how it affects your photo. If you don't see the photo colors change, check to make sure the box beside 'Preview' is ticked. If you don't want to keep those color changes, simply click 'Cancel', and your image will return to normal. You can also use the Edit > Undo feature. Colors > Colorize will allow you to tint the entire photo with a particular color. This is most often used to give a photo a 'sepia tint' – which makes the image look old and degraded. Rather than give you a red, green and blue slider, this window gives you a 'Hue' slider – which goes from red, through to green, to blue, and back to red. Tinker with the sliders until you get a nice orangey-brown color, and you'll see what I mean about the sepia tint. Colors > Brightness – Contrast will let you simply brighten or darken the image. This can come in handy for simple effects. If I brighten the image quite a bit, and slide the contrast up, I can make the dull original image look like it was taken on a scorching sunny day (which it definitely wasn't!). We've already looked at Levels, but let's look into it a bit more. Click Colors > Levels, and you'll see that familiar window – yes, it's the one with the magic 'Auto' button – but this time let's focus on the sliders below the graph. The black, grey and white sliders can move left to right, and represent the shadows, midtones and highlights, respectively. They allow you to manually color-correct the image. The 'Output Levels' is almost like a brightness control, but with three sliders for the shadow, midtone and highlights. At the top of the window is a dropdown menu which says 'Value' – you can click that and select one of your red, green or blue channels to fine-tune. This is quite a powerful window, so play around in there and get familiar with it. And remember: if you don't see your image updating as you move a slider, tick the 'Preview' checkbox. Colors > Curves is similar to the Levels window (which we just looked at), but uses curves, plotted on a graph, to give more control over your colors. Again, at the top of the window is the dropdown menu, which lets you alter the image as a whole, or just alter one of the color channels. To edit the curves, you click on the curve (to create a point), then move the point up, down, left, or right, to alter the colors. The most basic color correction in the curves window is the 'S-curve' – where you make an 'S' shape with the curve. Again, this gives pretty much the same result as the Color > Levels 'Auto' button. Color > Desaturate will remove all color from your photo, leaving you with a black and white image. You have three options before it removes the color, each giving you slightly different results, so it's best to try each to see which you prefer. As ever, try an option; if you don't like it, use Edit > Undo to revert back to the original. Color > Invert will give you the negative of the photo, just like you'd get with traditional camera film. Clicking Color > Invert again will return the image to normal. There are many other items in the Color menu, but the ones I have mentioned are the most important ones, and probably the most used. Next month, we will use the zoom and selection tools to expand your manipulation skills.