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Qt Graphics and Performance - OpenGL

Published on Wednesday January 06, 2010 by TomCooksey in OpenGL Performance Painting Graphics Dojo | Comments


Here's the next instalment of the graphics performance blog series. We'll begin by looking at some background about how OpenGL and QPainter work. We'll then dive into how the two are married together in OpenGL 2 Paint Engine and finish off with some advice about how to get the best out of the engine. Enjoy!

Why OpenGL?

Before I dive into the OpenGL paint engine, I want to make sure we all understand the motivation for the OpenGL 2.0 paint engine. I've talked about this before in my article about hardware acceleration, but we still frequently get questions like “Why not implement a Direct2D paint engine?”.

Everyone knows OpenGL means fast graphics right? Well, this is actually a bit of a misconception. What makes graphics fast is a bit of hardware dedicated to computer graphics called a GPU (Graphics Processing Unit). OpenGL 2.x is a software library which often (but not always) uses a particular class of GPU to help satisfy drawing operations (Note: OpenGL 1.x used a different class of GPU). A modern programmable GPU (e.g. nVidia GTX 295) can usually be programmed via both OpenGL, Direct3D and OpenCL. The only difference then is that Direct3D is only available on the Windows platform and OpenCL is not universally supported.

So the reason we are investing our time and effort into OpenGL, rather than Direct3D or OpenCL, is that OpenGL 2.0 is sufficient to give us access to all the GPU features we currently want to use. It is also available on more platforms, especially if you limit yourself to the ES sub-set. We are also looking into restricting ourselves further to only use APIs in OpenGL 3.2 Core Profile.

This might change in the future if we see a new class of GPU, like ones designed for 2D vector graphics which can't be abstracted by OpenGL 2.0 very well (enter OpenVG), or, if we want to start using GPU features which OpenGL (ES) 2.0 doesn't give us access to. Having said that, OpenGL is very good at exposing new GPU features through extensions.


Qt has had an OpenGL paint engine since early Qt 4.0 days. This engine was designed for the fixed-function hardware available at the time. As time went on and manufacturers added newer bits of hardware to their GPUs, the OpenGL paint engine was adapted to use those features through OpenGL extensions. Over the last 4 years, lots of people have hacked on the engine and added support for things like ARB fragment programs and even adapted the engine to work on OpenGL ES 1.1. The engine is pretty stable and has lots of fall-backs (or original code-paths, depending on how you look at them) for old hardware missing GL extensions the engine can utilise. But, fundamentally, it is an OpenGL 1.x engine.

In early 2008, around the time of the Falcon project (the Falcon Project was an internal project started for Qt 4.5 which focused on painting performance and architecture), it became increasingly clear that Qt needed to support hardware acceleration using the OpenGL ES 2.0 API which was starting to appear on embedded System-On-Chips like the OMAP3. There were two options available: Extend the existing OpenGL paint engine further still, or develop a new paint engine from scratch. When looking at the existing engine, there was a major problem – although it supported fragment programs, it was heavily reliant on fixed-function vertex processing. A further consideration was that the Falcon project had just kicked off and the future of the QPaintEngine API was uncertain. Both of these factors resulted in a new paint engine being written from scratch for OpenGL ES 2.0. This new engine had a distinct advantage over the existing engine: everything I wanted to use from OpenGL was in the core OpenGL ES 2.0 API. This meant I didn't need to add fallbacks in case of missing functionality, leading to much cleaner and leaner code.

Another point about OpenGL ES 2.0 is that it doesn't have much in the way of fixed function features – forcing you to write shader programs. While annoying at the time, this is apparently the best way to do things even on desktop GPUs. This point is important because it quickly became apparent that although the engine was designed for GLES2, not only would it also work on desktop OpenGL 2.0, but it would use that API in a way better suited for modern programmable GPUs. So, in Qt 4.6, the new engine is used by default on both GLES2 and on desktop systems which support OpenGL 2.0.

What does OpenGL (ES) 2 provide?

As I've already mentioned, OpenGL ES 2.0 is a pretty lean and mean API which models programmable GPUs. The "programmable" bit is fundamental to the API. It means that you write small programs known as shaders, ask OpenGL to compile and then run them on the GPU to process the data you give it. There are two types of shaders: one type processes positions (vertices) and another type processes pixels (fragments), called the vertex shader and fragment shader, respectively. The idea is that you tell OpenGL you want to draw some triangles and the vertex shader is run to determine the position of each of those triangles. Then, the GPU turns each triangle into a bunch of pixels and the fragment shader is run to determine the colour of each of those pixels. The API provides various ways of passing data from the CPU to the GPU (from textures and lists of triangle positions to individual floats) and ways of passing data from the vertex shader to the fragment shader. That's basically it. All the complexity lives in the shaders you give to the GPU to run.

