Pdf – Reduce resolution,size,dpi,number of pixels in PDF images

slhck's answer and scruss' comment deserve to be updated : pdfimages now (at least since version 0.26.5) explicitely lists x-ppi and y-ppi. Here is an sample output :

$ pdfimages -list example.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image    2244  2244  cmyk    4   8  image  no       215  0   301   301  418K 2.1%
   2     1 image     900   600  rgb     3   8  image  no       324  0  1524  1525 35.5K 2.2%

On Debian (Wheezy) and Fedora (23), pdfimages is part of poppler-utils packages.

I modified mupdf's pdfdraw to support drawing in best fit mode, so I could state that the output needed to be 128x128 at most and it would fit the output in the box while maintaining the aspect ratio. Before I did that the only way was to use pdfinfo to get the page size and then do the calcuations to fit it in a box and then ask pdfdraw to draw it with that scale factor (dots per inch).

Well, after that long story the process to do that is rather simple:

1. get the page size of the page to render (in pdf terms the media box) this can be done via pdfinfo and grep and will appear in pts (points, 1/72th of an inch) or via a pdf library like pyPDF like:

   import pyPdf
   p = pyPdf.PdfFileReader(file("/home/dan/Desktop/Sieve-JFP.pdf", "rb"))
   x,y,w,h = p.pages[0]['/MediaBox']
   

2. for a box fit do dpi = min( A/(w/72.), B/(h/72.) )
   where A is the maximum width and B is the maximum height; w and h are the width and height of the page.

3. pass dpi to convert -density $dpi

and as requested a slightly fudged git commit diff:

commit 0000000000000000000000000000000000000000
Author: Dan D.
Date:   Thu Jul 28 16:33:33 2011 -0400

    add options to pdfdraw to limit the output's width and height

    note that scaling must occur before rotation

diff --git a/apps/pdfdraw.c b/apps/pdfdraw.c
index 0000000..1234567 100644
--- a/apps/pdfdraw.c
+++ b/apps/pdfdraw.c
@@ -12,8 +12,10 @@
 #endif

 char *output = NULL;
-float resolution = 72;
+float resolution = -1;
 float rotation = 0;
+float width = -1;
+float height = -1;

 int showxml = 0;
 int showtext = 0;
@@ -47,6 +49,8 @@ static void usage(void)
        "\t\tsupported formats: pgm, ppm, pam, png, pbm\n"
        "\t-p -\tpassword\n"
        "\t-r -\tresolution in dpi (default: 72)\n"
+       "\t-w -\tmaximum width (default: no limit)\n"
+       "\t-h -\tmaximum height (default: no limit)\n"
        "\t-A\tdisable accelerated functions\n"
        "\t-a\tsave alpha channel (only pam and png)\n"
        "\t-b -\tnumber of bits of antialiasing (0 to 8)\n"
@@ -150,13 +154,39 @@ static void drawpage(pdf_xref *xref, int pagenum)

    if (output || showmd5 || showtime)
    {
-       float zoom;
+       float zoom = 1.0;
        fz_matrix ctm;
        fz_bbox bbox;
        fz_pixmap *pix;
+       float W, H;

-       zoom = resolution / 72;
-       ctm = fz_translate(0, -page->mediabox.y1);
+       ctm = fz_identity;
+       ctm = fz_concat(ctm, fz_translate(0, -page->mediabox.y1));
+       ctm = fz_concat(ctm, fz_rotate(page->rotate));
+       ctm = fz_concat(ctm, fz_rotate(rotation));
+       bbox = fz_round_rect(fz_transform_rect(ctm, page->mediabox));
+
+       W = bbox.x1 - bbox.x0; 
+       H = bbox.y1 - bbox.y0;
+       if (resolution != -1)
+           zoom = resolution / 72;
+       if (width != -1) 
+       {
+           if (resolution != -1)
+               zoom = MIN(zoom, width/W);
+           else
+               zoom = width/W;
+       }
+       if (height != -1)
+       {
+           if (resolution != -1 || width != -1)
+               zoom = MIN(zoom, height/H);
+           else
+               zoom = height/H;
+       }
+
+       ctm = fz_identity;
+       ctm = fz_concat(ctm, fz_translate(0, -page->mediabox.y1));
        ctm = fz_concat(ctm, fz_scale(zoom, -zoom));
        ctm = fz_concat(ctm, fz_rotate(page->rotate));
        ctm = fz_concat(ctm, fz_rotate(rotation));
@@ -295,7 +325,7 @@ int main(int argc, char **argv)
    fz_error error;
    int c;

-   while ((c = fz_getopt(argc, argv, "o:p:r:R:Aab:dgmtx5")) != -1)
+   while ((c = fz_getopt(argc, argv, "o:p:r:R:w:h:Aab:dgmtx5")) != -1)
    {
        switch (c)
        {
@@ -303,6 +333,8 @@ int main(int argc, char **argv)
        case 'p': password = fz_optarg; break;
        case 'r': resolution = atof(fz_optarg); break;
        case 'R': rotation = atof(fz_optarg); break;
+       case 'w': width = atof(fz_optarg); break;
+       case 'h': height = atof(fz_optarg); break;
        case 'A': accelerate = 0; break;
        case 'a': savealpha = 1; break;
        case 'b': alphabits = atoi(fz_optarg); break;
@@ -321,6 +353,10 @@ int main(int argc, char **argv)
    if (fz_optind == argc)
        usage();

+   if (width+height == -2)
+       if (resolution == -1)
+           resolution = 72;
+
    if (!showtext && !showxml && !showtime && !showmd5 && !output)
    {
        printf("nothing to do\n");