Images and Clips

What Is An Image?

Image Effects process images (funny that), this chapter describes images and clips of images, how they behave and how to deal with them.

Firstly some definitions…

  • an image is a rectangular array of addressable pixels,
  • a clip is a contiguous sequence of images that vary over time.

Images and clips contain pixels, these pixels can currently be of the following types…

  • a colour pixel with red, green, blue, alpha components
  • a colour pixel with red, green and blue components
  • single component ‘alpha’ images

The components of the pixels can be of the following types…

  • 8 bit unsigned byte, with the nominal black and white points at 0 and 255 respectively,
  • 16 bit unsigned short, with the nominal black and white points at 0 and 65535 respectively,
  • 32 bit float, with the nominal black and white points at 0.0f and 1.0f respectively, component values are not clipped to 0.0f and 1.0f.

Components are packed per pixel in the following manner…

  • RGBA pixels as R, G, B, A
  • RGB pixels as R, G, B

There are several structs for pixel types in ofxCore.h <> that can be used for raw pixels in OFX.

Images are always left to right, bottom to top, with the pixel data pointer being at the bottom left of the image. The pixels in a scan line are contiguously packed.

Scanlines need not be contiguously packed. The number of bytes between between a pixel in the same column, but separated by a scan line is known as the rowbytes of an image. Rowbytes can be negative, allowing for compositing systems with a native top to bottom scanline order to trivially support bottom to top images.

Clips and images also have a pixel aspect ratio, this is how much an actual addressable pixel must be stretched by in X to be square. For example PAL SD images have a pixel aspect ratio of 1.06666.

Images are rectangular, whose integral bounds are in Pixel coordinates, with the image being X1 <= X < X2 and Y1 <= Y < Y2, ie: exclusive on the top and right. The bounds represent the amount of data present in the image, which may be larger, smaller or equal to the Region of Definition of the image, depending on the architecture supported by the plugin. The kOfxImagePropBounds property on an image holds this information.

An image also contains it’s RoD in image coordinates, in the kOfxImagePropRegionOfDefinition property. The RoD is the maximum area that an image may have pixels in, t he bounds are the actual addressable pixels present in an image. This allows for tiled rendering an so on.

Clips have a frame rate, which is the number of frames per second they are to be displayed at. Some clips may be continously samplable (for example, if they are connected to animating geometry that can be rendered at arbitrary times), if this is so, the frame rate for these clips is set to 0.

Images may be composed of full frames, two fields or a single field, depending on its source and how the effect requests the image be processed. Clips are either full frame sequences or fielded sequences.

Images and clips also have a premultiplication state, this represents how the alpha component and the RGB/YUV components may have interacted.

Defining Clips

During an the effect’s describe in context action an effect must define the clips mandated for that context, it can also define extra clips that it may need for that context. It does this using the :cpp:func`OfxImageEffectSuiteV1::clipDefine` function, the property handle returned by this function is purely for definition purposes only. It has not persistance outside the describe in context action and is distinct to the clip property handles used by instances. The name parameter is how you can later access that clip in a plugin instance via the :cpp:func`OfxImageEffectSuiteV1::clipGetHandle` function.

During the describe in context action, the plugin sets properties on a clip to control its use. The properties that can be set during a describe in context call are…

Plugins must indicate which pixel depths they can process by setting the kOfxImageEffectPropSupportedPixelDepths on the plugin handle during the describe action.

Pixel Aspect Ratios, frame rates, fielding, components and pixel depths are constant for the duration of a clip, they cannot changed from frame to frame.


Getting Images From Clips

Clips in instances are retrieved via the :cpp:func`OfxImageEffectSuiteV1::clipGetHandle` function. This returns a property handle for the clip in a specific instance. This handle is valid for the duration of the instance.

Images are fetched from a clip via the OfxImageEffectSuiteV1::clipGetImage() function. This takes a time and an optional region to extract an image at from a given clip. This returns, in a property handle, an image fetched from the clip at a specfic time. The handle contains all the information relevant to dealing with that image.

Once fetched, an image must be released via the OfxImageEffectSuiteV1::clipReleaseImage() function. All images must be released within the action they were fetched in. You cannot retain an image after an action has returned.

