Creating a waterfall plot with the spectrogram

Hi,

I need to create a waterfall plot where data is represented incrementally by horizontal 'bars' moving down the plot and the intensity represented by varying colors across each horizontal bar. It looks like the sprectrogram would be perfect for this application, and I am able to create and run the example without any problems, but I do not understand how I would modify it to get the horizontal bars moving down the plot over time that I need to show. I am totally new to using raster plots and do not have a good enough understanding of how the data is handled to get my data displaying properly. If anybody has created an app with similiar requirements using the spectrogram or any other widget I would appreciate any advice and/or a piece of sample code to get me started. Thank you in advance...

tim

I recommend to start with displaying your data at a specific moment. Then you will have understood how the spectrogram item works and how you can connect your raster data to it. Then have a look at the cpuplot example, that shows how to implement a moving time axis.

Rendering raster data is much more expensive than vector data ( like curves ). So don't expect a refresh rate of 25Hz. Consider using Qwt from SVN trunk, because in this version the spectrogram is able to render the image multithreaded with a linear effect on the performance on multicore systems.

Uwe

Sounds like something I did recently. In my case re-rendering the entire spectrogram was too slow. So I added a caching layer to get the speed I needed.

Code:

---

class Waterfall : public QwtPlotSpectrogram
{
public:
	Waterfall(const QString &title = QString::null) :
        	QwtPlotSpectrogram(title)
	{
	}
 
	virtual ~Waterfall()
	{
	}
 
protected:
	virtual QImage renderImage(const QwtScaleMap &xMap,
				   const QwtScaleMap &yMap,
				   const QRectF &rect) const
	{
		QRect rect = transform(xMap, yMap, area);
 
		// I don't know why this adjustment is necessary, but
		// without it qwt plots a different area than we cache
		rect.adjust(-2, -2, -3, -3);
		QwtScaleMap xxMap = xMap;
		QwtScaleMap yyMap = yMap;
 
		const QSize res(QwtPlotSpectrogram::rasterHint(area));
		if ( res.isValid() )
		{
			rect.setSize(rect.size().boundedTo(res));
		}
 
		int px1 = rect.x();
		int px2 = rect.x() + rect.width();
		if ( xMap.p1() > xMap.p2() )
		qSwap(px1, px2);
 
		double sx1 = area.x();
		double sx2 = area.x() + area.width();
		if ( xMap.s1() > xMap.s2() )
		qSwap(sx1, sx2);
 
		int py1 = rect.y();
		int py2 = rect.y() + rect.height();
		if ( yMap.p1() > yMap.p2() )
		qSwap(py1, py2);
 
		double sy1 = area.y();
		double sy2 = area.y() + area.height();
		if ( yMap.s1() > yMap.s2() )
		qSwap(sy1, sy2);
 
		xxMap.setPaintInterval(px1, px2);
		xxMap.setScaleInterval(sx1, sx2);
		yyMap.setPaintInterval(py1, py2);
		yyMap.setScaleInterval(sy1, sy2);
 
		// Redraw everything if the scale or size changed
		if (cachedImage_.isNull()
			|| xMap.pDist() != cachedXMap_.pDist()
			|| xMap.sDist() != cachedXMap_.sDist()
			|| yMap.pDist() != cachedYMap_.pDist()
			|| yMap.sDist() != cachedYMap_.sDist()
			|| area.width() != cachedArea_.width()
			|| area.height() != cachedArea_.height())
		{
			cachedImage_ = QwtPlotSpectrogram::renderImage(xMap, yMap, area);
			cachedXShift_ = 0.0;
			cachedYShift_ = 0.0;
		}
		else
		{
			QRect xRect, yRect;
 
