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The OpenGL Pipeline Newsletter - Volume 004

Table of Contents
Previous article: Longs Peak Update: Buffer Object Improvements
Next article: Transforming OpenGL Debugging to a “White Box” Model

Another Object Lesson

"The object of the superior man is truth" -- Confucius

The OpenGL Longs Peak object model is substantially defined now, and we have a good notion of what a Longs Peak program will look like at a high level. Many smaller details are still being filled in, but after reading this article you should understand Longs Peak in considerable detail. For a background refresher, refer to "The New Object Model" in OpenGL Pipeline Volume 002, and "Using the Longs Peak Object Model" in OpenGL Pipeline Volume 003.

What's In A Namespace? Or, a GL by any other prefix would smell as sweet.

An important decision is that the OpenGL Longs Peak API will exist in a new namespace. Originally we thought Longs Peak could continue to use "gl" prefixed functions, "GL" prefixed types, and "GL_" prefixed tokens, but as we wrote up object specifications, we realized there were too many collisions. For example, both OpenGL 2.1 and Longs Peak have a Map Buffer entry point, but they take different parameters. We haven't chosen the namespace prefix yet; it's a marketing and branding issue, not a technical issue. As a placeholder until that's decided, we're using "lp" as the prefix.

The Object Hierarchy

"A mental act is cognitive only in the sense that it takes place in reference to some object, which is said to be known" -- Samuel Alexander

The objects defined in Longs Peak fall into several different categories depending on their behavior and semantics. In a true object-oriented language binding of the API, these categories would be abstract classes from which the concrete classes inherit behavior. Since our C API doesn't support inheritance, the categories are useful primarily as a conceptual tool for understanding the API. In any event, the categories are as follows:

  • Templates are client state, meaning they exist in the client(application) address space.All the other categories are server state, existing in the Longs Peak driver address space. Templates are fully mutable,meaning that any of their properties can be changed at any time; this makes it easier to reuse them for generating multiple objects. Templates, and the APIs to create and use them, are described more fully in OpenGL Pipeline 003.
  • State Objects contain a group of closely related attributes defining the behavior of some part of the graphics pipeline. They are fully immutable once created, which allows the driver to pre-cache derived state and otherwise optimize use of these objects, and they may be shared by multiple contexts.State objects are typically small. State object classes described below include format objects, shader objects, and texture filter objects.
  • Data Objects have an immutable structure (organization) defined when they are created, and a fully mutable data store filling out that structure. They may be shared by multiple contexts, although there are some remaining issues regarding when changes made in one context to the data store of an object will be visible to another context using the same object. Data object classes described below include buffer objects, image objects, and several types of sync objects (fences and queries).
  • Container Objects have one or more mutable attachments, which are references to other data, state, or container objects. They also have immutable attachment properties, which describe how to interpret their attachments. Container objects may not be shared by multiple contexts, mostly because the side effects of changing their attachments may be costly.For example, changing a shader attachment of a program object in use by another context could invalidate the state of that context at worst, and force time-consuming and unexpected relinking and validation at best. Container object classes described below include frame buffer objects, program objects, and vertex array objects.

Concrete Object Descriptions

"An object is not first imagined or thought about and then expected or willed, but in being actively expected it is imagined as future and in being willed it is thought" -- Samuel Alexander

Each of the concrete object classes mentioned above is explained in somewhat more detail here. The descriptions are organized according to the dependencies of the object graph, to avoid backwards references.

Format Objects fully resolve data formats that will be used in creating other types of objects. Such an object's defined usage must either match or be a subset of the usage supported by its format object. Format objects are a powerful generalization of the internal format parameter used in specifying texture and pixel images in OpenGL 2.1. In addition to the raw data format, format objects include:

  • intended usage: pixel, texture, and/or sample image, and which texture dimensionalities (1D, 2D, 3D, cube map, and array), vertex, and/or uniform buffer
  • minimum and maximum allowed texture or pixel image size
  • mipmap pyramid depth and array size
  • and whether data can be mipmapped, can be mapped to client address space, or is shareable.

Buffer Objects replace vertex arrays and pixel buffers, texture images, and render buffers from OpenGL 2.1. There are two types of buffer objects. Unformatted buffers are used to contain vertex data (whose format and interpretation may change depending on the vertex buffer object they're bound to) or uniform blocks used by shaders. Images are formatted buffers with a size, shape (dimensionality), and format object attachment. Changing buffer contents is done with APIs to load data (lpBufferData and lpImageData[123]D) and to map buffers in and out of client memory with several options allowing considerable flexibility in usage. See the article "Longs Peak Update: Buffer Object Improvements" earlier in this issue for more details.

