Saves an image to a specified file

cv.imwrite(filename, img)
cv.imwrite(filename, img, 'OptionName', optionValue, ...)

Input

• filename Name of the file.
• img Image to be saved.

Options

• FlipChannels in case the input is color image, flips the color order from MATLAB's RGB/RGBA to OpenCV's BGR/BGRA order. default true

The following format-specific save parameters are currently supported:

• JpegQuality For JPEG, it can be a quality from 0 to 100 (the higher is the better). Default value is 95.
• JpegProgressive Enable JPEG features, 0 or 1, default is false.
• JpegOptimize Enable JPEG features, 0 or 1, default is false.
• JpegResetInterval JPEG restart interval, 0 - 65535, default is 0 (no restart).
• JpegLumaQuality Separate luma quality level, 0 - 100, default is 0 (don't use).
• JpegChromaQuality Separate chroma quality level, 0 - 100, default is 0 (don't use).
• PngCompression For PNG, it can be the compression level from 0 to 9. A higher value means a smaller size and longer compression time. Default value is 3.
• PngStrategy For PNG; used to tune the compression algorithm. These flags will be modify the way of PNG image compression and will be passed to the underlying zlib processing stage. The strategy parameter only affects the compression ratio but not the correctness of the compressed output even if it is not set appropriately. One of:
  • Default (default) Use this value for normal data.
  • Filtered Use this value for data produced by a filter (or predictor). Filtered data consists mostly of small values with a somewhat random distribution. In this case, the compression algorithm is tuned to compress them better. The effect of Filtered is to force more Huffman coding and less string matching; it is somewhat intermediate between Default and HuffmanOnly.
  • HuffmanOnly Use this value to force Huffman encoding only (no string match).
  • RLE Use this value to limit match distances to one (run-length encoding). RLE is designed to be almost as fast as HuffmanOnly, but give better compression for PNG image data.
  • Fixed Using this value prevents the use of dynamic Huffman codes, allowing for a simpler decoder for special applications.
• PngBilevel Binary level PNG, 0 or 1, controls packing of pixels per bytes. If false, PNG files pack pixels of bit-depths 1, 2, and 4 into bytes as small as possible. default is false.
• PxmBinary For PPM, PGM, or PBM, it can be a binary format flag, 0 or 1, to specify ASCII or binary encoding. default is true.
• WebpQuality For WEBP, it can be a quality from 1 to 100 (the higher is the better). By default (without any parameter) and for quality above 100 the lossless compression is used.

For advanced uses, you can directly pass a vector of paramters:

• Params Format-specific save parameters encoded as pairs: [paramId_1, paramValue_1, paramId_2, paramValue_2, ...].

The function imwrite saves the image to the specified file. The image format is chosen based on the filename extension (see cv.imread for the list of extensions). Only 8-bit (or 16-bit unsigned uint16 in case of PNG, JPEG 2000, and TIFF) single-channel or 3-channel (with RGB channel order) images can be saved using this function. If the format, depth or channel order is different, use cv.cvtColor to convert it before saving. Or, use the universal cv.FileStorage I/O functions to save the image to XML or YAML format.

(If the chosen encoder does not support the depth of the input image, the image will be implicitly cast to 8-bit).

If the image cannot be saved (because of IO errors, improper permissions, unsupported or invalid format), the function throws an error.

It is possible to store PNG images with an alpha channel using this function. To do this, create 8-bit (or 16-bit) 4-channel image RGBA, where the alpha channel goes last. Fully transparent pixels should have alpha set to 0, fully opaque pixels should have alpha set to 255/65535.

Example

The sample below shows how to create such a RGBA image and store to PNG file. It also demonstrates how to set custom compression parameters:

% Create mat with alpha channel
nrows = 480; ncols = 640;
[I,J] = ndgrid(1:nrows, 1:ncols);
img = zeros(nrows, ncols, 4, 'uint8');
img(:,:,1) = uint8(255 * (nrows-I+1)/nrows);     % red
img(:,:,2) = uint8(255 * (ncols-J+1)/ncols);     % green
img(:,:,3) = uint8(255);                         % blue
img(:,:,4) = uint8(0.5 * sum(img(:,:,[1 2]),3)); % alpha

% save PNG file with alpha data
%imwrite(img(:,:,1:3), 'alpha.png', 'Alpha',img(:,:,4))
cv.imwrite('alpha.png', img, 'PngCompression',9, 'PngStrategy','RLE');
imfinfo('alpha.png')

% show image with alpha transparency
figure('Menubar','none', 'Color','k')
image(img(:,:,1:3), 'AlphaData',img(:,:,4))
axis image off