			// Shift the cached image to compensate for the area change
			// and calculate the x and y boxes to redraw
			//
			// 0
			//  +------------------------+  ---
			//  |         -yRect         |   |  yShift (negative)
			//  +---+----------------+---+  ---
			//  | - |                | + |   |
			//  | x |                | x |   |
			//  | R | Cached/shifted | R |   |  height - abs(yShift)
			//  | e |     image      | e |   |
			//  | c |                | c |   |
			//  | t |                | t |   |
			//  |   |                |   |   |
			//  +---+----------------+---+  ---
			//  |         +yRect         |   |  yShift (positive)
			//  +------------------------+  ---
			//
			//  |---|                |---|
			//  xShift (negative)    xShift (positive)
 
			double xScale = xxMap.pDist() / xxMap.sDist();
			double yScale = yyMap.pDist() / yyMap.sDist();
			double xShift = -(cachedArea_.left() - area.left()) * xScale + cachedXShift_;
			double yShift = (cachedArea_.top() - area.top()) * yScale + cachedYShift_;
			cachedXShift_ = xShift - ceil(xShift);
			cachedYShift_ = yShift - ceil(yShift);
			xShift = ceil(xShift);
			yShift = ceil(yShift);
 
			// Just redraw everything if the shift size is greater than the image
			if (abs(xShift) > rect.width() || abs(yShift) > rect.height())
			{
				cachedImage_ = QwtPlotSpectrogram::renderImage(xMap, yMap, area);
				cachedXShift_ = 0.0;
				cachedYShift_ = 0.0;
			}
			else
			{
				if (yShift < 0)
				{
					yRect.setCoords(rect.left(), rect.top(), rect.right(), rect.top() - yShift);
 
					if (xShift < 0)
					{
						xRect.setCoords(rect.left(), yRect.bottom(), rect.left() - xShift, rect.bottom());
					}
					else if (xShift > 0)
					{
						xRect.setCoords(rect.right() - xShift, yRect.bottom(), rect.right(), rect.bottom());
					}
				}
				else if (yShift > 0)
				{
					yRect.setCoords(rect.left(), rect.bottom() - yShift, rect.right(), rect.bottom());
					if (xShift < 0)
					{
						xRect.setCoords(rect.left(), rect.top(), rect.left() - xShift, yRect.top());
					}
					else if (xShift > 0)
					{
						xRect.setCoords(rect.right() - xShift, rect.top(), rect.right(), yRect.top());
					}
				}
				else
				{
					if (xShift < 0)
					{
						xRect.setCoords(rect.left(), rect.top(), rect.left() - xShift, rect.bottom());
					}
					else if (xShift > 0)
					{
						xRect.setCoords(rect.right() - xShift, rect.top(), rect.right(), rect.bottom());
					}
				}
 
				// Without this re-adjustment the scale is off by 1 pixel in each direction
				rect.adjust(0, 0, 1, 1);
				if (!yRect.isNull() || !xRect.isNull())
				{
					QImage newImage(rect.size(), cachedImage_.format());
					QPainter painter(&newImage);
					newImage.fill(0);
 
					if (!yRect.isNull())
					{
						QImage Image;
						QRectF sArea = invTransform(xxMap, yyMap, yRect);
						Image = QwtPlotSpectrogram::renderImage(xxMap, yyMap, sArea);
						painter.drawImage(yRect, Image);
					}
 
					if (!xRect.isNull())
					{
						QImage image;
						QRectF sArea = invTransform(xxMap, yyMap, xRect);
						image = QwtPlotSpectrogram::renderImage(xxMap, yyMap, sArea);
						painter.drawImage(xRect, image);
					}
 
					painter.drawImage(rect.translated(-xShift, -yShift), cachedImage_, rect);
					cachedImage_ = newImage;
				}
			}
		}
 
		cachedXMap_ = xMap;
		cachedYMap_ = yMap;
		cachedArea_ = area;
		cachedRect_ = rect;
 
		return cachedImage_;
	}
 
private:
	mutable QImage cachedImage_;
	mutable QwtScaleMap cachedXMap_;
	mutable QwtScaleMap cachedYMap_;
	mutable QRectF cachedArea_;
	mutable QRect cachedRect_;
	mutable double cachedXShift_;
	mutable double cachedYShift_;
};

---

If you need it pixel perfect you might have to fiddle with it a bit. I think it still had an off by one problem in the cached image.

