matplotlib-devel Mailing List for matplotlib

John Hunter wrote:
> It is useful to store the final pixel buffer (eg in a PNG) as RGBA 
> because some people like to have some parts of their figure 
> transparent to composite the figure with other images.
fair enough, and that's probably a really cool feature when you need it!
ken wrote:
> I was talking about the image-from-a-buffer business not helping us
> with WX 2.4/2.6 due to the RGBA to RGB conversion.
But it looks like RendererAgg has this a agg.tostring_rgb() method, so 
we should be able to do change :
image.SetData(agg.tostring_rgb())
to
image.SetDataBuffer(agg.tostring_rgb())
If we make sure to keepthe string around. I haven't looked at your C++ 
code, but does it do something faster than RendererAgg.tostring_rgb() ?
Another thing that would be nice (for all Agg back-ends, I imagine), is 
if we could replace this:
 # agg => rgb -> image => bitmap => clipped bitmap => image
 return wx.ImageFromBitmap(_clipped_image_as_bitmap(image, bbox))
with a RendererAgg._clipped_tostring_rgb(bbox)
So that we don't copy a bunch of RGB data we don't need.
even if we don't do that, I think
_clipped_image_as_bitmap()
could use wx.Image.GetSubImage(), rather than creating a bimtp of the 
whole thing and blitting. untested code:
def _clipped_image_as_bitmap(image, bbox):
 """
 Convert the region of a wx.Image described by bbox to a wx.Bitmap.
 """
 l, b, width, height = bbox.get_bounds()
 return wx.BitmapFromImage(image.GetSubImage(wxRect((l,b),(w,h))))
 > RendererAgg appears to already have a buffer_rgba() method.
So we're all set for wxPython 2.7 -- very nice! I hope it doesn't make a 
copy.
Is there a numpy_array_rgba method -- that could be nice, and would work 
as a buffer, too. Maybe when we are ready to dump Numeric and numarray.
-Chris
-- 
Christopher Barker, Ph.D.
Oceanographer
 		
NOAA/OR&R/HAZMAT (206) 526-6959 voice
7600 Sand Point Way NE (206) 526-6329 fax
Seattle, WA 98115 (206) 526-6317 main reception
Chr...@no...

>>>>> "Ken" == Ken McIvor <mc...@ii...> writes:
 Ken> On 08/31/06 13:43, Christopher Barker wrote:
 >> Ken McIvor wrote:
 >> 
 >> a wxBitmap is the same format as the native rendering
 >> system. While most systems use 24b RGB (or 32b RGBA), people
 >> can still run displays at 16bpp or whatever, so it's still
 >> needed.
 Ken> I can understand why it's still necessary, although it's nice
 Ken> to sometimes pretend that everyone's running 24-bit color
 Ken> displays. I hope I didn't sound too judgemental!
 >>> I don't think we're going to be able to get performance
 >>> similar to that of the accelerator using straight Python code
 >> But whether it's Python or C++, you still need to do the
 >> Image->Bitmap conversion -- so if we can get rid of the data
 >> copying from Agg buffer to wxImage in Python, we don't need
 >> C++.
 Ken> I think we got some wires crossed at some point in the
 Ken> conversation, although it could be that I'm wearing the
 Ken> Stupid Hat today. I was talking about the
 Ken> image-from-a-buffer business not helping us with WX 2.4/2.6
 Ken> due to the RGBA to RGB conversion.
 >> And it has. For wxPython 2.7 (and now in CVS) there are methods
 >> for dumping 32 bit RGBA data directly into a wxBitmap with no
 >> copying, if the data source is a Python Buffer object. I think
 >> I posted a note about this here yesterday.
 Ken> Yes, you did mention it. I agree completely with this
 Ken> analysis of the situation. When I replied I wasn't thinking
 Ken> in terms of wxPython 2.7.
 >> To really get it to work, the 24bit RGB Agg buffer needs to be
 >> a Python Buffer object -- is it now? I'm sorry I don't have the
 >> time to mess with this now -- maybe some day.
 Ken> I guess Guido lets John borrow his time machine, because
 Ken> RendererAgg appears to already have a buffer_rgba() method.
Guido has been very generous with us :-)
 >> You can alpha composite into a non-alpha background. You just
 >> lose the alpha there, so that the background couldn't be
 >> alpha-composited onto anything else -- but does it ever need to
 >> be?
 Ken> I thought that the buffer's accumulated alpha played a role
 Ken> in compositing new pixels onto it, but I apparently
 Ken> misunderstood. 
It does: here is agg's rgba pixel blending routing
 static AGG_INLINE void blend_pix(value_type* p, 
 unsigned cr, unsigned cg, unsigned cb,
 unsigned alpha, 
 unsigned cover=0)
 {
 calc_type r = p[Order::R];
 calc_type g = p[Order::G];
 calc_type b = p[Order::B];
 calc_type a = p[Order::A];
 p[Order::R] = (value_type)(((cr - r) * alpha + (r << base_shift)) >> base_shift);
 p[Order::G] = (value_type)(((cg - g) * alpha + (g << base_shift)) >> base_shift);
 p[Order::B] = (value_type)(((cb - b) * alpha + (b << base_shift)) >> base_shift);
 p[Order::A] = (value_type)((alpha + a) - ((alpha * a + base_mask) >> base_shift));
 }
 Ken> Images. Anyway, if the buffer's alpha channel isn't used,
 Ken> then the whole situation does seem a bit odd. Could the
 Ken> information be retained for PNGs or something?
It is useful to store the final pixel buffer (eg in a PNG) as RGBA
because some people like to have some parts of their figure
transparent to composite the figure with other images.
JDH

