Get an image matrix.
void pdfGetImageMatrix(PDFHandle pdf, int idx, double mat[6])
This function returns the image matrix for the idxth image on the page specified in the last call to pdfGetImages.

The returned matrix is the PDF/PostScript-style matrix used to transform a 1x1 square to the target region where the image will be drawn. This information can be used to derive the horizontal and vertical image resolution (assuming a straight rotation with no shearing) - see the example code below.

double mat[6]; double hRes, vRes; int nImgs, i, mask, colorSpace, w, h; pdfGetImages(pdf, 1); nImgs = pdfGetNumImages(pdf); for (i = 0; i < nImgs; ++i) { pdfGetImageMatrix(pdf, i, mat); hRes = (w / sqrt(mat[0] * mat[0] + mat[1] * mat[1])) * 72; vRes = (h / sqrt(mat[2] * mat[2] + mat[3] * mat[3])) * 72; printf("image %d: %d x %d pixels per inch\n", i, hRes, vRes); }