Hi Uwe,

Thank you for such a quick reply. Trying to work with data at one instance in time is what I have been trying to do using the qwt example and some of the other examples I have found here but I think.....no, I'm sure of it....that I just don't understand how the raster is used at all. I have set up a spectrogram plot with an overall area of for example, 500 X 500 and then try to put a horizontal 'band' of say 50 pixels high by 500 pixels wide in the plot with the color intensity based on my data (contained in an array of doubles which for simplicity right now all have the same value...) and using the next neighbor sampling method described in some of the other posts. So what I'm expecting/looking for is a 500 x 50 area of some solid color based on the value of my data, and the other 500x450 area of the plot remaining the same original background color since only one rectangular subset region has been created. The actual size of the 'bands' will be some constant area per data sample received. But what I continually get is a 'rainbow' of the colors displayed in the color bar filling the entire plot area of 500x500. Everything else looks correct in the plot...scale values, colorbar legend etc...I just don't know how to set up and control how my data is plotted using the raster object. Fortunately the data rate is very slow so I don't think the refresh rate will be an issue, at least not right now. I downloaded the latest from the svn trunk per your suggestion and will definately need the help provided in those examples when I get to that point. Thanks again.

tim
tim

Thank you xicoy, I'll give this a try...

It looks like you created this without using the raster item at all on your spectrogram? Just created the image and rendered it...is that correct or is this just a small snippet of the code? What does the transform method used on line 18 do....I didn't see it in the Qt or Qwt docs so I'm guessing its homegrown?

tim

transform is part of QwtPlotItem. It transforms the data coordinates to paint coordinates:

I still had to create and attach my custom raster data class of course. I made this class so I didn't have recalculate every point just because everything shifted down by one or two pixels.
You may have no problems replotting everything on each update depending on how dense you pack your data. In may case I would have had to reprocess nearly 1GB of data each second after my buffer had filled. I probably should have built my raster data class to create ripmaps, but this image caching worked well enough.
The job of this class was to shift the position of the cached image if needed, and call the default spectrogram draw functions but only for the slivers of data on the sides that really needed to be calculated. So it could potentially help improve your refresh rates later down the line.

As Uwe said, you really do need to get it working for a particular point in time first. So just forget about this class for now
Maybe you don't have your value() function in your raster data class set up right. You can write some quick tests for it.
Is the data getting stretched across the entire plot? Then maybe you need to add a call to setBoundingRect.

xilcoy,

Thank you again for your replies. I'm trying to get just a particuliar point in time done to start as you and Uwe suggest, but it seems no matter what I do with or without using a bounding rect I get the entire plot filled with bars spanning the spectrum of my colormap, instead of just the one horizontal 'bar' filling the coordinates of the bounding rect I am trying to fill in. The value function I am using is from one of the examples here on this forum (I think one of Uwe's) and uses uses a simple rounding for the re-sampling. The data fills the entire plot....seems to ignore the bounding rect I define. Maybe I'm defining it too late in the code after the raster area is already created...I'm not sure...

tim

The spectrogram item requests the bounding rectangle from the raster data object each time it is repainted. Did you forget to copy the assigned bounding rectangle in your implementation of YourRasterData::copy() ?

Uwe

PS: All these nasty copy/clone methods are already removed in the development branch.

Hi Uwe,Xilcoy,

I managed to get just a solid horizontal bar displayed on the spectrogram and have control over where it is drawn in the vertical direction (for step 1), but I still don't think I understand how the raster/ spectrogram plot works, and the way I'm drawing the bar is extremely slow....it takes in the order of 23 seconds to draw a screen with only one bar drawn (no animation even attempted yet).