On 08/31/06 13:43, Christopher Barker wrote:
> Ken McIvor wrote:
> 
> a wxBitmap is the same format as the native rendering system. While most 
> systems use 24b RGB (or 32b RGBA), people can still run displays at 
> 16bpp or whatever, so it's still needed.
I can understand why it's still necessary, although it's nice to sometimes 
pretend that everyone's running 24-bit color displays. I hope I didn't sound 
too judgemental!
>>I don't think we're going to be able to get performance similar to that 
>>of the accelerator using straight Python code
>
> But whether it's Python or C++, you still need to do the Image->Bitmap 
> conversion -- so if we can get rid of the data copying from Agg buffer 
> to wxImage in Python, we don't need C++.
I think we got some wires crossed at some point in the conversation, although 
it could be that I'm wearing the Stupid Hat today. I was talking about the 
image-from-a-buffer business not helping us with WX 2.4/2.6 due to the RGBA to 
RGB conversion.
> And it has. For wxPython 2.7 (and now in CVS) there are methods for 
> dumping 32 bit RGBA data directly into a wxBitmap with no copying, if 
> the data source is a Python Buffer object. I think I posted a note about 
> this here yesterday.
Yes, you did mention it. I agree completely with this analysis of the 
situation. When I replied I wasn't thinking in terms of wxPython 2.7.
> To really get it to work, the 24bit RGB Agg buffer needs to be a Python 
> Buffer object -- is it now? I'm sorry I don't have the time to mess with 
> this now -- maybe some day.
I guess Guido lets John borrow his time machine, because RendererAgg appears 
to already have a buffer_rgba() method.
> You can alpha composite into a non-alpha background. You just lose the 
> alpha there, so that the background couldn't be alpha-composited onto 
> anything else -- but does it ever need to be?
I thought that the buffer's accumulated alpha played a role in compositing new 
pixels onto it, but I apparently misunderstood. It must be time to read 
"Compositing Digital Images. Anyway, if the buffer's alpha channel isn't 
used, then the whole situation does seem a bit odd. Could the information be 
retained for PNGs or something?
> However, there is something to be said for just using alpha everywhere, 
> and as we'll soon be able to dump RGBA data straight into a wx.Bitmap, 
> this should work great.
Yes, it will be a great improvement over the current situation.
Ken

