如何使用PDFlib精确放置裁剪和调整大小的图像？

Question

Wrestled with this for a couple of hours and just can't figure out how PDFlib works. I have this image that we'll say has the size of 1000 x 300 pixels:

Now I want to place a part of the image into a box of 20 x 12 millimeters (at 300 dpi). The resizing must be such that if no cropping were done the image would take up 70 x 21 mm on the page. 3 mm of the image must be cropped away from the top, and 2 mm cropped away from the left side.

The examples and documentation are too weak for me to understand exactly how the parameters for fit_image() work. How do I accomplish this?

Note: I'm doing this in PHP but I guess the main concern is what parameters PDFlib needs, which is language independent.

Answer 1

There are a few steps required to get this solved, but of course it's easily possible. Your question contains various task which must be solved:

1. scale down the input image to a given area
2. clip this scaled down image
3. place the image scaled and clipped to a given position. (this was not exactly clear from your question, so you might adjust the x/y parameters of the fit_image() depending to your needs.

This is one way to solve it:

# Load the image
$image = $p->load_image("auto", $imagefile, "");

# First we retrieve the dimension and the resolution for the loaded image
$imagewidth = $p->info_image($image, "imagewidth", "");
$imageheight = $p->info_image($image, "imageheight", "");
$dpix = $p->info_image($image, "resx", "");
$dpiy = $p->info_image($image, "resy", "");

# Calculate the scale factor, to fit the image to a width/height of 70 x 21 mm.
# Use a helper function to calculate the mm-values to the PDF points
$scalex = mm2pt(70) / $imagewidth; 
$scaley = mm2pt(21) / $imageheight; 

# For demonstrating the correct placing, fit the loaded image with a
# size of 70x21 mm with a light opacity (scaling it to this dimension
# might distort the image ratio) (final code would not include this)
$gstate = $p->create_gstate("opacityfill=.4");
$optlist = sprintf("gstate=%d scale {%f %f} dpi=72",
                $gstate, $scalex, $scaley);
$p->fit_image($image, mm2pt(10), mm2pt(250), $optlist);

# Use dpi=72 to ignore the internal DPI value and interpret each image 
# pixel without scaling.
# Now, specify the partial area with a matchbox clipping (remember that
# those values are the positions within the 70x21, and y goes from bottom to top)
$optlist = sprintf("scale {%f %f} matchbox={clipping={%f %f %f %f}} dpi=72", 
                    $scalex, $scaley, mm2pt(2)/$scalex, mm2pt(6)/$scaley, 
                    mm2pt(22)/$scalex, mm2pt(18)/$scaley);

# Set the reference point, so the origin of the clipped image will be the 
# same as for the original image
$p->fit_image($image, mm2pt(10)+mm2pt(2), mm2pt(250)+mm2pt(6), $optlist);

function mm2pt($mm){
    return $mm*2.83465;
}

so when using this code and one of the PDFlib sample Images to place the partially image on top of the original image, I get the following output: