Exercise #7. GIMP - color management, text on the image

Transcription

1 dr inż. Jacek Jarnicki, dr inż. Marek Woda Institute of Computer Engineering, Control and Robotics Wroclaw University of Technology {jacek.jarnicki, Exercise #7 GIMP - color management, text on the image The aim of this exercise is to demonstrate the applicability of the program GIMP to work with a color image. There will be considered the following issues: photographic image quality improvement, picture modes and their mutual conversions, addition text to an image 1. Image quality improvement Image enhancement tools allow for photographic images correction, on which at first glance it is difficult to even imagine. Of course, it cannot help us visualize parts of the image, which is apparently missing, because, for example, it was damaged by a bad exposure. But in many cases, corrective actions allow you for almost complete elimination the effects of exposure errors made when photos were taken, or the effects of bad lighting. The following example shows how to use, a few, simple operations that can significantly improve quality of badly exposed pictures. Example procedure for quality improvement of photographic image The aim of this task is to improve the image shown on Fig. 6. As you can see the picture is not perfect. Blue color is clearly dominant. The picture is also somewhat blurred.

2 Fig. 1. Source (original) image You can try to improve the image using the following procedure. Load up source image from Foto_1.jpg file. Execute command Levels from Colors menu and set the Input levels value to be between 1.15 and observe all the time the effect. Accept the selection by pressing the OK button. Remove the blue color dominant by running Color Balance from Colors menu, and moving Yellow - Blue slider to -30 position. Execute Hue and Saturation command from Colors menu, and change the saturation to roughly 20. Use Brightness and contrast command from Colors menu Tools, to improve both parameters at their best extent. Further improve the image using one of the many available filtering algorithms such as filters Filters, Enhance, Sharpen. The processed image shown on Fig. 2

3 Fig. 2. Corrected image Of course, the procedure presented above is not mandatory, but organizes the conduct of the proceedings and it is worth to remember. Colors and Filters menu entries offer many more possibilities to shape the image, but their optimal usage is not easy. Application of color enhancement tools can be learnt only by independent experiments with different images. In order to better memorize previous point, please to repeat the task without instructions. 2. Image modes, color mode conversions There are three so-called modes in GIMP (in fact these are color models that represent colors in images). Color modes correspond to three types of images considered in second exercise, which are named respectively: RGB Grayscale Indexed The first mode is used to encode the image using the RGB color model, the second grayscale ("Shades of Gray") is applicable in the case of monochrome images, and indexed mode allowing saving images with colors indexed, in order to save computer memory and file storage. Changing the recorded image mode After loading the image up, it is that type, in which it was previously saved. For example, after conducting the steps (as in the previous exercise) File, Open,..., Foto_2.jpg a window with the loaded image up will appear (Fig. 3) and on its main bar, after the file name and resolution, a color model is printed. In this case, it is RGB, each pixel of the image is specified by three numbers defining pixels color according to RGB color model.

4 Fig. 3. Color image in RGB mode Conversion from "RGB" mode to "Grayscale" is simple. You need to run the commands from Image menu, Mode, Grayscale and color image will be converted to monochrome one. The image after the conversion can be saved to a file in the selected format. Of course after reading the file saved as a monochrome image, restoration of a color is no longer possible, because in the process of conversion color information was lost. You can of course convert a monochrome image into an image saved in RGB mode using the command Image, Mode, RGB. In this case, the image will still be monochromatic, but the image is encoded as an image in RGB mode. Each point of such an image will be described using three numbers defining the content of the three primary colors, so that these numbers for each pixel will be equal. A more interesting case is the conversion using indexed mode. In this case, the image after the conversion is stored using two arrays, the array of colors called a palette and indexes array that describe the assignment of color palettes to the image pixels (see task 2). To convert a color image stored in in RGB mode into an indexed image, run the following commands from Image Menu: Mode, Indexed... and then a window, such as on Fig. 4 will appear.

5 Fig. 4. Indexed color conversion window As you can see, there are pretty much choices; you can generate "optimized" color palette using the built-in algorithm. You can use a palette optimized for the web and finally you can use any range, e.g. taken from another image and stored in the appropriate file. The maximum number of colors generated by the algorithm may be 256. Color dithering (spreading, scattering) notion used in the dropdown is a way of full color palette approximation by usage of a limited number of available colors, by the scattering pixels in available colors across the image to simulate greater color number than it is in fact used. GIMP offers three dithering algorithms. You now may test image conversion from RGB to indexed mode, each time observing how the image and program generated color palette change. You can see color palette, after you run following commands from Windows menu Dockable dialogs, Colormap. You can edit colors palette - the general principles of color change applies to these operations. Usage a color map to colorize a monochrome image Task to perform at this point, is quite specific, and involves a simulation of technique used a few decades ago, at a time when color photography was not yet known. Black & white photo (made on matte paper) was colored using pastel crayons, yielding in very aesthetic color pictures. The final effect of course was dependent on the skill of the artist. Currently, a similar process can be carried out not necessarily with crayons and but with computer programs usage. There are companies, on the market, that specialize in such operations and their services are not low-priced. The coloring process is carried out in three stages. First, you have to convert provided to colorize black and white image to RGB mode. Secondly, you need to find a suitable color image, save it as the indexed one and then retrieve generated in this way color map. In the

