Monkeybread Software - DynaPDF Manual

DynaPDF Manual - Page 648

Function Reference

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The above values are already pre-computed and can be taken from the structure TPDFImage.

An image can be used several times in a document, e.g. if it represents a logo or other repeating

contents. To avoid the extraction of unnecessay duplicates a duplicate check should be performed

before extracting an image. If the image is not an inline image, the member ObjectPtr of the structure

TPDFImage represents a unique pointer to the image object.

Inline images are fully defined in the content stream and cannot be used repeatly. A duplicate check

is not required for these images. The only way to compare two inline images is to compare the

image data.

A duplicate check should also be performed for templates. Templates are normally used for

repeating contents such as fixed page backgrounds, logos and so on. To skip a template the

TBeginTemplate calback function must return 1.

Physical organization of images

The visible appearance and the physical structure how an image is stored in a PDF file is sometimes

somewhat confusing. Images can be split into bands or tiles. There are various reasons why this can

be done but if an image was split into smaller pieces then it is very difficult to restore the original

image. DynaPDF does not provide algorithms which try to identify pieces of a larger image.

It is usually best to ignore images which are less than two units high. For example, applications like

Microsoft Word split images often into separate scan lines or smaller pieces if the image contained

transparent areas. The resulting PDF file contains then hundreds or thousands of very small images.

Because such small pieces are not really meaningful when viewed alone, the application can either

try to reconstruct the original image, or if this is not possible, such images should be skipped.

If the content parser returns lots of very small images then it is usually best to provide a fallback

that renders the entire page with RenderPage() or RenderPageToImage().

Image coordinate space

An image occupies a rectangle in image space w units wide and h units high, where w and h are the

width and height of the image in samples or pixels. Each sample occupies one square unit. The

coordinate origin (0, 0) is at the upper-left corner of the image, with coordinates ranging from 0 to w

horizontally and 0 to h vertically.

The image’s sample data is ordered by row, with the horizontal coordinate varying most rapidly.

The upper-left corner of the first sample is at coordinates (0, 0), the second at (1, 0), and so on

through the last sample of the first row, whose upper-left corner is at (0, 0) and whose upper-right

corner is at (w - 1, 0). The next samples after that are at coordinates (0, 1), (1, 1), and so on to the final

sample of the image, whose upper-left corner is at (0, 0) and whose lower-right corner is at (w-1, h-

1).

The correspondence between image space and user space is constant: the unit square of user space,

bounded by user coordinates (0, 0) and (1, 1), corresponds to the boundary of the image in image