Porting Krita to OpenGL 3.1/ES 2.0

Krita was the first painting application with an OpenGL accelerated canvas. We had that before Photoshop… Which also meant that the code was getting quite old fashioned. These days, life is supposed to be better. More flexible in any case. However, even though a 2D canvas is a simple thing, once you factor in rotation, zooming, panning and so on, the potential for bugs is quite big, and we’ve been fixing bugs for ages in the old code.

So I didn’t want to throw that away, but have as clean and straightforward as possible a port from the old code to start with. The old code mostly looked like this (for painting the transparency checkers background):

KisCoordinatesConverter *converter = coordinatesConverter();

QTransform textureTransform;
QTransform modelTransform;
QRectF textureRect;
QRectF modelRect;

converter->getOpenGLCheckersInfo(&textureTransform, &modelTransform, &textureRect, &modelRect);

KisConfig cfg;
GLfloat checkSizeScale = KisOpenGLImageTextures::BACKGROUND_TEXTURE_CHECK_SIZE / static_cast(cfg.checkSize());

textureTransform *= QTransform::fromScale(checkSizeScale / KisOpenGLImageTextures::BACKGROUND_TEXTURE_SIZE,
                                            checkSizeScale / KisOpenGLImageTextures::BACKGROUND_TEXTURE_SIZE);

glViewport(0, 0, width(), height());
glOrtho(0, width(), height(), 0, NEAR_VAL, FAR_VAL);



glBindTexture(GL_TEXTURE_2D, m_d->openGLImageTextures->backgroundTexture());


glTexCoord2f(textureRect.left(), textureRect.bottom());
glVertex2f(modelRect.left(), modelRect.bottom());

glTexCoord2f(textureRect.left(), textureRect.top());
glVertex2f(modelRect.left(), modelRect.top());

glTexCoord2f(textureRect.right(), textureRect.bottom());
glVertex2f(modelRect.right(), modelRect.bottom());

glTexCoord2f(textureRect.left(), textureRect.top());
glVertex2f(modelRect.left(), modelRect.top());

glTexCoord2f(textureRect.right(), textureRect.top();
glVertex2f(modelRect.right(), modelRect.top());

glTexCoord2f(textureRect.right(), textureRect.bottom());
glVertex2f(modelRect.right(), modelRect.bottom());


glBindTexture(GL_TEXTURE_2D, 0);

In other words, we set a projection, a transformation matrix for the texture and for the model/view and then start drawing the vertices. Pretty simple. I was rather surprised when I did not find any clear tutorial on converting code like this through google. I’ve read a bunch of modern opengl books and tutorials by now, and they pretty much all have the same order of explanation, the same things they emphasize and the same “advanced” topics. But I couldn’t figure out how to draw my checkers or the tiles for my image. Yeah, I’m a linguist, not a mathematician, and I probable read these tutorials wrong or something.

In any case, after going through the Qt OpenGL examples, the tutorials on Wikibooks, the Red, Orange and Blue books, the Matt Gattis’ notes on porting to WebGL and more confused questions and more confusing answers on Stack Overflow than I care to count, I finally got something that works, and is as straight a translation of the old code as possible.

Purists will cavil at my use of attribute arrays and glDrawArrays — but the alternative, as far as I can tell, would be to redo all the matrix calculation and use matrices to place my tiles in the right location or to update and send new vertex buffer objects all the times. This works — and in the future it might even be pretty.

So, for posterity, and because there might be others in the same spot as me (to wit, tasked with porting OpenGL 1.3 code to OpenGL ES 2.0 or OpenGL 3.1 without compatibility profile), here’s a summary of my current code.

The vertex shader:

uniform mat4 modelViewProjection;
uniform mat4 textureMatrix;

attribute highp vec4 a_vertexPosition;
attribute mediump vec4 a_textureCoordinate;

varying vec4 v_textureCoordinate;

void main()
    gl_Position = modelViewProjection * a_vertexPosition;
    v_textureCoordinate = textureMatrix * a_textureCoordinate;

The fragment shader (needs to be expanded to handle color correction):

uniform sampler2D texture0;

varying mediump vec4 v_textureCoordinate;

void main() {
    gl_FragColor = texture2D(texture0, v_textureCoordinate.st);

And finally the code. The shader programs are all done using Qt’s shader classes, and I don’t show that code here — it’s in the calligra git repo anyway.