I am not doing anything with the copy method...should I be? Right now it is left as it was in the example I started with and just returns a pointer to the Spectrogram data object. I'm setting the bounding rect in the QwtRasterData method. To draw the horizontal line I have just been working with how the points are written to the value method. Admittedly this spectrogram plotting has me confused....Uwe, have you ever considered writing a tutorial book on Qwt? I'd buy it.

tim

Quote:

---

I managed to get just a solid horizontal bar displayed on the spectrogram and have control over where it is drawn in the vertical direction (for step 1), but I still don't think I understand how the raster/ spectrogram plot works, and the way I'm drawing the bar is extremely slow....it takes in the order of 23 seconds to draw a screen with only one bar drawn (no animation even attempted yet).

---

Well, when the spectrogram item is requested ( by the plot widget ) to draw itself, it iterates over all pixels of the target device ( the widget ). For each pixel it calculates the corresponding scale coordinates (x/y) and requests a value ( QwtRasterData::value() ) for this position. Then it maps the value into a color ( using a QwtColorMap ) and puts it to a QImage. When all pixels are collected the image is painted to the target device.

So your implementation of YourRasterData::value() needs to be pretty fast as it is called often ( width * height ! ). If you needs 23 seconds you are doing something very, very expensive in YourRasterData::value() ( or in your color map, when you have implemented your own ).

Quote:

---

Uwe, have you ever considered writing a tutorial book on Qwt? I'd buy it.

---

Writing a tutorial is one of my priorities, when Qwt 6.0.0 has been done.

Uwe

Hello!

I have wrote simple program which captures sound samples from sound card in one thread and writes them to circular buffer. Another thread takes samples from buffer and process them (calculate FFT). The output (double*) is added in beggining of QList<double* > (last item is removed) and than signal is emmited to main thread that new data is ready to display.
In main thread i have two plots: simple qwtplot which plots current spectrum and second qwtspectrogram which plots spectrogram from QList.
The problem is that it works very slow - about 2 FPS. When I remove spectrogram from GUI and leave only qwtplot I have about 22 FPS.
So my question is: how to speed up generating/drawing spectrogram ?

Misza

Quote:

---

Originally Posted by m15ch4

So my question is: how to speed up generating/drawing spectrogram ?

---

Implement YourRasterData::rasterHint().

Uwe

I have gotten a waterfall plot working using the spectrogram widget thanks to a lot of help from Uwe and Xicoy above.... However, until now I have been using all positive values.....fe...x axis: 0-4096 samples, y axis: 0-250 ms, sample values between 0-40, colorbar scale 0-40. Now I need to modify it so that the x axis has +/- values (-2048 to 2048 for example), and data values from -200 to 200. Can that be done with this spectrogram widget? My attempts have yielded a colorbar which is still fine for positive values, but all negative values just remain at the color used for 0. And any negative data values default to the color for zero on the colorbar. Also values only plot on the positive side of the x axis, negative data values seem to be ignored and not plotted at all. I've tried adjusting range and axis values etc...but so far have had no success. Any thoughts or advice would be appreciated.
I am using qwt 6.0.

Thank you,
tim

There is no limitation for ranges - the problem is on your side. Maybe the bounding rectangle of your data is wrong.

Uwe

Can the labels on the x-axis scale major ticks be changed to a different string other than the bounding rect interval values it defaults to? My data is displayed fine now thanks to your tip above, but I would like the major tick labels to show something like ...50Mhz 100Mhz 150Mhz....? I don't want to change anything about how the spectrogram data is plotted, just the label strings displayed.

tim

Quote:

---

Can the labels on the x-axis scale major ticks be changed to a different string other than the bounding rect interval values it defaults to?

---

Derive from QwtScaleDraw and implement "QwtText YourScaleDraw::label((double) const".
F.e. see the cpuplot example.

