Output of -RGBimage seems to have changed, any help?

I have worked on a function (applyhatch_plusC) which has been on the file exchange now for 15years. Recently, the functionality fails.

What seems to be the issue is the print(gcf,'-RGBimage') does not behave as before, and appears to be doing some internal dithering that I can't seem to turn off. In previous MatLab versions, even back to <2011 with the funtion hardcopy, an RGB copy of a figure (say, containing a red, green, and blue object) would output a cdata matrix of the image, with 5 unique colores (rgb and b&w). I do the same thing now, on the same image, and I get a whole slew of shades, maybe occuring on a few dozen pixels only.

Now, as my work exploits the unique colors to convert 1 color to soemthing else, this no longer works. hardcopy no longe exists, and I don't see how to get print(gcf,'-RGBimage') to accept options. I would like to avoid passing through a temp file, as this requires write access and isn't very elegant.

This is rather frustrating, as this undocumented MatLab change ruins a nice little function that's been download thousands of times...

[KALYAN ACHARJYA]

Share the file exchange link? Which Matlab version you are using?

[Brian Katz]

My function is applyhatch_plusC https://fr.mathworks.com/matlabcentral/fileexchange/26797-applyhatch_plusc

Currently running MatLab 2017b.

[Brian Katz]

Hello? Can anyone help with this question?

[Adam Danz]

The following code produces a [830 x 1184 x 3] array of RGB values in r2019a. Do you expect otherwise?

surf(peaks); 
cdata = print(gcf,'-RGBImage');
size(cdata)

When I run the same code in r2014a, I get the error message,

Error using inputcheck>LocalCheckOption (line 342)
Illegal option '-RGBImage' given.
Error in inputcheck (line 86)
                opIndex = LocalCheckOption( cur_arg, options );
Error in print (line 166)
    [pj, devices, options ] = inputcheck( pj, inputargs{:} ); 

The first mention of "-RGMImage" in the documentation for print() was in r2014b (as you can see, it's missing in the documentation for r2014a).

So, that conflicts with the idea that this flag worked in 2011, doesn't it?

[Brian Katz]

Dear Adam,

The Matlab Central submissionin question is https://www.mathworks.com/matlabcentral/fileexchange/26797-applyhatch_plusc.

This was written some time ago, using the hidden function hardcopy to get an RGB image dump of a figure window. This disappeared in 2017 (I think) at which point I moved to print with the option -RBGimage. However, as I noted in my original question, the output of this is not the same as with hardcopy, as it seems to perform dithering on the image.

An an example:

bar(rand(1,3)); create simple 1 color bar graph

bits = print(gcf,'-RGBImage');

bits_r = bits(:,:,1);bits_g = bits(:,:,2);bits_b = bits(:,:,3);bits_rgb = [bits_r(:) bits_g(:) bits_b(:)];

unique(bits_rgb,'rows')

cdata should be an RGB version of the orginal figure, comprising only 3 colors (white [background], black [lines], and blue [the bars]). Instead, it contains over 126 different colors, or actually black, white, blue and the rest are shades of gray.

My question is, how to arrive at the previous functionality of hardcopy now that it is gone, and been replaced by print option which doesn't perform the same.

This was my question, not about when the flag RGBimage option appeared in print.

Looking forward to your response to that I can bring this function back to use (unless there is a new MatLab function for hatch-filling polygons).

Cordially,

-Brian

[Brian Katz]

Hello? Can anyone help with this question? I want an undithered RGB image dump, as was possible in the past.

