Function output 'y' cannot be an mxArray in this context. Consider preinitializing the output variable with a known type

function y = fcn(u)
coder.extrinsic('imwrite');
y = imwrite(u,'testOutput.bmp');

I want to convert a jpg image to bmp image in simulink model. Tried doing through mathlab function but it is throwing this error. Can you please help me through this!

Answer 1

[Walter Roberson]

imwrite() never returns a value. The way the imwrite() function is defined prevents it from returning a value.

[Niranjan Ravi]

I have seen examples of imwrite() returning the image output. I have updated the code to rectify above error by

function y = fcn(u)
coder.extrinsic('imwrite');
y = coder.nullcopy(zeros(size(u)));
y = imwrite(u,'testOutput.bmp');

But now i get an error as:

Caused by:
Too many output arguments.

Could you please help me with this?

[Walter Roberson]

You can look at the imwrite() code yourself:

dbtype imwrite

The first few lines are

1     function imwrite(varargin)
2     %IMWRITE Write image to graphics file.
3     %   IMWRITE(A,FILENAME,FMT) writes the image A to the file specified by
4     %   FILENAME in the format specified by FMT.
5     %

Notice the complete lack of "=" and output variables on the "function" line. imwrite() can never return anything.

[Walter Roberson]

This mythical output argument for imwrite: what is it?

1. Is it the file name? File names are character vectors which cannot be returned in Simulink.
2. Is it a copy of the image you pass to imwrite()? If so, why bother to make it an output?
3. Is it a copy of the image you pass to imwrite(), but converted to RGB? If so then why, considering that the original might be grayscale or logical and most image files are happy to write that
4. Is it a copy of the image you pass to imwrite(), but if the data was uint16 it is returned the same, and if the data was uint8 then it is returned the same, but if the data was double then the result is as from im2uint8(), reflecting the data written to the file in the case where it had to be converted? Potentially useful, but No -- if you cared about getting that back then you could convert the data type before imwrite()
5. Is it a copy of the binary file written by imwrite(), complete with headers? Ummm, No -- and no other MATLAB operation returns raw file contents for write operations either.
6. Is it a copy of what you get by doing an imread() after the imwrite(), and thereby reflecting the data after it might have undergone any lossy compression? That could potentially be useful, but if you need it then you can imread()

[Niranjan Ravi]

Yes, you are right. It will not return any value. At the same time, it can be used to convert an image from one format to other format? My purpose of using this function is to convert it from jpg to bmp format. But when i use the code of imwrite() in command window it works.

But when deployed in simulink it does not. I dont understand the reason behind it.

[Walter Roberson]

You can imread() in any format and imwrite() into any format that supports the same datatype.

function y = fcn(u)
coder.extrinsic('imwrite');
imwrite(u,'testOutput.bmp');
y = 0;

The input to this would have to be the array of data to write out.

[Niranjan Ravi]

Thank you. Worked like a charm!

[Niranjan Ravi]

Hi ,

I tried to use imresize to change height and width of image.

function y = fcn(u)

coder.extrinsic('imresize','imwrite');

imresize(u,[640 480]);

imwrite(u,'valli270.bmp');

y = u;

But i am unable to do it. I took a portrait image as an input

[Walter Roberson]

function y = fcn(u)
coder.extrinsic('imresize','imwrite');
u640 = zeros(640, 480, size(u,3));
u640 = imresize(u,[640 480]);
imwrite(u640,'valli270.bmp');
y = u640;

[Niranjan Ravi]

The image is getting rotated and saved when i implement this code

[Walter Roberson]

How do you know the input image is portrait ? If you imread() it and image() it at the MATLAB level does it show up as portrait?

The reason I ask is that JPEG images in particular can have an Orientation EXIF header that tells the presentation software that data that is stored as landscape should be displayed as portrait. When you have a landscape image and you use a tool to rotate the image by 90 degrees, sometimes instead of rewriting the data matrix, the tools just write in a new Orientation header. But imread() does not pay any attention to the Orientation header and would use the way the data is stored.

[Nagendar Reddy]

Hi, I am also facing same problem "function output cannot be a mxarray, Consider preinitializing the output variable with a known type". In my application I am assigning output of "cat /dev/serial" output to M-function output. My command "cat /dev/serial" always returns 20 bytes data. So how i have to define length & type of my function output