Texture Filter Objects replace the state set with glTexParameter in OpenGL 2.1 controlling how sampling of textures is performed, such as minification and magnification filters, wrap modes, LOD clamps and biases, border colors, and so on. In Longs Peak, texture images and texture filters have been completely decoupled; a texture filter can be used with many different image objects, and an image can be used with many different texture filter objects.

Shader Objects are a (typically compiled) representation of part or all of a shader program, defined using a program string. A shader object may represent part or all of a stage, such as vertex or fragment, of the graphics pipeline.

Program Objects are container objects which link together one or more shader objects and associate them with a set of images, texture filters, and uniform buffers to fully define one or more stages in the programmable graphics pipeline. There is no incremental relinking; if a shader needs to be changed, simply create a new program object.

Frame buffer Objects are containers which combine one or more images to represent a complete rendering target. Like FBOs in OpenGL 2.1, they contain multiple color attachments, as well as depth and stencil attachments. When image objects are attached to an FBO, a single 2D image must be selected for attachment. For example, a 3D mipmap could have a particular mipmap level and Z offset slice selected, and the resulting 2D image attached as a color attachment. Similarly, a specific cubemap face could be selected and attached as a combined depth/stencil attachment. Each attachment point has an associated format object for determining image compatibility. When an image is bound to an FBO attachment, the format object used to create the image and the format object associated with the attachment point must be the same format object or validation fails. This somewhat draconian constraint greatly simplifies and speeds validation.

Vertex Array Objects are containers which encapsulate a complete set of vertex buffers together with the interpretation (stride, type, etc.) placed on each of those buffers. Geometry is represented in Longs Peak with VAOs, and unlike OpenGL 2.1, VAOs are entirely server state. That means no separate client arrays or enables! It also becomes very efficient to switch sets of vertex buffers in and out, since only a single VAO need be bound -- in contrast to the many independent arrays, and their interpretation, that have to be set in OpenGL 2.1 when switching VAOs. (The vendor extension GL_APPLE_vertex_array_object provides similar efficiency today, but is only available in Apple's implementation of OpenGL.)

Sync Objects are semaphores which may be set, polled, or waited upon by the client, and are used to coordinate operations between the Longs Peak server and all of the application threads associated with Longs Peak contexts in the same share group. Two subclasses of sync objects exist to date. Fence Syncs associate their semaphore with completion of a particular command (set with lpFence) by the graphics hardware, and are used to indicate completion of rendering to a texture, completion of object creation, and other such events. Query Syncs start a region with lpBeginQuery, and keep count of fragments rendered within that region. After lpEndQuery is called to end the query region, the semaphore is signaled once the final fragment count is available within the query object. In the future we will probably define other types of syncs associated with specific hardware events -- an example would be a sync associated with monitor vertical retrace -- as well as ways to convert syncs into equivalent platform-specific synchronization primitives, such as Windows events or pthreads semaphores.

The remaining objects making up Longs Peak are still being precisely defined. They are likely to include: display list objects, which capture the vertex data resulting from a draw call for later reuse; per-sample operation objects, which capture the remaining fixed-functionality state used for scissor test, stencil test, depth test, blending, and so on; and perhaps a "miscellaneous state" object containing remaining bits of state that don't have an obvious better home, such as edge flag enables, point and line smooth enables, polygon offset parameters, and point size.

Context is Important

"One context to rule them all, one context to bind them"
-- with apologies to J.R.R. Tolkien

Just as in OpenGL 2.1, the Longs Peak graphics context encapsulates the current state of the graphics pipeline. Unlike OpenGL 2.1, most context state is encapsulated in attributes of server objects. A small number of objects are required to define the pipeline state. These objects are bound to the context (see figure 1); changing a binding to refer to another object updates the graphics hardware state to be consistent with that object's attributes.

Changing state by binding objects can be very efficient compared to the OpenGL 2.1 model, since we are changing large groups of state in one operation, and much of that state may have already been pre-validated while constructing the object being bound. This approach will also be useful for applications and middleware layers performing complex state management. It is both more general and more powerful than either the glPushAttrib/glPopAttrib commands or encapsulating state changes in GL display lists, which are the only ways to change large groups of state in one operation today.