What does QPainter require?

The rest of this blog assumes you are familiar with the QPainter API (if not, go check the QPainter docs) ). It might also be a good idea to read through Gunnar's post about how the Raster engine works.

So, the QPainter API provides more than just triangles. It is therefore the GL paint engine's job to turn the whole of the QPainter API into "just a bunch of triangles". To understand its task a little better, you have to split QPainter up into chunks which map better to OpenGL. A great example of this is drawRect(). In QPainter terms, this is a single primitive, but in GL engine terms, it is actually two: A rectangle (the fill) and a (possibly quite complex) line round the outside (the stroke). The OpenGL paint engine tries to keep a fairly clean separation between the shape of something which is drawn and its fill. So, here's the list of primitives (shapes) QPainter requires the engine to draw:

  • Simple primitives (Rectangles, convex polygons, ellipses, etc.)
  • Text
  • Pixmaps
  • Strokes
  • Complex vector paths (QPainterPath)

In addition to this, we have various fills which we can use on our primitives provided by QBrush:

  • Solid colour
  • Linear gradients
  • Radial gradients
  • Conical gradients
  • Bitmap patterns
  • Textures

Not only do we have different types of fill, but we also support a full 3x3 transformation matrix on the brushes. This allows you to draw a rectangle but use it as a kind-of stencil over (for example) a perspective transformed texture.

Finally, QPainter also requires the engine to implement clipping, different composition modes and support it's state stack (QPainter::save() & QPainter::restore()).

Engine Operation

Primitive Rendering

  • Simple Primitives: To render convex primitives such as rounded rectangles, we just generate a GL triangle fan and render it using glDrawArrays
  • Text: For large text, we convert it to a complex path and render is as such. However, for smaller font sizes, we rasterize the individual font glyphs and upload them as a texture (8-bit texture for bitmap & anti-aliased glyphs and 24-bit RGB for sub-pixel anti-aliased glyphs). This glyph texture is used as a mask in the engine's pixel pipeline (see below). So, in terms of primitives, text is actually rendered as a set of rectangles - one rectangle for each glyph. When rendering with sub-pixel anti-aliased glyphs, it is possible that the engine will need to do two passes (if the brush is not a solid colour). This is because the engine uses a clever trick and sets the brush's colour as the glBlendColor and outputs the RGB mask in the fragment shader. It is then able to set a glBlendFunc which combines the two and gives per-sub-pixel blending. If you set a more complex brush, the engine has to do two passes - first apply the mask to the destination, then a second pass to apply the brush, with glBlendFunc set to give the correct result.
  • Pixmaps: A pixmap is actually just a rectangle.
  • Strokes: Strokes can be very complex - just take a look at the pathstoke demo! However, even the most complex dashed pattern with rounded joins and end caps can be turned into a GL triangle strip relatively easily. This is done by the QTriangulatingStroker.
  • Complex vector paths: This is where things get tricky. QPainterPaths can have lots of things which break the "turn lineTo, moveTo and curveTo into verticies and render as triangle fan" algorithm...

Rendering Using Stencil Technique

Take the following path as an example:

Convex Path (1)

Here we have a seemingly trivial path with only 4 points. To draw this with GL, you could just convert the path's points to verticies and draw it as a triangle fan, which results in two triangles: Triangle 1: ABC and Triangle 2: ACD. The problem is that just looks like a solid triangle, not the path we wanted:

Convex Path (2)

So, to overcome this difficulty, we drop to a 2-pass rendering method which uses the stencil buffer as a temporary scratchpad. So first off, we clear the stencil buffer to all zeros (represented as white):

Stencil Buffer (Clear)

Next, we set the stencil operation to invert, which means instead of setting the stencil value to '1' when a triangle touches a pixel, invert the existing value instead. So 0->1 & 1->0. First we render the first triangle (ABC). As all the pixels are currently 0, every pixel touched by the triangle turns to 1 (represented as black):

Stencil Buffer (Triangle 1)

Next, we draw the second triangle (ACD). Note: We are inverting the stencil's value, so black pixels touched by the second triangle turn to white and white pixels turns to black:

Stencil Buffer (Triangle 2)

So now the stencil buffer contains the silhouette of our path. All we do now is draw a rectangle into the destination window, but with the stencil test enabled.

In addition to the stencil technique, we are also adding experimental support for triangulating QPainterPaths and caching the triangulation. While this is slower for paths which change often or are zoomed in & out, paths which are relatively static can be triangulated once and rendered multiple times without having to re-triangulate.