[Walter Roberson]

I wonder if it helps to turn off graphics smoothing?

https://www.mathworks.com/help/matlab/creating_plots/using-graphics-smoothing.html

[Brian Katz]

This reduced the amount of dithering, but did not toally eliminate it. Here is an example, where you can see the assumed black, blue, and white, but also 2 gray shades being produced (ie some smoothing still occuring):

>> bar(rand(1,3)); % create simple 1 color bar graph
>> set(gcf,'GraphicsSmoothing','off')
>> fr = getframe(gca);
>> getf_r = fr.cdata(:,:,1);getf_g = fr.cdata(:,:,2);getf_b = fr.cdata(:,:,3);getf_rgb = [getf_r(:) getf_g(:) getf_b(:)];
>> [uniqueEntries,numberOfOccurences] = countEntries(getf_rgb,true)

uniqueEntries =

  5×3 uint8 matrix

     0     0     0
     0     0   255
    38    38    38
   240   240   240
   255   255   255


numberOfOccurences =

        2099
       72531
        1623
           1
       73386

[Walter Roberson]

Try the below both with graphics smoothing on and off.

The [38 38 38] you are seeing is due to the default XColor and YColor for the tick marks and text.

Your countEntries appears to be from a file exchange submission https://www.mathworks.com/matlabcentral/fileexchange/23661-violin-plots-for-plotting-multiple-distributions-distributionplot-m so I substituted standard code for that part.

fig = gcf;
ax = axes(fig);
bar(ax, rand(1,3)); % create simple 1 color bar graph
%set(fig,'GraphicsSmoothing','off')
ax.XColor = [0 0 0];
ax.YColor = [0 0 0];

fr = getframe(ax);

getf_r = fr.cdata(:,:,1);getf_g = fr.cdata(:,:,2);getf_b = fr.cdata(:,:,3);getf_rgb = [getf_r(:) getf_g(:) getf_b(:)];
[uniqueEntries,~,G] = unique(getf_rgb,'rows');
numberOfOccurences = accumarray(G, 1);

uniqueEntries

uniqueEntries = 3×3

0 0 0 0 114 189 255 255 255

●

numberOfOccurences

numberOfOccurences = 3×1

2319 47846 111103

●

Answer 1

[Brian Katz]

Looking more closely, it appears that the no smoothing option (when also applied to the axis Font) works pretty well. I have tested it in combination with print('-RGBImage').

In doing so, I also discovered that the default color for axis text and box is not BLACK, but [0.15 0.15 0.15]. Setting these object properties (XColor,YColor,GridColor,...) finally resolves the whole problem.

Thank you to all for chiming in. I will update by FileExchange submission soon to reflect all this.

Answer 2

[Image Analyst]

Maybe try the attached MaximizeFigureWindow function first, and then call either saveas(), imwrite(), or (on the File Exchange) export_fig

Answer 3

[Image Analyst]

Try

theImage = getimage(gca);

or else

frame = getframe(gcf)
figure;
imshow(frame.cdata);
axis off

[Brian Katz]

Looking at the results of this function, the image is still dithered and not the original image. Looking at the exmaple of a single color bar graph, the present colors are blue, white, and black. However, using getimage produces an image with 9 colors, being interpolated colors at/near borders between colors:

uniqueEntries =

  9×3 uint8 matrix

     0     0     0
     0     0   255
    19    19   146
    38    38    38
    64    64    64
    92    92    92
   146   146   146
   240   240   240
   255   255   255

numberOfOccurences =

        1240
       39222
           3
        1618
           6
           4
          21
         343
      106748

The older function did not do this, but actually created a "true" bitmap of the data, not an interpolated/dithered image.

Can you apply getimage, or an equivalent, without dithering?

[Image Analyst]

Can you give a small example with an actual RGB image that you synthesize? Be aware that if you use something like plot() over the image, it will antialias the line so that line won't be that one single color that you drew it in.

[Brian Katz]

Please see my original post for a very simple example using BAR.

What I am looking for is a replacement of the behaviour of the now defunct "hardcopy" function which would create a BMP version of a figure, without dithering/antialias processing, at a desired DPI (so not even a screendump).

For image processing of the figure, to replace colored regions as in the function I wrote, this is no longer possible because the number of colors is both incorrect and unpredictable.