Equivalent of -colors 2.

[rpatelob]

I'm using ImageMagick 7.0.5-5 Q16 x86_64

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convert -size 20x640 gradient: -rotate 90 -colors 2 -colorspace gray -auto-level test.jpg

C code

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    MagickWand * m_wand;
    PixelWand *PW1, **pixels;
    m_wand = NewMagickWand();
    PW1 = NewPixelWand();

    PixelSetColor(PW1,"none");
    MagickNewImage(m_wand, 20, 640, PW1);
    MagickSetImageMatte(m_wand, MagickTrue);
    MagickRotateImage(m_wand, PW1,90);
    MagickTransformImageColorspace(m_wand, GRAYColorspace);
    MagickAutoLevelImage(m_wand);
    PixelIterator *iterator = NULL;
    size_t x,y;
    int gray;
    char hex[128];
  	iterator=NewPixelIterator(m_wand);
  	for(y=0;y<20;y++) {
  		pixels=PixelGetNextIteratorRow(iterator,&x);
  		for(x=0;x<640;x++) {
  			gray = x*255/640;
  			sprintf(hex,"#%02x%02x%02x",gray,gray,gray);
        		PixelSetColor(pixels[x],hex);
  		}
  		PixelSyncIterator(iterator);
  	}
    MagickWriteImage(m_wand, "testing.jpg");

I'm trying to convert above CLI into C and there is my code. What I don't understand is -colors 2. What is the equivalent of it?
And another thing is I need to generate the gradient, and for that I have taken two loops according to image's dimentions and got the result. Is it a good solution? If not then Is there any other way to do it?

[snibgo]

What I don't understand is -colors 2. What is the equivalent of it?

I don't know, but MagickQuantizeImage() seems likely.

And another thing is I need to generate the gradient, and for that I have taken two loops according to image's dimentions and got the result. Is it a good solution? If not then Is there any other way to do it?

Your solution will make a maximum of 256 different values, across an image that is 640 pixels wide. If you are using Q16, a correct gradient would have 640 different values.

If you are using Q8, having only 256 values is reasonable, but I don't think code should be dependant on a particular Q-number. Hint: I would use "QuantumRange" instead of "255".

[rpatelob]

Thanks @snibgo. "MagickQuantizeImage" looks similar. I'll try that.

[el_supremo]

This works with V6.9.7-10:

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    MagickSetSize(m_wand,20,640);
    MagickReadImage(m_wand,"gradient:white-black");
    MagickRotateImage(m_wand, PW1,90);

    // In V7 the 5th argument is a DitherMethod rather than a boolean - try NoDitherMethod
    MagickQuantizeImage(m_wand,2,GRAYColorspace,0,MagickTrue,MagickFalse);

    MagickAutoLevelImage(m_wand);
    MagickWriteImage(m_wand, "gradient_colors.jpg");

With V7, the 5th argument to MagickQuantizeImage can be one of NoDitherMethod, RiemersmaDitherMethod or FloydSteinbergDitherMethod. I presume that NoDitherMethod will work.

Pete

[el_supremo]

I've just installed V7. NoDitherMethod doesn't work and the tree depth needs to be 1 instead of 0.

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	MagickQuantizeImage(m_wand,2,GRAYColorspace,1,FloydSteinbergDitherMethod,MagickFalse);

Pete

[rpatelob]

Hello @el_supremo,

The expected result is not the same. It looks like similar but not the same.
Please check two images

Using CLI
https://drive.google.com/file/d/0B-HZjm ... sp=sharing

Using C Code
https://drive.google.com/file/d/0B-HZjm ... sp=sharing

[rpatelob]

Hello @el_supremo,
One quick question, there are two ways to achieve gradient using PixelIterator and just using MagickReadImage(m_wand,"gradient:white-black");
Which one is the recommend solution for gradient?

[snibgo]

Please check two images

Using CLI
Using C Code

Yes, the images are different. Perhaps they used different dithering methods.

What was your command? What was your C code?

[rpatelob]

Yes, the images are different. Perhaps they used different dithering methods.

What was your command? What was your C code?

Please check command and C code.

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convert -size 20x640 gradient: -rotate 90 -colors 2 -colorspace gray -auto-level test.jpg

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  MagickWand  * wand;
    PixelWand * PW1;
    size_t newHeight = 640, newWidth = 20;

    wand = NewMagickWand();
    PW1 = NewPixelWand();

    PixelSetColor(PW1,"none");
    MagickSetSize(wand,20,640);
    MagickReadImage(wand,"gradient:white-black");
    MagickRotateImage(wand, PW1, 90);
    MagickQuantizeImage(wand, 2, GRAYColorspace, 1, FloydSteinbergDitherMethod, MagickFalse);
    MagickAutoLevelImage(wand);
    MagickWriteImage(wand, "test1.jpg");

[snibgo]

There are only two dithering methods: FloydSteinberg and Riemersma. As I suspected, you have used different methods. So the results are different.

[rpatelob]

There are only two dithering methods: FloydSteinberg and Riemersma. As I suspected, you have used different methods. So the results are different.

With RiemersmaDitherMethod I got the same result. Just a quick question. How do I know that which method I need to use?
Because in one of my examples I have tried all to get the result.

[snibgo]

With RiemersmaDitherMethod I got the same result.

In what? The command or C code?

Using your command, I used "-dither XXX" with the two methods. The two results I got were the same as your two results.

[rpatelob]

In what? The command or C code?

Using C code. The CLI result was perfect, I wanted the same in C. So the RiemersmaDitherMethod worked.