Source codepngpdf. Keywords: python, matplotlib, pylab, example, codex see Search examples. This Page Show Source. Quick search. The emphasis in this demo is on showing how to make contours register correctly on images, and on how to get both of them oriented as desired. In particular, note the usage of the "origin" and "extent" keyword arguments to imshow and contour.

To avoid discretization error, use either this number or a large number such as the default If we want lines as well as filled regions, we need to call contour separately; don't try to change the edgecolor or edgewidth of the polygons in the collections returned by contourf.

Use levels output from previous call to guarantee they are the same. We are making a thick green line as a zero contour. Specify the zero level as a tuple with only 0 in it. Note that the contour lines don't extend to the edge of the box. This is intentional. The Z values are defined at the center of each image pixel each color block on the following subplotso the domain that is contoured does not extend beyond these pixel centers.

Last updated on May 10, Created using Sphinx 1.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. With the following code I am producing an obstacle on a 2D Cartesian grid used later as input for some simulations by defining the area where the obstacle is located as 1 and 0 everywhere else.

I wanted to plot it using a contour plot, but had some troubles producing a binary filled contour plot side question: how to achieve that? The resulting image looks as follows, with the contour plot on the left and the imshow plot on the right the center of the obstacle is marked with a yellow star. Obviously, the two plots are different. What is the reason for this, i. The origin for matplotlib's imshow function is the top left corner. If you change the relevant line to:.

Learn more. How to use matplotlib's imshow and contour plot on the same numpy array? Ask Question. Asked 3 years, 5 months ago. Active 3 years, 5 months ago. Viewed 3k times. Here is the code: import numpy as np, matplotlib. Alf Alf 1, 1 1 gold badge 16 16 silver badges 35 35 bronze badges. I don't understand it though Hm, I thought you asked how to produce a filled contour plot. The rounded corners are happening because matplotlib builds a set of curves based on a raster image and, naturally, does some smoothing in the process.

Active Oldest Votes. If you change the relevant line to: ax2. Robbie Robbie 3, 1 1 gold badge 13 13 silver badges 20 20 bronze badges. No problem. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.

The Overflow Blog. Podcast Ben answers his first question on Stack Overflow. The Overflow Bugs vs. Featured on Meta.John Hunter Excellence in Plotting Contest submissions are open! Entries are due June 1, Click here to download the full example code. Test combinations of contouring, filled contouring, and image plotting. For contour labelling, see also the contour demo example. The emphasis in this demo is on showing how to make contours register correctly on images, and on how to get both of them oriented as desired.

In particular, note the usage of the "origin" and "extent" keyword arguments to imshow and contour. Keywords: matplotlib code example, codex, python plot, pyplot Gallery generated by Sphinx-Gallery. Version 3. Show Page Source. Note Click here to download the full example code. To avoid discretization error, use either this number or a large number such as the default If we want lines as well as filled regions, we need to call contour separately; don't try to change the edgecolor or edgewidth of the polygons in the collections returned by contourf.

Use levels output from previous call to guarantee they are the same. We are making a thick green line as a zero contour. Specify the zero level as a tuple with only 0 in it. Note that the contour lines don't extend to the edge of the box.

This is intentional. The Z values are defined at the center of each image pixel each color block on the following subplotso the domain that is contoured does not extend beyond these pixel centers.If you find this content useful, please consider supporting the work by buying the book!

Sometimes it is useful to display three-dimensional data in two dimensions using contours or color-coded regions. There are three Matplotlib functions that can be helpful for this task: plt.

## Density and Contour Plots

This section looks at several examples of using these. We'll start by setting up the notebook for plotting and importing the functions we will use:. A contour plot can be created with the plt. It takes three arguments: a grid of x values, a grid of y values, and a grid of z values.

The x and y values represent positions on the plot, and the z values will be represented by the contour levels. Perhaps the most straightforward way to prepare such data is to use the np.

Notice that by default when a single color is used, negative values are represented by dashed lines, and positive values by solid lines. Alternatively, the lines can be color-coded by specifying a colormap with the cmap argument. Here, we'll also specify that we want more lines to be drawn—20 equally spaced intervals within the data range:. Here we chose the RdGy short for Red-Gray colormap, which is a good choice for centered data.

Matplotlib has a wide range of colormaps available, which you can easily browse in IPython by doing a tab completion on the plt. Our plot is looking nicer, but the spaces between the lines may be a bit distracting.

We can change this by switching to a filled contour plot using the plt. Additionally, we'll add a plt. The colorbar makes it clear that the black regions are "peaks," while the red regions are "valleys.

One potential issue with this plot is that it is a bit "splotchy. This could be remedied by setting the number of contours to a very high number, but this results in a rather inefficient plot: Matplotlib must render a new polygon for each step in the level. A better way to handle this is to use the plt. Finally, it can sometimes be useful to combine contour plots and image plots.

For example, here we'll use a partially transparent background image with transparency set via the alpha parameter and overplot contours with labels on the contours themselves using the plt.

The combination of these three functions— plt.

For more information on the options available in these functions, refer to their docstrings. If you are interested in three-dimensional visualizations of this type of data, see Three-dimensional Plotting in Matplotlib. Now let's look at this with a standard line-only contour plot:. The following code shows this:. There are a few potential gotchas with imshowhowever: plt. This must be changed when showing gridded data.Contours are defined as the line joining all the points along the boundary of an image that are having the same intensity.