Uwe

Quote:

---

Originally Posted by xilcoy

Sounds like something I did recently. In my case re-rendering the entire spectrogram was too slow. So I added a caching layer to get the speed I needed.

[..]

If you need it pixel perfect you might have to fiddle with it a bit. I think it still had an off by one problem in the cached image.

---

Hi xilcoy,
I've a couple of questions about your class Waterfall and in particular about renderImage method.
First of all i d'dn't understand why the third parameter is const QRectF; shouldn't it be const QwtDoubleRect instead?
Then, the other thing that is not clear to me is if the area must be different each time or if it's always the same. In this case, in fact, xShift and yShift are 0. If area should change, how can you change it?

Thx

Quote:

---

Originally Posted by stilgar

Hi xilcoy,
First of all i d'dn't understand why the third parameter is const QRectF; shouldn't it be const QwtDoubleRect instead?

---

A QwtDoubleRect is a QRectF. From the Qwt docs:

Code:

---

typedef QPointF 	QwtDoublePoint
typedef QSizeF 	QwtDoubleSize
typedef QRectF 	QwtDoubleRect

---

Quote:

---

Originally Posted by d_stranz

A QwtDoubleRect is a QRectF. From the Qwt docs:

Code:

---

typedef QPointF 	QwtDoublePoint
typedef QSizeF 	QwtDoubleSize
typedef QRectF 	QwtDoubleRect

---

---

Yes, it is true, but my question was not exactly what I meant. The real question was: the third parameter shouldn't be area instead of rect?

Thx

I'm using Qt-4.7.2 and qwt-6.0.0 and I need a waterfall plot.
If you have a implementation and you want share it, I would be pleased and it would save me much time.

Many thanks.

Quote:

---

Originally Posted by Uwe

Writing a tutorial is one of my priorities, when Qwt 6.0.0 has been done.Uwe

---

Uwe v6.0.2 is out - if this tutorial exists, please post the location. It is sorely needed by Qwt newbies - as are working practical examples.

Qwt 6.1-rc1 is out too and all I have done so far is buying a book about DocBook.
Concerning the examples: what topics would you like to see that are not covered by the existing ( 6.1 ) examples already ?

Uwe

Reviewing v6.1 examples now.

In general generic examples that do nothing are not useful in practice for newbies to Qwt. On topic, an example of using Qwt to make a waterfall widget that displays sound captured (from say a jack audio input client) would be an outstandingly practical example for anyone as long as it works.

Quote:

---

In general generic examples that do nothing are not useful in practice for newbies to Qwt.

---

I completely disagree, examples like simpleplot are exactly what a newbie needs. The complete example has 42 lines of code and shows how easy it is to set up an application with a curve chart. This is the relevant information a user needs to know to get a project started.

Quote:

---

On topic, an example of using Qwt to make a waterfall widget that displays sound captured (from say a jack audio input client) would be an outstandingly practical example for anyone as long as it works.

---

Nothing could be more off topic than this. A waterfall plot qualifies as a demo like the oscilloscope - a cool application showing the power of Qwt, but leaving a newbie completely clueless, when reading the code.

But don't get me wrong: I would like to have a demo for a waterfall plot too, but the problem is the generation of the data:

• examples have to run on all target platforms without external dependencies - nobody installs and runs jack only to see how Qwt works.
• you can't blow up the size of a Qwt tarball with recorded data ( like the cpuplot example does for systems without a proc file system )

That's why in the end examples often end with synthetic data.

So if you have a specification for a waterfall plot and an idea how to generate the data I would be willing to implement such a demo. Also I could imagine to offer a more high level set of widgets - if I would have reasonable specifications - like:

• QwtOscilloscope
• QwtWaterfallPlot
• ...

This is not the first thread about a waterfall example, but unfortunately ideas like this usually die as soon as I'm asking for any type of contribution ...

Uwe