Additive (not linear) compositing when using GLX

[Carsten Haitzler (The Rasterman)]

On Sun, 17 Oct 2010 18:03:28 -0500 Rendaw <rendaw at zarbosoft.com> said:

x uses premultiplied alpha ARGB colorspace. (google for the info) but the
simple thing here is
  r = r *a, g =g * a, b = b* a
that means r can NEVER be bigger than a, same with g and b. otherwise rendering
booboos happen as you saw. you are rendering with opengl - but you are not
producing a premultiplied alpha result. fix that and presto. :)


>   I'm using xcompmgr for compositing.  A window I paint using OpenGL is 
> much brighter than a window painted the same way using Cairo.  
> Specifically, with Red=1, Alpha=0, everything has full red.  Using 
> Cairo, there is no coloration.
> 
> I'm not using glxcompmgr, but I noticed the line glBlendFunc(GL_ONE, 
> GL_ONE_MINUS_SRC_ALPHA); in the code.  Compositing like that would cause 
> the output I showed above.
> 
> Does anyone know from which package this behavior might stem?  Since 
> xcompmgr is pretty small, I doubt it is differentiating between GL 
> rendered windows and xrender/whatever rendered ones, but that would have 
> been my first guess.
> 
> (I'm using Arch linux: xcompmgr 1.1.5-1, xf86-video-intel 2.12.0-1, 
> xorg-server 1.8.1.902-1)
> 
> Here's the code to reproduce.  You'll have to change TargetVisual to use 
> some RGBA supporting visual.
> #include <GL/glx.h>
> #include <GL/gl.h>
> #include <cairo/cairo-xlib.h>
> #include <unistd.h>
> #include <cassert>
> #include <string>
> 
> const float ClearColor[4] = {1, 0, 0, 0.0f};
> int TargetVisual = 0x5e;
> 
> void CairoDraw(Display *Context, Window Canvas, XVisualInfo *VisualData)
> {
>      cairo_surface_t *BaseSurface = cairo_xlib_surface_create(Context, 
> Canvas, VisualData->visual, 100, 100);
>      cairo_t *CairoContext = cairo_create(BaseSurface);
> 
>      cairo_set_operator(CairoContext, CAIRO_OPERATOR_SOURCE);
>      cairo_set_source_rgba(CairoContext, ClearColor[0], ClearColor[1], 
> ClearColor[2], ClearColor[3]);
>      cairo_paint(CairoContext);
> 
>      cairo_surface_write_to_png(BaseSurface, "Test.png");
> }
> 
> void GLDraw(Display *Context, Window Canvas, XVisualInfo *VisualData)
> {
>      GLXContext GLContext = glXCreateContext(Context, VisualData, NULL, 
> GL_TRUE);
>      glXMakeCurrent(Context, Canvas, GLContext);
> 
>      glClearColor(ClearColor[0], ClearColor[1], ClearColor[2], 
> ClearColor[3]);
>      glClear(GL_COLOR_BUFFER_BIT);
>      glXSwapBuffers(Context, Canvas);
> }
> 
> int main(int argc, char **argv)
> {
>      Display *Context = XOpenDisplay(NULL);
> 
>      XVisualInfo Criteria, *FoundVisuals;
>      int FoundVisualCount;
>      Criteria.visualid = TargetVisual;
>      FoundVisuals = XGetVisualInfo(Context, VisualIDMask, &Criteria, 
> &FoundVisualCount);
>      assert(FoundVisualCount != 0);
> 
>      XSetWindowAttributes NewAttributes;
>      NewAttributes.colormap = XCreateColormap(Context, 
> RootWindow(Context, FoundVisuals->screen), FoundVisuals->visual, AllocNone);
>      NewAttributes.border_pixel = 0;
>      NewAttributes.event_mask = StructureNotifyMask;
> 
>      Window TestWindow = XCreateWindow(Context, RootWindow(Context, 
> FoundVisuals->screen),
>          0, 0, 100, 100, 0,
>          FoundVisuals->depth, InputOutput, FoundVisuals->visual,
>          CWBorderPixel | CWColormap | CWEventMask,
> &NewAttributes);
>      XMapWindow(Context, TestWindow);
> 
>      XEvent Poll;
>      do XNextEvent(Context, &Poll); while (Poll.type != MapNotify);
> 
>      if ((argc >= 2) && (std::string(argv[1]) == "cairo"))
>          CairoDraw(Context, TestWindow, FoundVisuals);
>      else GLDraw(Context, TestWindow, FoundVisuals);
> 
>      sleep(10);
> }
> 
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-- 
------------- Codito, ergo sum - "I code, therefore I am" --------------
The Rasterman (Carsten Haitzler)    raster at rasterman.com