KisCoordinatesConverter *converter = coordinatesConverter();

QTransform textureTransform;
QTransform modelTransform;
QRectF textureRect;
QRectF modelRect;

converter->getOpenGLCheckersInfo(&textureTransform, &modelTransform, &textureRect, &modelRect);

// XXX: getting a config object every time we draw the checkers is bad for performance!
KisConfig cfg;
GLfloat checkSizeScale = KisOpenGLImageTextures::BACKGROUND_TEXTURE_CHECK_SIZE / static_cast(cfg.checkSize());

textureTransform *= QTransform::fromScale(checkSizeScale / KisOpenGLImageTextures::BACKGROUND_TEXTURE_SIZE,
                                            checkSizeScale / KisOpenGLImageTextures::BACKGROUND_TEXTURE_SIZE);


QMatrix4x4 projectionMatrix;
projectionMatrix.ortho(0, width(), height(), 0, NEAR_VAL, FAR_VAL);

// Set view/projection matrices
QMatrix4x4 modelMatrix(modelTransform);
modelMatrix = projectionMatrix * modelMatrix;
m_d->checkerShader->setUniformValue("modelViewProjection", modelMatrix);

QMatrix4x4 textureMatrix(textureTransform);
m_d->checkerShader->setUniformValue("textureMatrix", textureMatrix);

//Setup the geometry for rendering
QVector vertices;
vertices << QVector3D(modelRect.left(),  modelRect.bottom(), 0.f)
            << QVector3D(modelRect.left(),  modelRect.top(),    0.f)
            << QVector3D(modelRect.right(), modelRect.bottom(), 0.f)
            << QVector3D(modelRect.left(),  modelRect.top(), 0.f)
            << QVector3D(modelRect.right(), modelRect.top(), 0.f)
            << QVector3D(modelRect.right(), modelRect.bottom(), 0.f); m_d->checkerShader->enableAttributeArray(PROGRAM_VERTEX_ATTRIBUTE);
m_d->checkerShader->setAttributeArray(PROGRAM_VERTEX_ATTRIBUTE, vertices.constData());

QVector texCoords;
texCoords << QVector2D(textureRect.left(), textureRect.bottom())
            << QVector2D(textureRect.left(), textureRect.top())
            << QVector2D(textureRect.right(), textureRect.bottom())
            << QVector2D(textureRect.left(), textureRect.top())
            << QVector2D(textureRect.right(), textureRect.top())
            << QVector2D(textureRect.right(), textureRect.bottom()); m_d->checkerShader->enableAttributeArray(PROGRAM_TEXCOORD_ATTRIBUTE);
m_d->checkerShader->setAttributeArray(PROGRAM_TEXCOORD_ATTRIBUTE, texCoords.constData());

    // render checkers
glBindTexture(GL_TEXTURE_2D, m_d->openGLImageTextures->checkerTexture());

glDrawArrays(GL_TRIANGLES, 0, 6);

glBindTexture(GL_TEXTURE_2D, 0);

For Krita, there are quite a few todo’s left:

  • Restore the opengl outline cursor — now we use the qpainter one
  • Render onto a framebuffer object so this code can be integrated in Krita Sketch
  • Update the image texture tiles in a thread
  • Update the projection in a thread
  • Restore the colormanagement using OCIO
  • Check whether it runs on Windows, OSX (and Android)
  • Maybe move the layer composition to OpenGL using the GPUImage shader code?

Especially the testing on Windows is interesting, since the old opengl canvas never worked on Windows.

More Vacation Fun: smoother lines in Krita!

Ever since sigetch created a smooth inking pen for Gimp Painter, artists have been asking us to implement something similar in Krita. It’s a feature particularly in demand for inking comics, and since comics is one of Krita’s focus areas…

It basicaly works by looking at the history of the stroke — the last twenty or fifty of positions or so, the speed with which you paint and then calculates a new position, one that makes the line smoother. I don’t pretend to understand the mathematics — but it was pretty easy to figure out from Alexia Death’s clean and clear implementation in Gimp 2.8, so I took that as a model. Free software rocks more!

I actually couldn’t believe it when I was done, and Timothee Giet reported that it worked as per spec… But here’s a sketch from his hand that proves it!

A New Krita Feature: Flipbooks

I’m on vacation, but I cannot stop hacking… But instead of soul-destroying Windows build work, detective work implementing PSD export support or some hair-pulling bug-fixing, I wanted to have some clean, innocent fun…

Create a nice new feature. So over the past couple of days, I’ve implemented a flipbook.

