1 Load and Display an Image {#tutorial_display_image} 2 ========================= 3 4 Goal 5 ---- 6 7 In this tutorial you will learn how to: 8 9 - Load an image (using @ref cv::imread ) 10 - Create a named OpenCV window (using @ref cv::namedWindow ) 11 - Display an image in an OpenCV window (using @ref cv::imshow ) 12 13 Source Code 14 ----------- 15 16 Download the source code from 17 [here](https://github.com/Itseez/opencv/tree/master/samples/cpp/tutorial_code/introduction/display_image/display_image.cpp). 18 19 @include cpp/tutorial_code/introduction/display_image/display_image.cpp 20 21 Explanation 22 ----------- 23 24 In OpenCV 2 we have multiple modules. Each one takes care of a different area or approach towards 25 image processing. You could already observe this in the structure of the user guide of these 26 tutorials itself. Before you use any of them you first need to include the header files where the 27 content of each individual module is declared. 28 29 You'll almost always end up using the: 30 31 - *core* section, as here are defined the basic building blocks of the library 32 - *highgui* module, as this contains the functions for input and output operations 33 34 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp includes 35 36 We also include the *iostream* to facilitate console line output and input. To avoid data structure 37 and function name conflicts with other libraries, OpenCV has its own namespace: *cv*. To avoid the 38 need appending prior each of these the *cv::* keyword you can import the namespace in the whole file 39 by using the lines: 40 41 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp namespace 42 43 This is true for the STL library too (used for console I/O). Now, let's analyze the *main* function. 44 We start up assuring that we acquire a valid image name argument from the command line. Otherwise 45 take a picture by default: "HappyFish.jpg". 46 47 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp load 48 49 Then create a *Mat* object that will store the data of the loaded image. 50 51 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp mat 52 53 Now we call the @ref cv::imread function which loads the image name specified by the first argument 54 (*argv[1]*). The second argument specifies the format in what we want the image. This may be: 55 56 - IMREAD_UNCHANGED (\<0) loads the image as is (including the alpha channel if present) 57 - IMREAD_GRAYSCALE ( 0) loads the image as an intensity one 58 - IMREAD_COLOR (\>0) loads the image in the RGB format 59 60 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp imread 61 62 @note 63 OpenCV offers support for the image formats Windows bitmap (bmp), portable image formats (pbm, 64 pgm, ppm) and Sun raster (sr, ras). With help of plugins (you need to specify to use them if you 65 build yourself the library, nevertheless in the packages we ship present by default) you may 66 also load image formats like JPEG (jpeg, jpg, jpe), JPEG 2000 (jp2 - codenamed in the CMake as 67 Jasper), TIFF files (tiff, tif) and portable network graphics (png). Furthermore, OpenEXR is 68 also a possibility. 69 70 After checking that the image data was loaded correctly, we want to display our image, so we create 71 an OpenCV window using the @ref cv::namedWindow function. These are automatically managed by OpenCV 72 once you create them. For this you need to specify its name and how it should handle the change of 73 the image it contains from a size point of view. It may be: 74 75 - *WINDOW_AUTOSIZE* is the only supported one if you do not use the Qt backend. In this case the 76 window size will take up the size of the image it shows. No resize permitted! 77 - *WINDOW_NORMAL* on Qt you may use this to allow window resize. The image will resize itself 78 according to the current window size. By using the | operator you also need to specify if you 79 would like the image to keep its aspect ratio (*WINDOW_KEEPRATIO*) or not 80 (*WINDOW_FREERATIO*). 81 82 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp window 83 84 Finally, to update the content of the OpenCV window with a new image use the @ref cv::imshow 85 function. Specify the OpenCV window name to update and the image to use during this operation: 86 87 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp imshow 88 89 Because we want our window to be displayed until the user presses a key (otherwise the program would 90 end far too quickly), we use the @ref cv::waitKey function whose only parameter is just how long 91 should it wait for a user input (measured in milliseconds). Zero means to wait forever. 92 93 @snippet cpp/tutorial_code/introduction/display_image/display_image.cpp wait 94 95 Result 96 ------ 97 98 - Compile your code and then run the executable giving an image path as argument. If you're on 99 Windows the executable will of course contain an *exe* extension too. Of course assure the image 100 file is near your program file. 101 @code{.sh} 102 ./DisplayImage HappyFish.jpg 103 @endcode 104 - You should get a nice window as the one shown below: 105 106  107 108 \htmlonly 109 <div align="center"> 110 <iframe title="Introduction - Display an Image" width="560" height="349" src="http://www.youtube.com/embed/1OJEqpuaGc4?rel=0&loop=1" frameborder="0" allowfullscreen align="middle"></iframe> 111 </div> 112 \endhtmlonly 113
