Re: plot data stream in realtime

["Amish S. Dave" <dave amish gmail com>]

Just some thoughts on this topic. Basically, it gets at the idea of 
trying to handle different representations for the input data.  This is 
assumed to be representable by static arrays of gfloats.  That's fine, 
but it means that all applications that don't start with that 
representation need to adapt their representation to what gtkdatabox wants.

It should be fairly straightforward to implement a slightly different 
scheme that could allow the inputs arrays to be

1. the current linear arrays [x0 x1 x2 .. xn] [y0 y1 y2 .. yn]
2. represent a ring-buffer e.g. [xn-1 xn x0 x1 x2 ... xn-2]
3. dynamically growing arrays
4. interleaved X&Y values  [x0 y0 x1 y1 x2 y2 ... xn yn]

I'm thinking of something like this:

gtk_databox_graph_data_update(graph, int XYallocpts, void *X, void *Y, 
int startpt, int numpts, int xstridebytes, int ystridebytes, GType 
XYdatatype);

If this function is never called, backwards compatibility is assured by 
assuming that
XYdatatype == G_TYPE_FLOAT;
xstridebytes  = ystridebytes = sizeof(float);
startpt = 0;
numpts = XYallocpts

But, if this function *is* called, the graph replaces the existing X and 
Y with the new pointers, and it forces a redraw, and updates any rulers 
as needed.

For the example of a realtime plot of something where new data is coming 
in and the graph is adding points to the rightward end, the calling 
program could allocate an array at the beginning large enough to hold 
the final size (XYallocpts), but by calling this function periodically 
with numpts incremented to reflect number of points currently available, 
it would have the effect of adding new data to the right of the 
screen.   No new allocations, and no unnecessary copying of arrays.

If you wanted to scroll the data, you could keep numpts fixed once it 
reached your number_of_points_to_graph, and then start incrementing 
startpt such that the most recently acquired point was startpt + numpts 
- 1...

If you need to realloc() a larger array, then this function will note 
the change in the XYallocpts value, or the X or Y pointers, and realize 
that new arrays are being passed...

If the arrays were actually ring buffers, then calling
...
gtk_databox_graph_data_update(ptsgraph, 10, Xpts, Ypts, 0, 10, 
sizeof(float), sizeof(float), G_TYPE_FLOAT);
Ypts[9] = get_data_from_adc();
gtk_databox_graph_data_update(ptsgraph, 10, Xpts, Ypts, 1, 10, 
sizeof(float), sizeof(float), G_TYPE_FLOAT);
Ypts[0] = get_data_from_adc();
gtk_databox_graph_data_update(ptsgraph, 10, Xpts, Ypts, 2, 10, 
sizeof(float), sizeof(float), G_TYPE_FLOAT);
Ypts[1] = get_data_from_adc();
gtk_databox_graph_data_update(ptsgraph, 10, Xpts, Ypts, 3, 10, 
sizeof(float), sizeof(float), G_TYPE_FLOAT);
Ypts[2] = get_data_from_adc();
...
shows 10 points each time but the latest point is always on the right of 
the graph.  The X Axis could be kept constant, or the application could 
update it if so desired by modifying Xpts[x].

This scheme is also an opportunity to add support for types other than 
float...  I'd suspect most of the time, GtkDatabox is being grafted onto 
another already developed application that has it's own data 
representations.  It's annoying to have to convert doubles/ints/whatever 
to floats for example.  Most of what needs to be done to support 
non-float data is in one function: gtk_databox_values_to_pixels... and 
the modification is not hard.

What do people think?

Sincerely,
--
Amish

On 11/12/2012 02:30 PM, Johannes Deutsch wrote:
> Hi all,
>
> I'm very thankful for all of your echos. I received interesting
> suggestions and was particularly pleased that the mailing list is very
> active!
>
> I already wrote a little demo that simulates plotting of data in
> realtime. By now the code is messy but i will pretty it up
> tomorrow and post i on this list.
>
> Maybe it is a valid example. If not i would be thankful for any
> comments to improve it further...
>
> With best regards and thanks for this great project
> _______________________________________________
> gtkdatabox-list mailing list
> gtkdatabox-list gnome org
> https://mail.gnome.org/mailman/listinfo/gtkdatabox-list