Image Handling

ABCpdf embeds images into the PDF at their original resolution. Images may be displayed at different sizes but the underlying image is exactly the same. If your image looks grainy when you print you'll need to find a higher resolution (bigger) image.

Suppose you have an image 300 pixels wide by 600 pixels high. If you wanted this image to print well at 300 dpi then you would need to add it into a rectangle no larger than 1 inch by 2 inches. If you wanted to view it at a screen resolution of 75 dpi you could place it into a rectangle up to 4 inches by 8 inches.

You can present images to ABCpdf in one of two ways.

You can operate in pass-through mode and add an image directly into your Doc object. The Doc.AddImageFile, Doc.AddImageData operate in pass-through mode.

Alternatively you can operate in indirect mode and draw your data into an Image object before adding the Image object to your document. The Doc.AddImageObject method defaults to indirect mode.

Indirect mode has a number of advantages over pass-through mode.

Because each image is fully decoded when it is presented to the Image object, image corruption can be caught at this stage. Corrupt images are not uncommon and if you don't detect the corruption before the data is inserted you may end up with a corrupt PDF.

Images are color corrected which means that color profiles do not need to be embedded and file size may be reduced.

Pass-through mode has advantages for some types of images.

Because the image may not have to be decompressed and re-compressed, using pass-through mode can be much faster than indirect mode. This is particularly true for scanned TIFF images.

Because the original image compression is maintained there is no possibility of expansion due to re-compression.

Images are inserted in their native color space together with any color profiles - this guarantees fidelity of color reproduction.

Colors can be specified in a number of ways - most commonly in terms of RGB or CMYK values. These values are literal - RGB values relate directly to the brightness of the red, green and blue phosphors on a display - CMYK values relate directly to the amounts of Cyan, Magenta, Yellow and Black inks applied to a piece of paper.

These literal values do not always equate to the same perceived color. Monitors vary and an image displayed on one may look quite different to the same image displayed on another. Similarly a CMYK value applied with one printer to one type of paper may look quite different to a CMYK value applied with a different printer to a different type of paper.

The International Color Consortium (ICC) provides specifications to allow an independent definition of color. An ICC color profile is designed to complement your raw color data. So if you have a CMYK image you also specify an color profile which tells you more about the intended destination of the image. This allows you to adjust the colors for your intended output medium.

In order to maintain flexibility and color fidelity it is important to preserve both the raw image data and the ICC profile associated with it. This can only be done if you provide images in pass-through rather than direct mode.

ABCpdf will accept RGB TIFF, CMYK TIFF, LAB TIFF, Grayscale JPEG, RGB JPEG and CMYK JPEG direct. This means whatever colors you pass into ABCpdf will go direct into the PDF without any modification. Additionally any ICC profile will be preserved and inserted appropriately so that the colors can be adjusted accurately for any output.

So what types of compression does the Image object use?

The Image object generally uses Flate compression. This is the same compression type as is used for PNG images. It is a lossless method which ensures that the quality of your original image is maintained.

However if your source image is black and white then ABCpdf will use CCITT G4 fax compression as this typically results in reduced file sizes for this type of image.