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Create PDF with (resized) PNG images using Pycairo - rescaling Surface issue

I have som PNG image links that I want to download, "convert to thumbnails" and save to PDF using Python and Cairo.

Now, I have a working code, but I don't know how to control image size on paper. Is there a way to resize a PyCairo Surface to the dimensions I want (which happens to be smaller than the original)? I want the original pixels to be "shrinked" to a higher resolution (on paper).

Also, I tried Image.rescale() function from PIL, but it gives me back a 20x20 pixel output (out of a 200x200 pixel original image, which is not the banner example on the code). What I want is a 200x200 pixel image plotted inside a 20x20 mm square on paper (instead of a 200x200 mm square as I am getting now)

My current code is:

#!/usr/bin/python

import cairo, urllib, StringIO, Image   # could I do it without Image module?

paper_width = 210
paper_height = 297
margin = 20

point_to_milimeter = 72/25.4

pdfname = "out.pdf" 
pdf = cairo.PDFSurface(pdfname , paper_width*point_to_milimeter, paper_height*point_to_milimeter)
cr = cairo.Context(pdf)
cr.scale(point_to_milimeter, point_to_milimeter)

f=urllib.urlopen("http://cairographics.org/cairo-banner.png")
i=StringIO.StringIO(f.read())
im=Image.open(i)

# are these StringIO operations really necessary?

imagebuffer = StringIO.StringIO()  
im.save(imagebuffer, format="PNG")
imagebuffer.seek(0)
imagesurface = cairo.ImageSurface.create_from_png(imagebuffer)


### EDIT: best answer from Jeremy, and an alternate answer from mine:
best_answer = True      # put false to use my own alternate answer
if best_answer:
    cr.save()
    cr.scale(0.5, 0.5)
    cr.set_source_surface(imagesurface, margin, margin)
    cr.paint()
    cr.restore()
else:
    cr.set_source_surface(imagesurface, margin, margin)
    pattern = cr.get_source()
    scalematrix = cairo.Matrix()    # this can also be used to shear, rotate, etc.
    scalematrix.scale(2,2)          # matrix numbers seem to be the opposite - the greater the number, the smaller the source
    scalematrix.translate(-margin,-margin)  # this is necessary, don't ask me why - negative values!!
    pattern.set_matrix(scalematrix)  
    cr.paint()  

pdf.show_page()

Note that the beautiful Cairo banner does not even fit the page... The ideal result would be that I could control the width and height of this image in user space units (milimeters, in this case), to create a nice header image, for example.

Thanks for reading and for any help or comment!!

Solution

  • Try scaling the context when you draw the image.

    E.g.

    cr.save()    # push a new context onto the stack
cr.scale(0.5, 0.5)    # scale the context by (x, y)
cr.set_source_surface(imagesurface, margin, margin)
cr.paint()
cr.restore()    # pop the context

    See: http://cairographics.org/documentation/pycairo/2/reference/context.html for more details.