Filling Primitives

Now we know how all the different QPainter operations get turned into GL primitives, but we're still missing how they get filled. As already mentioned, the colour of a pixel is determined by the fragment shader. We therefore have lots of different fragment shaders for different types of fill. However, we also need to support text rendering with arbitrary fills (QPainter lets you fill text with a perspective transformed radial gradient). In the future, we also want to support composition modes which OpenGL doesn't provide. We've also found there are ways we can simplify the shaders for certain situations (and thus improve performance). The result is that Qt needs lots of different shaders. At last count, we'd need over 1000 different shaders to cover all situations. That's a lot of GLSL to maintain and test, far more than the resources we have available. So instead we split the shaders into different interchangeable "stages". This is achieved by having each stage in it's own GLSL function. As an example, lets take regular, non sub-pixel anti-aliased text rendering with a transformed radial gradient. Note, this is just an example to demonstrate how the engine operates and you probably shouldn't do it in performance critical situations.

We render gradients by pre-calculating a 1px high texture (like a 1D texture) on the CPU which we sample from in the fragment shader. However, we calculate the texture coordinates in the vertex shader and pass it to the fragment shader as a varying. This is because it's a good idea to do as much work as possible in the vertex shader rather than the fragment shader as it is called so much less frequently.

As already mentioned, we render (non sub-pixel) anti-aliased text by using an 8-bit mask texture. We then multiply the fragment colour by a sample taken from this mask. So, if we're on the edge of a glyph where the alpha value is <1, we adjust the alpha of the srcPixel by that amount (actually, we also adjust the RGB values too as we use pre-multiplied alpha pixel format internally).

If there was a non-standard composition mode, we'd then pass the masked pixel to another stage which would blend it with the background (although this isn't implemented yet!).

So you can see in the fragment shader, there's 3 different stages. The first stage (srcPixel) determines the brush colour of the fragment. The next stage (applyMask) modulates the pixel by a mask to achieve anti-aliased text rendering. The final stage (compose) then blends the pixel with the background. We also have a similar staging technique for the vertex shader. All this complexity is nicely abstracted by the QGLEngineShaderManager. The paint engine tells the shader manager what it wants to draw and the shader manager selects an appropriate selection of shaders. One final note on this: While desktop OpenGL 2 supports linking multiple fragment shaders in a single program, OpenGL ES 2.0 does not. This means that we actually use the different stages by appending them to a single string of GLSL we pass to GL. This also gives the GL implementation the best chance to inline the different stages (without which, performance would suck).

Texture Management

The OpenGL paint engine makes heavy use of gradients. For example, even though it's perfectly possible to calculate colours for gradients in the fragment shader, we still use a texture as a look-up-table as it is so much faster. Repeatedly uploading textures every time we need them would ruin performance. So instead, we keep a per-context cache of what QPixmap/QImage is already present in texture memory. If two contexts are sharing then we also detect this and don't duplicate the textures. This functionality is available publicly in QGLContext::bindTexture() too.

On Linux/X11 platforms which support it, Qt will use glX/EGL texture-from-pixmap extension. This means that if your QPixmap has a real X11 pixmap backend, we simply bind that X11 pixmap as a texture and avoid copying it. You will be using the X11 pixmap backend if the pixmap was created with QPixmap::fromX11Pixmap() or you're using the "native" graphics system. Not only does this avoid overhead but it also allows you to write a composition manager or even a widget which shows previews of all your windows.


The OpenGL paint engine uses OpenGL multisampling to provide anti-aliasing. Typically, this will be 4x/8x FSAA, meaning 4/8 levels of coverage, which is worse quality than the raster engine, which always uses 256 levels of coverage. However, as the DPI of modern displays increases, you can get away with lower-quality anti-aliasing.

Using multisampling also doesn't affect text rendering as text is anti-aliased using masks rather than multisampling (for smaller font sizes). So text rendered with the OpenGL engine should look almost as good as text rendered with the raster engine (which also does gamma-correction). The only drawback of using multisampling is that some OpenGL implementations don't support switching multisampling off. Indeed, the OpenGL ES 2.0 specification doesn't even provide the API to switch it off. The consequence is that non-anti-aliased (a.k.a. aliased) rendering can be broken (Everything gets anti-aliased even when the QPainter::Antialiasing hint isn't set). There's little we can do about this. :-(


QPainter supports setting an arbitrary clip, including complex QPainterPaths. Qt uses the GL stencil buffer (or more specifically the lower 7 bits of the stencil buffer) to store the clip. The clip is written to in the same way as we render any other primitive, even using the stencil technique for complex paths. However, instead of filling pixel colours into a colour buffer, we fill stencil values into the stencil buffer. The actual value we use depends on the current QPainter stack depth (how many times save() was called minus the number of time restore() was called). This means that if you restrict yourself to intersect clips (Qt::ClipOperation == Qt::IntersectClip), the engine only needs to write to the part of the stencil buffer which is being clipped to. What's more, the engine doesn't need to write to the stencil buffer at all when you call restore() - it just changes the value at which the stencil test passes.