Contours come handy in shape analysis, finding the size of the object of interest, and object detection. OpenCV has findContour function that helps in extracting the contours from the image.

It works best on binary images, so we should first apply thresholding techniques, Sobel edges, etc. Output: We see that there are three essential arguments in cv2. First one is source image, second is contour retrieval mode, third is contour approximation method and it outputs the image, contours, and hierarchy.

Each individual contour is a Numpy array of x, y coordinates of boundary points of the object. Contours Approximation Method — Above, we see that contours are the boundaries of a shape with the same intensity.

It stores the x, y coordinates of the boundary of a shape. But does it store all the coordinates? That is specified by this contour approximation method. If we pass cv2.

## matplotlib: difference between pcolor, pcolormesh and imshow

But actually, do we need all the points? For eg, if we have to find the contour of a straight line. We need just two endpoints of that line. This is what cv2.

It removes all redundant points and compresses the contour, thereby saving memory. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute. See your article appearing on the GeeksforGeeks main page and help other Geeks.

**9: Surfaces and Contour Plots - Valuable Vector Calculus**

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below. Writing code in comment? Please use ide. Let's load a simple image with 3 black squares. Canny gray, 30 Use a copy of the image e. Check out this Author's contributed articles.

Load Comments. We use cookies to ensure you have the best browsing experience on our website. Canny gray, 30cv2.Except as noted, function signatures and return values are the same for both versions. X and Y must both be 2-D with the same shape as Z e. If not given, they are assumed to be integer indices, i. If array-like, draw contour lines at the specified levels. The values must be in increasing order. If Falseany quad touching a masked point is masked out. If Trueonly the triangular corners of quads nearest those points are always masked out, other triangular corners comprising three unmasked points are contoured as usual.

The colors of the levels, i. The sequence is cycled for the levels in ascending order. If the sequence is shorter than the number of levels, it's repeated. As a shortcut, single color strings may be used in place of one-element lists, i.

This shortcut does only work for color strings, not for other ways of specifying colors. A Colormap instance or registered colormap name. The colormap maps the level values to colors. If a colormap is used, the Normalize instance scales the level values to the canonical colormap range [0, 1] for mapping to colors. If not given, the default linear scaling is used. If not Noneeither or both of these values will be supplied to the Normalize instance, overriding the default color scaling based on levels.

Determines the orientation and exact position of Z by specifying the position of Z[0, 0]. This is only relevant, if XY are not given. If origin is not Nonethen extent is interpreted as in imshow : it gives the outer pixel boundaries. In this case, the position of Z[0, 0] is the center of the pixel, not a corner. If origin is Nonethen x0y0 is the position of Z[0, 0], and x1y1 is the position of Z[-1, -1].

The locator is used to determine the contour levels if they are not given explicitly via levels. Defaults to MaxNLocator. Determines the contourf -coloring of values that are outside the levels range. If 'neither', values outside the levels range are not colored. If 'min', 'max' or 'both', color the values below, above or below and above the levels range. Note that most colormaps do not have dedicated colors for these by default, so that the over and under values are the edge values of the colormap.

You may want to set these values explicitly using Colormap. An existing QuadContourSet does not get notified if properties of its colormap are changed. Therefore, an explicit call QuadContourSet.

The explicit call can be left out, if a colorbar is assigned to the QuadContourSet because it internally calls QuadContourSet.

### Find and Draw Contours using OpenCV | Python

Override axis units by specifying an instance of a matplotlib. Enable antialiasing, overriding the defaults.Except as noted, function signatures and return values are the same for both versions.

X and Y must both be 2-D with the same shape as Z e. If not given, they are assumed to be integer indices, i. If an int nuse n data intervals; i. The level heights are automatically chosen. If array-like, draw contour lines at the specified levels. The values must be in increasing order. If Falseany quad touching a masked point is masked out. If Trueonly the triangular corners of quads nearest those points are always masked out, other triangular corners comprising three unmasked points are contoured as usual.

Defaults to rcParams["contour. The colors of the levels, i. The sequence is cycled for the levels in ascending order. If the sequence is shorter than the number of levels, it's repeated. As a shortcut, single color strings may be used in place of one-element lists, i. This shortcut does only work for color strings, not for other ways of specifying colors. By default value Nonethe colormap specified by cmap will be used.

A Colormap instance or registered colormap name. The colormap maps the level values to colors. Defaults to rcParams["image. If a colormap is used, the Normalize instance scales the level values to the canonical colormap range [0, 1] for mapping to colors. If not given, the default linear scaling is used. If not Noneeither or both of these values will be supplied to the Normalize instance, overriding the default color scaling based on levels. Determines the orientation and exact position of Z by specifying the position of Z[0, 0].

This is only relevant, if XY are not given. If origin is not Nonethen extent is interpreted as in imshow : it gives the outer pixel boundaries. In this case, the position of Z[0,0] is the center of the pixel, not a corner.

If origin is Nonethen x0y0 is the position of Z[0,0], and x1y1 is the position of Z[-1,-1]. This argument is ignored if X and Y are specified in the call to contour. The locator is used to determine the contour levels if they are not given explicitly via levels. Defaults to MaxNLocator. Determines the contourf -coloring of values that are outside the levels range. If 'neither', values outside the levels range are not colored. If 'min', 'max' or 'both', color the values below, above or below and above the levels range.

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