Images may be fetched from an attached clip in the following situations…

and kOfxActionEndInstanceChanged actions with a kOfxPropChangeReason of kOfxChangeUserEdited

A host may not be able to support random temporal access, it flags its ability to do so via the kOfxImageEffectPropTemporalClipAccess property. A plugin that wishes to perform random temporal access must set a property of that name on the plugin handle and the clip it wishes to perform random access from.


  • it is an error for a plugin to attempt random temporal image access if the host does not support it
  • it is an error for a plugin to attempt random temporal image access

if it has not flagged that it wishes to do so and the clip it wishes to do so from.

Premultiplication And Alpha

All images and clips have a premultiplication state. This is used to indicate how the image should interpret RGB (or YUV) pixels, with respect to alpha. The premultiplication state can be…


Used to flag the alpha of an image as opaque

The image is opaque and so has no premultiplication state, but the alpha component in all pixels is set to the white point


Used to flag an image as premultiplied

The image is premultiplied by it’s alpha


Used to flag an image as unpremultiplied

The image is unpremultiplied.

This document won’t go into the details of premultiplication, but will simply state that OFX takes notice of it and flags images and clips accordingly.

The premultiplication state of a clip is constant over the entire duration of that clip.

Clips and Pixel Aspect Ratios

All clips and images have a pixel aspect ratio, this is how much a ‘real’ pixel must be stretched by in X to be square. For example PAL D1 images have a pixel aspect ratio of 1.06666.

The property kOfxImageEffectPropSupportsMultipleClipPARs is used to control how a plugin deals with pixel aspect ratios. This is both a host and plugin property. For a host it can be set to…

  • 0 - the host only supports a single pixel aspect ratio for all clips, input or output, to an effect,
  • 1 - the host can support differing pixel aspect ratios for inputs and outputs

For a plugin it can be set to…

  • 0 - the plugin expects all pixel aspect ratios to be the same on all clips, input or output
  • 1 - the plugin will accept clips of differing pixel aspect ratio.

If a plugin does not accept clips of differing PARs, then the host must resample all images fed to that effect to agree with the output’s PAR.

If a plugin does accept clips of differing PARs, it will need to specify the output clip’s PAR in the kOfxImageEffectActionGetClipPreferences action.

Allocating Your Own Images

Under OFX, the images you fetch from the host have already had their memory allocated. If a plug-in needs to define its owns temporary images buffers during processing, or to cache images between actions, then the plug-in should use the image memory allocation routines declared in OfxImageEffectSuiteV1. The reason for this is that many host have special purpose memory pools they manage to optimise memory usage as images can chew up memory very rapidly (eg: a 2K RGBA floating point film plate is 48 MBytes).

For general purpose (as in less than a megabyte) memory allocation, you should use the memory suite in ofxMemory.h

OFX provides four functions to deal with image memory. These are,

A host needs to be able defragment its image memory pool, potentially moving the contents of the memory you have allocated to another address, even saving it to disk under its own virtual memory caching scheme. Because of this when you request a block of memory, you are actually returned a handle to the memory, not the memory itself. To use the memory you must first lock the memory via the imageMemoryLock call, which will then return a pointer to the locked block of memory.

During an single action, there is generally no need to lock/unlock any temporary buffers you may have allocated via this mechanism. However image memory that is cached between actions should always be unlocked while it is not actually being used. This allows a host to do what it needs to do to optimise memory usage.

Note that locks and unlocks nest. This implies that there is a lock count kept on the memory handle, also not that this lock count cannot be negative. So unlocking a completely unlocked handle has no effect.

An example is below….

// get a memory handle
OfxImageMemoryHandle memHandle;
gEffectSuite->imageMemoryAlloc(0, imageSize, &memHandle);

// lock the handle and get a pointer
void *memPtr;
gEffectSuite->imageMemoryLock(memHandle, &memPtr);

... // do stuff with our pointer

// now unlock it

// lock it again, note that this may give a completely different address to the last lock
gEffectSuite->imageMemoryLock(memHandle, &memPtr);

... // do more stuff

// unlock it again

// delete it all