Graphics Context Bindings
Figure 1: Graphics Context Bindings. The Longs Peak context contains bindings for geometry (a vertex array object), programs (a program object), a rendering target (framebuffer object), sample operations state, and remaining fixed-functionality state affecting rasterization, hints and other miscellaneous state. In this diagram, yellow objects are containers, green objects are state objects, blue objects are data objects, red blocks represent attributes of container and state objects, and arrows represent attachments to objects or bindings to the context. The context itself, while not strictly speaking an object, is shown in yellow-red to indicate that it takes on aspects of a container object. View Closeup

Drawing Conclusions

Once all required objects are bound to the context, we can draw geometry. The drawing call looks very much like the OpenGL 2.1 glDrawArrays, but combines multiple draw array and primitive instancing parameters into a single call:

void lpDrawArrays(LPenum mode, LPint *first,
                  LPint *count, LPsizei primCount,
                  LPsizei instanceCount)

mode is the primitive type, just as in OpenGL 2.1. first and count define the range of indices to draw. primCount ranges are specified, so count[0] vertices starting at index first[0] will be drawn from the currently bound vertex array object and passed to the vertex program. Then count[1] vertices starting at index first[1], ending with count[primCount-1] vertices starting at index first[primCount-1]. Finally, instanceCount is used for geometry instancing; the entire set of ranges will be drawn instanceCount times, each time specifying an instance ID available to the vertex shader, starting at 0 and ending at instanceCount-1.

A similar variation of glDrawElements is also provided:

void lpDrawElements(LPenum mode, LPsizei *count,
                    LPsizeiptr *indices,
                    LPsizei primCount, 
                    LPsizei instanceCount)

The drawing calls are among the small number of Longs Peak entry points that do not take an object as an argument, since all the objects they use are already bound to the graphics context.

Outline for Success

"If somebody hits you with an object you should beat the hell out of them"
-- Charles Barkley

Finally, we've reached the point of outlining a Longs Peak sample program. The outline is not intended to be detailed source code, just to give a sense of the steps that will need to be taken to fully define the objects required for rendering. While this initialization looks complex, most of it is simple "boilerplate" code that can readily be encapsulated in utility libraries or middleware such as GLUT. It is also likely that at least some of the required objects can be predefined by the driver; for example, if the application is rendering to a window-system provided drawable, then a "default framebuffer object" will be provided.

// Create a framebuffer object to render to
// This is the fully general form for offscreen
// rendering, but there will be a way to bind a window-
// system provided drawable as a framebuffer object, or
// as the color image of an FBO, as well.
LPformat cformat, dformat, sformat = { create format 
objects for color, depth, and stencil buffers 
respectively }

LPframebuffer fbo = { create a framebuffer object, 
specifying cformat, dformat, and sformat as the 
required formats of color buffer 0, the depth buffer, 
and the stencil buffer respectively }

LPbuffer cimage, dimage, simage = { create image 
objects, specifying cformat, dformat, and sformat as 
the formats of the color image, depth image, and 
stencil image respectively }

Attach cimage, dimage, and simage to fbo at its color 
buffer 0, depth buffer, and stencil buffer attachment 
points respectively

// Create a program object to render with
LPshader vertshader, fragshader = { create shader 
objects for the vertex and fragment shader stages, 
specifying the shader program text for each stage as 
an attribute of the respective shader object}

LPprogram program = { create program object, 
specifying vertshader and fragshader as attributes of 
the program object}

LPbuffer vertbuffer, fragbuffer = { create unformatted 
buffer objects for the uniform storage used by the 
vertex and fragment shaders, respectively }

Attach vertbuffer and fragbuffer to program as the 
backing store for the uniform partitions of the vertex 
and fragment shaders, respectively

// Create vertex attribute arrays to render with
LPbuffer attribs = { create an unformatted buffer 
object containing all the attribute data required by 
the bound programs }

LPvertexArray vao = { create a vertex array object 
with specified size/type/stride/offset attributes for 
each required attribute array }

Attach attribs to vao at each attachment point for a 
required attributes

// Create miscellaneous required state objects
LPsampleops sampleops = { create sample operations 
object with specified fixed-function depth test, 
stencil test, blending, etc. attributes }

LPmiscstate misc = { create "miscellaneous state" 
object with specified rasterization settings, hints, 
etc. }

// Bind everything to the context
lpBindVertexArray(vao);
lpBindProgram(program);
lpBindFramebuffer(fbo);
lpBindSampleops(sampleops);
lpBindMiscState(misc);

// Finally, all required objects are defined and we 
// can draw a single triangle (or lots of them)
LPint first = 0, count = 3;
lpDrawArrays(LP_TRIANGLES, &first, &count, 1, 1);

While we still have a lot of work to do, and the final details may differ slightly, the ARB has now defined the overall structure of the Longs Peak API and the organization and definition of the object classes in the API. We'll continue to show you details of Longs Peak in future issues of OpenGL Pipeline, and when Longs Peak is released, we’ll expand these articles into a tutorial and sample code in the ARB's online SDK.

Jon Leech
OpenGL Spec Editor / ARB Ecosystem TSG Chair

(Subtitles in this article are thanks to the late-night availability of Google and www.brainyquote.com)


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