Custom transforms

Introduction

Mako ITransforms provide a method of applying common operations on DOM objects such as brushes, colors, colorspaces, glyphs, or paths. They can operate on individual nodes or entire trees. 

For example, there are many built-in transforms in Mako for:

• Image downsampling (IImageDownsamplerTransform)
• Color conversion (IColorConverterTransform)
• Color simplification (IComplexColorSimplifierTransform)
• Image merging (IImageMergerTransform)
• Remove non-visible Optional Content (IOptionalContentFixerTransformPtr)

Thus, transforms simplify complex operations to be applied to a number of DOM objects in one go, and despite their name, transforms don't have to make a change; they can be used to simply interrogate document content.

A custom transform enables a Mako SDK developer to take advantage of the ITransforms framework for their own purposes.

Custom transform header

A framework to enable development of custom transforms is provided as a header, customtransform.h.

It's part of the standard Mako 5.x distribution. (Should you need them for Mako 4.x., the headers for those versions can be downloaded from the link under the "Downloads" heading. Note that there are two versions: one for pre-Mako 4.8 and another for Mako 4.8.)

A custom transform implements call backs for the DOM objects listed below. All you need to do is override the cases you are interested in, providing one or more methods for actually doing the work.

For example, let's say you wanted to find out information for every image in a document. You could write a custom transform to override transformImageBrush(). Your code will be called every time an imagebrush (IDOMImageBush) is encountered, and from there get to the image (IDOMImage) and its frame (IDOMFrame) to obtain all the information you need (size, resolution, colorspace, and so forth). You would run the transform on a page and repeat for all pages in a document.

A more likely scenario is one where you want to change the object in some way. Your implementation can do this, returning the updated object or a NULL if you want it removed from the DOM tree.

CTransformState

Class for tracking the graphics state leading to the point where a transform is applied.

Consider for example the IImageDownsamplerTransform described in the API documentation (paste the full name into the Search box). In order to determine how to downsample an image, the transform needs to know how large the image will eventually be. The CTransformState provides this information by providing the combined transform that applies to the image based on the RenderTransforms of all the nodes entered leading to the point where the image is actually encountered.

Other transforms need access to other information, such as the approximate clip area, the current group colorspace, the renderingIntent (if present), the current antialiasing mode (edge mode) and/or how a brush is used.

CTransformState::transformPriv

New to Mako 4.8.0, this member can be used to track extra information needed by the transform process. Use it as you wish.

Implementing a custom transform

This GitHub example of a custom transform implementation converts glyphs to paths.

Note that:

• You need only to implement transforms for those objects you are interested in examining or changing
• Returning the default genericImplementation allows the transform to continue, possibly recursing further into the object
• The object you return will replace the object being transformed (and therefore if it's the same object, will have no effect)
• Returning a null or empty object will result in that object being removed from the tree

In the example, a path is returned which replaces the glyph object in the tree (that is, the glyph object is removed and the path added).

Creating and calling the transform

It is instantiated by the calling program:

		// Create a transform to convert glyphs to paths
		GlyphTransformImplementation implementation(jawsMako, strokeWidth, solidBrush);
		ITransformPtr outliner = ICustomTransform::create(jawsMako, &implementation, abort, true, true, true, true, true, true);

Once the instance exists, there is no need to create it again. Just call it as required. In the example it is called for each page:

        // Apply the transform to every page
        for (uint32 pageNum = 0; pageNum < pageCount; pageNum++)
        {
			IPagePtr page = document->getPage(pageNum);
        	IDOMFixedPagePtr fixedPage = page->edit();

			// Convert any glyphs
			bool result = false;
			outliner->transform(fixedPage, result);
        }

One advantage of custom transforms is that they can hide away complexity. Remember that you can implement call backs for as many objects types as you wish, in the one transform.

Object sharing

In general, if the custom transform callback has a “changed” parameter, then the object being passed in is not shared and can be edited at will, providing you set the “changed” result appropriately.

If the custom transform callback does not, then the object is potentially shared and you must make a clone before editing it. The internal transformation machinery detects changes if a different object is returned.

What are all those Booleans in the call to the transform?

They control caching behavior. Depending on the nature of the transform, it may not be necessary to process the same transform repeatedly. Setting one or more of these values to false may speed up a custom transform by allowing Mako to return a reference to a cached copy. Each relates to the properties of various object types, seen here in this excerpt from customtransform.h:

static JAWSMAKO_API ICustomTransformPtr create(const IJawsMakoPtr &jawsMako, 
                                                IImplementation *implementation,
                                                const IAbortPtr &abort = IAbortPtr (),
                                                bool dependsOnClipBounds = true,
                                                bool dependsOnGroupSpace = true,
                                                bool dependsOnRenderingIntent = true,
                                                bool dependsOnTransform = true,
                                                bool dependsOnBrushUsage = true,
                                                bool dependsOnEdgeMode = true,
                                                bool dependsOnUncoloredTilingBrush = true);

We recommend you experiment with these to determine their applicability to your implementation.

Downloads

Custom transform headers