In addition to using the stencil buffer for clipping, the OpenGL paint engine can also just use glScissor. This only allows a single, untransformed rectangle to be used as the clip, which can be quite restrictive. However, it is by far the fastest way to do clipping. So if performance is more important to you than utility, only ever use untransformed rectangular clips.


Interleaved Rendering

Unlike OpenGL, QPainter allows an arbitrary number of rendering contexts (QPainters) to be active in the same thread at the same time. For example, in your widget's paint event, you can begin a painter on your widget and begin another painter on a QPixmap and interleave rendering to them:

void Widget::paintEvent(QPaintEvent*)
QPainter widgetPainter(this);
widgetPainter.fillRect(rect(), Qt::blue);
QPixmap pixmap(256, 256);
QPainter pixmapPainter(&pixmap);
widgetPainter.drawPixmap(0, 0, &pixmap);

While this works ok with the OpenGL graphics system, having to switch from doing something with one painter to doing something with a different painter can be very costly and should be avoided whenever possible.

Mixing QPainter and Native OpenGL

As shown in several examples, it is possible to mix your own OpenGL rendering code with QPainter rendering code. However, as OpenGL is a giant state machine, it is very easy for you to accidently clobber Qt's GL state and vice-versa. To overcome this, we've added some new API to QPainter in Qt 4.6 - QPainter::beginNativePainting() and QPainter::endNativePainting(). To prevent artifacts, you must enclose your custom painting in beginNativePainting() and endNativePainting(). This is very important - even if you're not seeing any problems now, you might find your code starts failing in a future Qt release in which the GL paint engine works slightly differently. Also, as beginNativePainting and endNativePainting sets lots of OpenGL state, it can be quite expensive and thus you should try to use it sparingly. Try to batch up all your custom OpenGL code in a single block.

QGLWidget vs OpenGL Graphics-System

Unlike the raster & OpenVG paint engine, you don't have to use a specific graphics system to render widgets using the OpenGL paint engine. The QtOpenGL module provides several classes, including QGLWidget, which all use the OpenGL paint engine regardless of what graphics system is being used. QGLWidget is basically a regular widget which always has a native window ID and is always rendered to using OpenGL. You are free to choose whichever method you want to get OpenGL rendering (graphics system or QGLWidget). However, using the opengl graphics system can often be slower than using a QGLWidget, as Qt needs the contents of the "back buffer" (or QWindowSurface) to be preserved when flushing the render to the window system. OpenGL does not guarantee this and it is often not the case so Qt has to use either an FBO or a PBuffer as the back buffer. When the render needs to be flushed, the FBO or PBuffer is bound to a texture, rendered into the window and then the GL buffers are swapped. This extra overhead is avoided by using a QGLWidget, however as a consequence, it is not possible to redraw a sub-region of a QGLWidget: Whenever a QGLWidget is updated, the entire widget must be re-drawn.

It should also be noted that using the OpenGL paint engine isn't a silver bullet which makes everything faster. For example, the GL engine really sucks at drawing lots of small geometry with state changes between each drawing operation. While we're working on improving that use case at the moment, the raster paint engine will probably always be faster just because it has so much less overhead. So QGLWidget might be a great way to get the best of both worlds when combined with the raster graphicssystem - Use QGLWidget for operations which GL excels at and the raster engine for everything else.

Tips for Performance (fps)

As a general rule of thumb, OpenGL state changes are expensive. So, use the knowledge you now have of what's going on under QPainter and try to minimise the number of OpenGL state changes the paint engine needs to do. For example, if you implement a virtual keyboard, you now know that the engine uses a shader for text rendering and a different shader for pixmaps, so draw all the key pixmaps first, then draw all the text on top. That way, the engine only needs to change shaders twice per frame.

  • Never, ever use anything other than intersecting clips
  • Don't switch render target in the middle of a render
  • Try to use use untransformed rectangular clips whenever possible
  • Minimise changing the brush wherever possible
  • Render batches of primitives of the same types together.
  • Avoid drawing translucent pixels & blending (particularly important on mobile GPUs)
  • Try to cache QPainterPaths and re-use them rather than creating & discarding them in your paintEvent
  • Use QPainterPaths even when there's a QPainter convenience function. E.g. Rounded rects and elipses.
  • If you're drawing lots of small pixmaps, try bunching them up into a single, larger pixmap
  • Prefer to use power-of-two (2^n) widths & heights for QImages and QPixmaps (128x256, 256x256, 512x512, etc)
  • If using QGLWidget and don't need anti-aliasing, don't enable sample buffers in the QGLFormat
  • If rendering complex QPainterPaths, try to only use odd-even fill rule

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