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139 lines
6.1 KiB
139 lines
6.1 KiB
Image Pyramids {#tutorial_pyramids} |
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============== |
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Goal |
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---- |
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In this tutorial you will learn how to: |
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- Use the OpenCV functions @ref cv::pyrUp and @ref cv::pyrDown to downsample or upsample a given |
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image. |
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Theory |
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------ |
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@note The explanation below belongs to the book **Learning OpenCV** by Bradski and Kaehler. |
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- Usually we need to convert an image to a size different than its original. For this, there are |
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two possible options: |
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-# *Upsize* the image (zoom in) or |
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-# *Downsize* it (zoom out). |
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- Although there is a *geometric transformation* function in OpenCV that -literally- resize an |
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image (@ref cv::resize , which we will show in a future tutorial), in this section we analyze |
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first the use of **Image Pyramids**, which are widely applied in a huge range of vision |
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applications. |
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### Image Pyramid |
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- An image pyramid is a collection of images - all arising from a single original image - that are |
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successively downsampled until some desired stopping point is reached. |
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- There are two common kinds of image pyramids: |
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- **Gaussian pyramid:** Used to downsample images |
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- **Laplacian pyramid:** Used to reconstruct an upsampled image from an image lower in the |
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pyramid (with less resolution) |
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- In this tutorial we'll use the *Gaussian pyramid*. |
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#### Gaussian Pyramid |
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- Imagine the pyramid as a set of layers in which the higher the layer, the smaller the size. |
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- Every layer is numbered from bottom to top, so layer \f$(i+1)\f$ (denoted as \f$G_{i+1}\f$ is smaller |
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than layer \f$i\f$ (\f$G_{i}\f$). |
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- To produce layer \f$(i+1)\f$ in the Gaussian pyramid, we do the following: |
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- Convolve \f$G_{i}\f$ with a Gaussian kernel: |
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\f[\frac{1}{16} \begin{bmatrix} 1 & 4 & 6 & 4 & 1 \\ 4 & 16 & 24 & 16 & 4 \\ 6 & 24 & 36 & 24 & 6 \\ 4 & 16 & 24 & 16 & 4 \\ 1 & 4 & 6 & 4 & 1 \end{bmatrix}\f] |
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- Remove every even-numbered row and column. |
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- You can easily notice that the resulting image will be exactly one-quarter the area of its |
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predecessor. Iterating this process on the input image \f$G_{0}\f$ (original image) produces the |
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entire pyramid. |
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- The procedure above was useful to downsample an image. What if we want to make it bigger?: |
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columns filled with zeros (\f$0\f$) |
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- First, upsize the image to twice the original in each dimension, wit the new even rows and |
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- Perform a convolution with the same kernel shown above (multiplied by 4) to approximate the |
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values of the "missing pixels" |
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- These two procedures (downsampling and upsampling as explained above) are implemented by the |
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OpenCV functions @ref cv::pyrUp and @ref cv::pyrDown , as we will see in an example with the |
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code below: |
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@note When we reduce the size of an image, we are actually *losing* information of the image. |
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Code |
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---- |
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This tutorial code's is shown lines below. You can also download it from |
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[here](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/ImgProc/Pyramids.cpp) |
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@include samples/cpp/tutorial_code/ImgProc/Pyramids.cpp |
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Explanation |
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----------- |
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Let's check the general structure of the program: |
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- Load an image (in this case it is defined in the program, the user does not have to enter it |
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as an argument) |
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp load |
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- Create a Mat object to store the result of the operations (*dst*) and one to save temporal |
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results (*tmp*). |
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@code{.cpp} |
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Mat src, dst, tmp; |
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/* ... */ |
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tmp = src; |
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dst = tmp; |
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@endcode |
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- Create a window to display the result |
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp create_window |
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- Perform an infinite loop waiting for user input. |
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp infinite_loop |
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Our program exits if the user presses *ESC*. Besides, it has two options: |
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- **Perform upsampling (after pressing 'u')** |
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp pyrup |
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We use the function @ref cv::pyrUp with three arguments: |
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- *tmp*: The current image, it is initialized with the *src* original image. |
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- *dst*: The destination image (to be shown on screen, supposedly the double of the |
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input image) |
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- *Size( tmp.cols*2, tmp.rows\*2 )\* : The destination size. Since we are upsampling, |
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@ref cv::pyrUp expects a size double than the input image (in this case *tmp*). |
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- **Perform downsampling (after pressing 'd')** |
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp pyrdown |
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Similarly as with @ref cv::pyrUp , we use the function @ref cv::pyrDown with three arguments: |
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- *tmp*: The current image, it is initialized with the *src* original image. |
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- *dst*: The destination image (to be shown on screen, supposedly half the input |
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image) |
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- *Size( tmp.cols/2, tmp.rows/2 )* : The destination size. Since we are upsampling, |
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@ref cv::pyrDown expects half the size the input image (in this case *tmp*). |
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- Notice that it is important that the input image can be divided by a factor of two (in |
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both dimensions). Otherwise, an error will be shown. |
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- Finally, we update the input image **tmp** with the current image displayed, so the |
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subsequent operations are performed on it. |
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp update_tmp |
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Results |
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------- |
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- After compiling the code above we can test it. The program calls an image **chicky_512.jpg** |
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that comes in the *samples/data* folder. Notice that this image is \f$512 \times 512\f$, |
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hence a downsample won't generate any error (\f$512 = 2^{9}\f$). The original image is shown below: |
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- First we apply two successive @ref cv::pyrDown operations by pressing 'd'. Our output is: |
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- Note that we should have lost some resolution due to the fact that we are diminishing the size |
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of the image. This is evident after we apply @ref cv::pyrUp twice (by pressing 'u'). Our output |
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is now: |
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