Ken McIvor wrote:
> I think they added preliminary support for alpha channels in 2.5, in the 
> form of wx.Image.HasAlpha().
right, but it uses the old 24 bit RGB buffer, and separate Alpha buffer, 
it's kind of tacked on, rather than native.
 > My beef is that you have to convert
> everything to a wx.Bitmap before you can do anything useful with it.
And that DCs don't support alpha, even though the underlying device 
often does. This has been discuses a lot, but no one has done much about 
it yet -- wxTNG will have something better, but who knows how far out 
that is?
> As near as I can tell, the primary slowdown at this point is the way 
> wxWidgets distinguishes from RGB image data (wx.Image) as opposed to 
> displayed image data (wx.Bitmap). Right now you cannot draw a wx.Image 
> without first converting it into a wx.Bitmap, nor can you use a MemoryDC 
> to blit or otherwise munge a wx.Image directly. My impression is that 
> this made sense when wxWindows was getting started (Win16 and Motif), 
> but is more of an artificial distinction at this point.
a wxBitmap is the same format as the native rendering system. While most 
systems use 24b RGB (or 32b RGBA), people can still run displays at 
16bpp or whatever, so it's still needed. Also, I wouldn't be surprised 
if some less common systems use ARGB or something else weird.
> I don't think we're going to be able to get performance similar to that 
> of the accelerator using straight Python code
But whether it's Python or C++, you still need to do the Image->Bitmap 
conversion -- so if we can get rid of the data copying from Agg buffer 
to wxImage in Python, we don't need C++.
> unless something changes 
> in the wxWidgets' Image/Bitmap/MemoryDC department.
And it has. For wxPython 2.7 (and now in CVS) there are methods for 
dumping 32 bit RGBA data directly into a wxBitmap with no copying, if 
the data source is a Python Buffer object. I think I posted a note about 
this here yesterday.
> I'd love to be proven wrong! If you're interested in the gory details, 
> you should check out the pure-Python implementation of the image 
> conversion functions, at the end of `backend_wxagg.py'.
I did, and I suggested some improvements a couple messages back. To 
really get it to work, the 24bit RGB Agg buffer needs to be a Python 
Buffer object -- is it now? I'm sorry I don't have the time to mess with 
this now -- maybe some day.
>> I do have one question -- does the agg back-end really need to use an 
>> alpha channel for it's buffer? Isn't it the whole image anyway? What 
>> is is it going to get blended with?
> 
> I don't know enough about Agg to venture an educated guess. My 
> un-educated guess is that there's an RGBA buffer to support alpha in the 
> drawing operations... how can Agg alpha-composite new pixels into the 
> buffer when you draw something, unless you know the alpha values of the 
> existing pixels?
You can alpha composite into a non-alpha background. You just lose the 
alpha there, so that the background couldn't be alpha-composited onto 
anything else -- but does it ever need to be?
However, there is something to be said for just using alpha everywhere, 
and as we'll soon be able to dump RGBA data straight into a wx.Bitmap, 
this should work great.
-Chris
-- 
Christopher Barker, Ph.D.
Oceanographer
 		
NOAA/OR&R/HAZMAT (206) 526-6959 voice
7600 Sand Point Way NE (206) 526-6329 fax
Seattle, WA 98115 (206) 526-6317 main reception
Chr...@no...

