Optimizing images in a PDF document: compression, color space and resolution

Qoppa’s 100% Java PDF optimizer library, jPDFOptimizer, allows, among other things, to change the properties of images in a PDF document including the compression, the color space, and the DPI resolution.

This is done using the ImageHandler interface and implementing the convertImage() method which allows an application to define optimization option per image or per image type.

public class MyImageOptimizer implements ImageHandler
{
// this is the method where you can implement any image conversion / optimization
public ImageOutput convertImage(ImageInfo imageInfo)
{
// construct an image output that by default retains the same image properties as the image input
ImageOutput imageoutput = new ImageOutput(ImageOutput.CO_BW, CO_JBIG2);
// for gray or black and white images, use JBIG2 compressions
if(imageInfo.isGray() || imageInfo.isMonochrome())
{
imageoutput.setCompression(ImageOutput.CO_JBIG2);
imageoutput.setColorSpace(ImageOutput.CS_BW);
}
// for color images, use JPEG 2000 compression
if(imageInfo.isColor())
{
imageoutput.setCompression(ImageOutput.CO_JPEG2000);
imageoutput.setCompressionQuality(0.8f);
}
// downgrade image resolution to 200DPI if it's higher
if (imageInfo.getDPIX() > 200 || imageInfo.getDPIY() > 200)
{
// Calculate new dimensions to match DPI
float scale = Math.min(200 / imageInfo.getDPIX(), 200 / imageInfo.getDPIY());
int newWidth = (int)(imageInfo.getImageWidth() * scale + 0.5);
int newHeight = (int)(imageInfo.getImageHeight() * scale + 0.5);
imageoutput.setImageWidth(newWidth);
imageoutput.setImageHeight(newHeight);
}
// return the new image output
return imageoutput;
}

The imageHandler is set through the OptSettings class.

// Create the PDF Optimizer object with the path of the file to optimize
pdfOptimizer = new PDFOptimizer("C:\myfile.pdf", null);
// Set optimization settings
OptSettings options = new OptSettings();
// ... set standard optimization settings ...
// set the image handler
options.setImageHandler(new MyImageHandler());
// run the optimizer and save
pdfOptimizer.optimize(options, "C:\myfile_opt.pdf");
Method Options
Image Compression

ImageOutput.setCompression()

 Retain existing compression or convert any image to either a JPEG, JPEG2000, FLATE, or JBIG2.

  • CO_RETAIN – retain current compression
  • CO_JPEG – lossy JPEG compression that will reduce image quality and size
  • CO_JPEG2000 – less lossy JPEG compression that will reduce size and some quality
  • CO_FLATE – Lossless flate compression does not reduce quality but may not always reduce file size
  • CO_JBIG2 – Lossless JBIG2 compression that can only support black and white color space. Color images will be converted to black and white. Best for scanned documents with no images
Compression Quality

ImageOutput.setCompressionQuality()

 Allows the compression quality to be adjusted between 0.1 and 1 (corresponding to a quality from 10 to 100%).

  • This will only affect JPEG or JPEG2000 images as these are the only formats that support image quality settings.
Image Color Space

ImageOutput.setColorSpace()		 Choose to either retain or convert the image colors to Grayscale or B&W.

  • ImageOutput.CS_RETAIN – Retain current color space.
  • ImageOutput.CS_BW – Black and white color space, 1 pixel per bit. Only supported by FLATE or JBIG2. Best for scanned documents with no images
  • ImageOutput.CS_CMYK – The CMYK color space.
  • ImageOutput.CS_GRAY – Grayscale color space – Only supported with JPEG, JPEG2000, or FLATE. Best for full color images
  • ImageOutput.CS_RGB – The RGB color space.

*Note – When the retain option is selected CMYK images will be converted to RGB due to a regression bug in Java 1.7.
DPI ResolutionImageOutput.setImageWidth()
ImageOutput.setImageHeight()		 Adjust the image resolution by changing the imageWidth and imageHeight in pixels. The image bounds will stay the same when displayed on the page.
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