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2 ** |
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3 ** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies). |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \page x11overlays.html |
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44 \title How to Use X11 Overlays with Qt |
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45 |
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46 X11 overlays are a powerful mechanism for drawing |
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47 annotations etc., on top of an image without destroying it, thus saving |
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48 a great deal of image rendering time. For more information, see the highly |
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49 recommended book \e{OpenGL Programming for the X Window System} (Mark |
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50 Kilgard, Addison Wesley Developers Press 1996). |
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51 |
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52 \warning The Qt OpenGL Extension includes direct support for the |
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53 use of OpenGL overlays. For many uses of overlays, this makes the |
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54 technique described below redundant. The following is a discussion |
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55 on how to use non-QGL widgets in overlay planes. |
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56 |
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57 In the typical case, X11 overlays can easily be used together with the |
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58 current version of Qt and the Qt OpenGL Extension. The following |
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59 requirements apply: |
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60 |
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61 \list 1 |
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62 \i Your X server and graphics card/hardware must support overlays. |
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63 For many X servers, overlay support can be turned on with |
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64 a configuration option; consult your X server installation |
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65 documentation. |
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66 |
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67 \i Your X server must (be configured to) use an overlay visual as the |
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68 default visual. Most modern X servers do this, since this has the |
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69 added advantage that pop-up menus, overlapping windows etc., will |
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70 \e not affect underlying images in the main plane, thereby |
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71 avoiding expensive redraws. |
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72 |
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73 \i The best (deepest) visual for OpenGL rendering is in the main |
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74 plane. This is the normal case. Typically, X servers that support |
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75 overlays provide a 24-bit \c TrueColor visual in the main plane, |
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76 and an 8-bit \c PseudoColor (default) visual in the overlay plane. |
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77 \endlist |
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78 |
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79 Assuming that the requirements mentioned above are met, a |
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80 QGLWidget will default to using the main plane visual, while all |
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81 other widgets will use the overlay visual. Thus, we can place a |
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82 normal widget on top of the QGLWidget, and do drawing on it, |
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83 without affecting the image in the OpenGL window. In other words, |
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84 we can use all the drawing capabilities of QPainter to draw |
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85 annotations, rubberbands, etc. For the typical use of overlays, |
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86 this is much easier than using OpenGL for rendering annotations. |
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87 |
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88 An overlay plane has a specific color called the transparent |
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89 color. Pixels drawn in this color will not be visible; instead |
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90 the underlying OpenGL image will show through. |
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91 |
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92 To use this technique, you must not use the |
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93 QApplication::ManyColor or QApplication::TrueColor color |
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94 specification for QApplication, because this will force the |
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95 normal Qt widgets to use a \c TrueColor visual, which will |
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96 typically be in the main plane, not in the overlay plane as |
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97 desired. |
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98 */ |