Monkeybread Software - DynaPDF Manual

DynaPDF Manual - Page 678

Function Reference

Page 678 of 860

usually best to set the width or height to zero so that the function can calculate the missing

value to preserve the aspect ratio.

The pixel format and the output image format must be compatible. Because TIFF is the only image

format that supports different compression filters, the parameter Filter will be ignored for all other

output formats if the pixel format is not set to pxfGray. For gray images, the filter is also used to

determine whether the Floyd-Steinberg dithering algorithm should be applied (see 1 Bit Rendering).

If Resolution is greater zero the image size will be calculated as follows:

imageWidth

= pageWidth

* Resolution / 72.0

imageHeight = pageHeight * Resolution / 72.0

The page width and height is calculated according to the bounding boxes and orientation. When

rendering PDF pages to a specific resolution it is recommended to set also the maximum width and

height (as negative values) so that the image size can be restricted. This makes it possible to render

arbitrary PDF files in high resolutions without further considerations.

1 Bit Rendering

If the pixel format pxf1Bit is used, the page will be rendered with a simple 1 bit ordered dither

matrix algorithm. This kind of rendering is fast but not very accurate. Since this pixel format has no

alpha channel (which is sometimes required), the output can be wrong if the file uses transparency.

Dithering

To avoid issues with transparency it is recommended to render into a gray pixel format (pxfGray or

pxfGrayA) and to apply a dithering algorithm on the result. This can be achieved by combining the

compression filter with one of the available dithering algorithms:

• cfOrderedDithering

-> Simple ordered dithering

• cfDitherFloydSteinberg -> Floyd-Steinberg dithering

• cfConvGrayToOtsu

-> Optimal filter for OCR engines

In order to compress the image with CCITT Fax 4, for example, and to apply a simple ordered

dithering algorithm, combine the compression filter and dithering flag with a binary or operator,

e.g. TCompressionFilter(cfCCITT4 | cfOrderedDithering).

Anti-Aliasing should be disabled when creating 1 bit images to avoid anti-aliasing artifacts.

The Otsu filter is useful if an OCR scan should be applied on the resulting 1 bit image. Most OCR

engines produce best results with this filter.

If the output format is Bitmap (BMP) the compression filter is used only to determine whether a

dithering algorithm should be applied. The compression filter can be set to any of the above values.

Remarks:

This function is available in an Ansi and Unicode compatible version. Unicode file paths are

converted to UTF-8 on non-Windows operating systems.