It’s quite full-featured already, with flipbooks being available in the recent files list in Krita’s startup screen, a page for creating new flipbooks and finally a docker that shows the images and allows the user to flip between images in the flipbook, or save the current set of images as a flipbook.

There are bound to be some rough edges, and I’m wondering what kind of feature requests our users will think up to make the flipbook even more useful, but really, it’s already quite usable.

KDE on Windows

From Friday to Sunday, I attended the KDE on Windows sprint, hosted by Intevation, in Osnabrück. There were seven attendants, making it a nice and hacking-focussed sprint. Sure, there were discussions about improvements to the system and so on, as you can read on the meeting wiki page.

Important items are changing the versioning of the portage system by removing the version from the package names, improving the sandboxing of KDE applications (so they don’t overwrite each other settings, system configuration cache and so on) and integration of creatsing nsis installers in the emerge system.

We had some non-keyboard time as well, when on Friday, KDAB sponsored a german-style dinner, and on Saturday, KO an Arabian-style dinner, followed by drinks in the “unikeller”. Some pretty obscure German beer was had there, very tasty. And then some of us went back to the office of an all-night hack session…

See also Patrik SPendrin’s blog, with group picture….


Currently, I’m working with three or four build trees: one with Microsoft Visual C++, one mixed Microsoft Visual C++ and Intel Composer, one with Mingw, all natively on Windows. Dominik Schmidt demonstrated how easy it is to create 32 and 64 bit builds on Linux using Tomahawk as an example. I hadn’t heard of Tomahawk before, since my media player of choice is ogg123… But the build system looks very simple to use, and Tomahawk is pretty cool, too. Dominik spent a large part of the weekend integrating kdelibs, kde-runtime and kdepim into this cross-compiling system. This system depends heavily on the OpenSUSE build system, which I need to investigate anyway to make my CentOS packages easier to generate.

Check the tomahawk wiki for a basic cross-compiling howto: it really is utterly simple, once you realize that you need to do the same trick as tomahawk with a toolchain file for cmake definitions. That needs to be in order; from that point onwards, everything should just work. I’ll give it a try with Krita soon: the only dependencies still missing are exiv2 and eigen2.

This generates NSIS-based installers, but I also heard that other people have had success generating WIX-based MSI installers using mono on Linux and OSX, so that’s worth investigating as well.


Being a newcomer to the KDE Windows scene, I did some simple stuff, like creating a calligra package for the emerge system. If you put an emerge checkout on your windows system and follow the basic setup steps (really, just get the checkout, install python3, create an etc directory with copy of kdesettings.bat), you should be able to do an “emerge calligra” and get a working calligra installation. Of course, there might be problems along the way, but the nice people in #kde-windows know most of the answers!

I also started on packaging ilmbase, the basic dependency for OpenEXR. This was trickier, since ilmbase is an autotools-based package, so we had to add a CMake-based build system. That basically worked, so I added the package to emerge’s portage tree, and Patrick Spendrin then finished up and added OpenEXR. Maybe we should try to get the CMake system for those libraries upstreamed, though. The upshot of this is that Krita on Windows, once I’ve made new installers, will be able to handle 32-bits floating point/channel images. Not 16 bit floating point, though, since LittleCMS cannot handle that yet. But Marti has promised that for the next release…

Krita in history…

Krita in history…

“Azes is now being associated with the creation of the era of 58 BC that was to be known through the centures as the Krita, Malava or Vikramaditya, samvat, era. Possibly the era was also calculated for use in astronomy as the term krita, created, would suggest, but was given status by association with royalty.” Romila Thapar, The Penguin History of Early India, p. 220

Two worrying bugs

Krita uses OpenGTL, a very, very cool techology. We use it for filters and for colormodel definitions (mainly for floating point colorspaces). OpenGTL uses LLVM internally to compile from its domain specific language to fast native code, on the fly. Ultra-cool stuff, in short.

But recently, I’ve had two bug reports that worry me a lot, and I do not know how to handle them:

It looks like there’s problem having an application that links to llvm running on a system that uses the radeon driver. I’m not sure, llvm being black magic to me… I also don’t know who to approach in the gallium/radeon/llvm communities about this issue, and neither are the reporters. But if this is true, Krita has a big problem, and so have other applications that make use of the cool possibilities of llvm.

If you have any suggestions, please mail me directly!

Third Krita Sprint First Day

While I was forced by circumstances for forego the production of t-shirts for this sprint — and I feel bad about that! — the first day of the Third Krita Sprint has been great. Yesterday, our webmaster Bugsbane, a.k.a. Kubuntiac and Krita hacker Dmitry Kazakov arrived in Deventer already, and this morning we took the train to Amsterdam, to the the Blender studios. We’re crammed around the same glass table where last year selected Krita hackers saw a sneak preview of the Sintel movie. Feels weird!

Assembled today were artists David Revoy, Animtim, Silvio Heinrich and Kubuntiac, as well as hackers Adam, Dmitry, Sven, Lukas, Matus (tomorrow is his birthday!), Dmitry, Silvio (again, this guy puts in a double appearance). We got a surprise visit from a bunch of Danish game designers here for a game jam at the Binger Filmlab, who, of course, got a full demo.

The selection system in Krita was redesigned, bugs were filed and solved, lunch was consumed on a road-side terrace in the wonderful sunshine, dinner was had at the Blender team’s usual Thai restaurant — and we made plans for tomorrow.

We’ll start off around 10 with an artists session: all artists present are going to take about half an hour to work in Krita, a projector connected so we all can follow. We call it the “demo and gripe” sessions. That way the developers can learn about the way different artists really work, and we can spot glitches and issues with usability. We are going to try to record this session, both sound and screencast and make it available afterwards. No promises though, we might run into glitches there!

The afternoon is for the whole team to get a list of highly important issues that hamper artists most so we can set the priorities for the year following this sprint. After that — it’s hacking and discussion time!

Lamented unmaintained features in Krita

Krita 2.3 is really pretty good — although following the release we’ve found a number of rather serious issues, especially when working with images in the 50-100 megapixel range. But on a less important note, there are quite a few features in Krita that never really got any love after they were initially coded.

The pattern creation tool is one of those, and it’s getting more important now since it’s not possible to use patterns as a color source for freehand painting. The pattern creation popup was coded in 2006 by Bart Coppens (who is also responsible for quite a few other features in Krita but who’s too busy with his Phd these days to code for Krita).

As you can see, it’s quite simple:

There’s a preview, an update button, an use-it-now button and an add-to-predefined patterns button. If you try to make your pattern from a big image (like this 6000×4000 image), you get a warning, and the pattern will be downscaled to fit in 1000×1000. That’s because otherwise it’s pretty easy to run out of memory! Oh, and since yesterday you can select whether you want to create your pattern from the current layer or from the whole image.

What’s missing is using the selection, naming the pattern, exporting the pattern and who knows what else — I haven’t yet checked what other apps provide. Managing existing patterns is pretty primitive as well. And the layout of the popup is a bit of an embarrasment!

But there you have it — a nice small feature that’s suddenly become important. It was fine in 2006, but in 2011, we expect more of it. So… If you suddenly feel inspired to hack on this, join us on #krita on irc.freenode.net and we’ll get you started!

Speaking at Fosdem 2011

This year I’ll give a presentation at Fosdem. I’m not sure which day yet, though.. Updates will follow. The presentation will be very technical: I want to show how easy it is to build a new, custom interface on top of the Calligra Suite core engine — using QWidgets, QGraphicsView or even Qt Quick, for viewing and editing of documents.

I don’t think there’s any other office engine/application suite that makes it not just possible, but even relatively easy, to reuse the canvas, the file conversion code and even the tools in applications. Calligra is beginning to become sort of the webkit of the office document world.

There’s a lot of interest and a real need for this sort of possibility: I know for a fact that apart from Nokia’s FreOffice/Calligra Mobile, there are at least four projects working on a new Calligra user interface, using various technologies, some of them generic, some of them focussed on a particular target group, like medical information systems.

I’m really looking forward to it, but it’s going to be a weird kind of weekend for me, though, since Irina will be doing KDE booth duty, like last year, and I don’t think we have ever been away from the kids together for more than a day since they were small and still had grandparents who could care for them for a weekend.

Moving KOffice to Git

No, don’t get all worried yet! The KOffice community has been planning for a long time to move development to git, but it’s only now that all the pieces seem to fall into place. The KDE git project infrastructure is looking pretty good and ready for use.

The only thing missing for KOffice were up-to-date conversion rules and a timeline. KO GmbH decided to do something about the first, by sponsoring Ian Monroe to bring the rules up to date, run the conversion and put the first experimental KOffice git repo online:


What’s needed now is people cloning this repo, testing it, checking it for weirdnesses and getting a feel for git in general.

And the community has decided on a timeline for when development will move to Git, namely right after the branching for the 2.3 release, which should happen with the first release candidate. In other words, pretty soon!