6 last, most difficult step you need, using the chosen colors from color palette obtained in previous step, "manually" colorize black and white image with painting tools such as a brush. The third stage requires some experience and knowledge of tool options. Fig. 5. Original image and colored one The first step execution, namely getting a black and white image from a color one, is fairly simple. Just open Lena_color_256.jpg picture and convert it to grayscale image using the commands Image, Mode, Grayscale. However, further work requires a monochrome image but saved in RGB mode. To obtain such a picture is enough to re-convert it using the command Image, Mode, RGB. The second step consists in collecting the palette, is a little bit more complicated. First, one needs to find the image of an appropriate color scheme. In this example it is proposed as a source of downloadable palette - the image located on the Internet at: ?lang=en&context_space=aria_catalogs&context_id=Term_ _en After viewing an image you can save it and open it with GIMP and then: Convert image to indexed one. See how the palette looks like running the following commands from Windows menu Dockable dialogs, Colormap. It should look like one on Fig. 6

7 Fig. 6. A color palette taken from a sample image The next steps are about to save the created palette, in order to do so do the following: Run the command Dockable dialogs, Palettes. Click small triangle in the upper right Palettes menu in order to open submenu. Pick Palettes Menu, Import Palettes. New window pops up Import a New Palette. Select Image as a Select Source entry and from a drop down menu choose image name from which you are about to import color map. You may want to name a palette use Palette name field just under Import Options. The last step is to color black and white image saved in RGB mode with colors from the imported palette. So now re-open the black and white image and just imported palette, and then using available tools to color the image. It is advisable to do the coloring on a new layer. The basic tool for coloring will be a simple brush. When painting with a brush appropriate tip (size, shape), its opacity, pressure sensitivity and other parameters should be selected to obtain desired effect. When painting it is convenient also to use other tools used to : - filling with color or pattern - pencil drawing - erasing - painting with aerograph - calligraphy painting - blurring / sharpening - finger smearing - dodging / burning Any mistake can always reverted to a previous state using the sequence of commands Edit, Undo or use Undo History and arrow buttons. Pay special attention to this second possibility, where you can see the list of undertaken operations. Another possibility, it simply pressing CTRL+Z from keyboard.

8 3. Text on the image GIMP provides basic operations for application a text to an image. There are not so much operations like in Adobe Photoshop, however, existing ones allows for relatively easy and quick creation of simple text compositions. A useful tool for placing text on an image is a tool marked in the main window as. Writing text on image Writing text in an image is a not difficult task. Please follow the steps: Load up a source image from a file e.g. Foto_2.jpg Choose the writing tool in the main window, and set all the parameters i.e. Font, Size, Color, Formatting etc. In the window with image to set the mouse cursor roughly where text is to be entered and click. GIMP text editor window will pop up. In the editor window type the desired text and accept by pressing Close. Run command Dockable dialogs, Layers from Windows menu, to find out what happened on the layer list. In the main window, select the tool for layers and selections movement, and move written texts to the desired place on the image. Remember that option Move: Layer should be selected. Before saving the image with text to a file, you still need to execute a command Image, Flatten Image to merge layers. Processed image save to a file. The text filled with a gradient Previous task was not too complicated. More difficult is to write a text in which the letters are filled unevenly. This can be done using the so-called gradient, which allows you for filling a selection with any pre-defined pattern in the form of different colors segments. Example steps leading to the creation of a gradient filled inscription are as follows: Load up a source image from a file e.g.foto_2.jpg Open layers and channels preview panes using the commands Dockable dialogs, Layers and Channels from Windows menu. Channels is a new element containing information on individual color components of the image. Currently there are three channels defining three components Red, Green and Blue, because the image is saved in RGB mode. In accordance with the principles described in the previous task - write some text on the image. Pay attention to what happened after writing the text to Layers and

9 Channels. A new channel called Alpha Channel, appeared in the Channels window - it holds information about transparency. You could say that this is an additional monochrome image superimposed on the color image. The brightness point of such image can be treated as transparency. After adding a text picture should look like Fig. 7 Fig. 7. An image with typed in sample text In the Layers window, select text layer right-click mouse key, context menu appears on the screen, select "Alpha to Selection". Create New Layer from Layer menu, select it and then pick Gradient tool, Choose your Gradient. Drag a line across your text. Now the text should be filled in with your gradient. Remove selection Select, None. And then Flatten the image. The effect should be as on Fig. 8.

10 Fig. 8. A text filled with gradient color Repeat this task paying attention on different Gradient tool options.