On 08/28/06 12:19, Christopher Barker wrote:
>
> wx is really due for an update to support alpha properly. But I guess
> you'll always have problems with different data formats.
I think they added preliminary support for alpha channels in 2.5, in the form 
of wx.Image.HasAlpha(). My beef is that you have to convert everything to a 
wx.Bitmap before you can do anything useful with it.
> Anyway, this thread started because people were having binary 
> compatibility issues. Even if this doesn't speed up the accelerator, it 
> may be possible to get the same performance without using 
> wx-version-specific compiled code -- i.e. pure python.
As near as I can tell, the primary slowdown at this point is the way wxWidgets 
distinguishes from RGB image data (wx.Image) as opposed to displayed image 
data (wx.Bitmap). Right now you cannot draw a wx.Image without first 
converting it into a wx.Bitmap, nor can you use a MemoryDC to blit or 
otherwise munge a wx.Image directly. My impression is that this made sense 
when wxWindows was getting started (Win16 and Motif), but is more of an 
artificial distinction at this point.
I don't think we're going to be able to get performance similar to that of the 
accelerator using straight Python code unless something changes in the 
wxWidgets' Image/Bitmap/MemoryDC department. That being said, I'd love to be 
proven wrong! If you're interested in the gory details, you should check out 
the pure-Python implementation of the image conversion functions, at the end 
of `backend_wxagg.py'.
> I do have one question -- does the agg back-end really need to use an 
> alpha channel for it's buffer? Isn't it the whole image anyway? What is 
> is it going to get blended with?
I don't know enough about Agg to venture an educated guess. My un-educated 
guess is that there's an RGBA buffer to support alpha in the drawing 
operations... how can Agg alpha-composite new pixels into the buffer when you 
draw something, unless you know the alpha values of the existing pixels?
Ken

I don't know how (or if) this can be improved, but I will gladly consider 
patches.
On Thursday 31 August 2006 03:09, Michael Fitzgerald wrote:
> Hi all,
>
> I have a question about the PS backend (building on the thread
> "imshow with PS backend" from ~ a month ago). Evidently this backend
> is fixed at 72 dpi. This isn't a problem with vector information.
> However, it would seem that one would want to use a higher resolution
> when plotting figures that use imshow() for raster data, since this
> command has several choices for interpolation. As I understand, the
> AxesImage is sampled at this low-resolution when being written to PS/
> EPS. Subsequent interpolation is done when printing, or viewing with
> ghostview. For the (originally?) raster data, gv seems to use a
> nearest-neighbor scheme, making the image blocky. It would be nice
> to use matplotlib's interpolation instead. Is there a fundamental
> reason this needs to be fixed at 72 dpi? As some publishers ask for
> EPS files of e.g. 300 dpi, I would think it's theoretically possible
> to export at different resolutions. My understanding is that the
> _preview_ image in the file is supposed to be 72 dpi.
>
> One possible workaround is to scale up the size of the figure (in
> inches), but then fonts, line thickness, marker sizes, etc. must also
> be scaled, making it less-than-satisfactory.
>
> Thank you in advance for any enlightenment, and please forgive my
> ignorance -- I must admit I don't know that much about PS, nor about
> the specific scheme used in matplotlib for getting the image data
> into the postscript file, so I may be critically mistaken in the
> above assessment.

Hi all,
I have a question about the PS backend (building on the thread 
"imshow with PS backend" from ~ a month ago). Evidently this backend 
is fixed at 72 dpi. This isn't a problem with vector information. 
However, it would seem that one would want to use a higher resolution 
when plotting figures that use imshow() for raster data, since this 
command has several choices for interpolation. As I understand, the 
AxesImage is sampled at this low-resolution when being written to PS/ 
EPS. Subsequent interpolation is done when printing, or viewing with 
ghostview. For the (originally?) raster data, gv seems to use a 
nearest-neighbor scheme, making the image blocky. It would be nice 
to use matplotlib's interpolation instead. Is there a fundamental 
reason this needs to be fixed at 72 dpi? As some publishers ask for 
EPS files of e.g. 300 dpi, I would think it's theoretically possible 
to export at different resolutions. My understanding is that the 
_preview_ image in the file is supposed to be 72 dpi.
One possible workaround is to scale up the size of the figure (in 
inches), but then fonts, line thickness, marker sizes, etc. must also 
be scaled, making it less-than-satisfactory.
Thank you in advance for any enlightenment, and please forgive my 
ignorance -- I must admit I don't know that much about PS, nor about 
the specific scheme used in matplotlib for getting the image data 
into the postscript file, so I may be critically mistaken in the 
above assessment.
Best,
Mike
(P.S. please cc me, as I'm not subscribed)

Rob:
I am building Matplotlib on my Mac with Python 2.5 Release Candidate 1.
We use this for our astronomical data-reduction software.
I saw your post on matplotlib-devel and I think I can help a bit. 
Here are some patches that get it to compile: