Goocanvas - 坐标系统和缩放到光标

Question

我正在尝试在scrolledwindow中实现针对指针/光标的缩放，其中包含一个goocanvas。不幸的是，有关使用的坐标系统的文档不太理想。

那么，有人能解释一下为什么以下概念性代码不应该起作用吗？（有关详细信息，请参见内联代码注释）

方便结构体

 typedef struct {
     gdouble x;
     gdouble y;
 } Coords;
 typedef Sheet GooCanvas; // actually a subclass, but anyways

这只是一个方便的辅助函数（去除了测试）

gboolean sheet_get_adjustments (const Sheet *sheet, GtkAdjustment **hadj, GtkAdjustment **vadj)
{
GtkWidget *parent;
GtkScrolledWindow *scrolled;
parent = gtk_widget_get_parent (GTK_WIDGET (sheet));
scrolled = GTK_SCROLLED_WINDOW (parent);
*hadj = gtk_scrolled_window_get_hadjustment (scrolled);
*vadj = gtk_scrolled_window_get_vadjustment (scrolled);
return TRUE;
}

获取像素中的指针位置（已验证，按预期工作）

gboolean sheet_get_pointer_pixel (Sheet *sheet, gdouble *x, gdouble *y)
{
GtkAdjustment    *hadj, *vadj;
gdouble           x1, y1;
gint              _x, _y;
GdkDeviceManager *device_manager;
GdkDevice        *device_pointer;
GdkRectangle      allocation;

device_manager = gdk_display_get_device_manager (
                gtk_widget_get_display (GTK_WIDGET (sheet)));
device_pointer = gdk_device_manager_get_client_pointer (device_manager);
gdk_window_get_device_position (gtk_widget_get_window (GTK_WIDGET (sheet)),
                                    device_pointer,
                                    &_x, &_y, NULL);

gtk_widget_get_allocation (GTK_WIDGET (sheet), &allocation);

_x -= allocation.x;
_y -= allocation.y;

x1 = (gdouble) _x;
y1 = (gdouble) _y;

if (!sheet_get_adjustments (sheet, &hadj, &vadj))
    return FALSE;

x1 += gtk_adjustment_get_value (hadj);
y1 += gtk_adjustment_get_value (vadj);

*x = x1;
*y = y1;
return TRUE;
}

实际缩放校正时，我们获取指针在像素上的位置，该位置是恒定的且独立于 Goo 画布比例。基于指针在缩放应用于画布之前和之后的像素位置计算 Goo 画布位置。差异是指针在缩放后所处位置和所需位置之间的偏移量（我们希望将其夹紧到画布位置）。获取画布左上角在画布坐标系中的位置，应用偏移量并滚动到新的左上角位置（goo_canvas_scroll_to 总是向左上方滚动）。

void change_zoom(Sheet *sheet, gdouble f)
{
Coords a;
Coords pixel_pointer;
    Coords goo_pointer_before, goo_pointer_after;

GtkWidget *w;
GtkAdjustment *hadj, *vadj;

if (sheet_get_adjustments (sheet, &hadj, &vadj)) {
    a.x = gtk_adjustment_get_value (hadj);
    a.y = gtk_adjustment_get_value (vadj);
} else {
    a.x = a.y = 0.;
}

w = GTK_WIDGET (sheet);

//top left, not used, just to "visualize" the visible canvas range
Coords goo_tl = {a.x, a.y}; 
Coords pixel_tl = goo_tl;
goo_canvas_convert_to_pixels (GOO_CANVAS (sheet), &pixel_tl.x, &pixel_tl.y);
NG_DEBUG ("### top left [pixel]  %lf,%lf", pixel_tl.x, pixel_tl.y);


sheet_get_pointer_pixel (sheet, &pixel_pointer.x, &pixel_pointer.y);
goo_pointer_before = pixel_pointer;
goo_canvas_convert_from_pixels (GOO_CANVAS (sheet), &goo_pointer_before.x, &goo_pointer_before.y);
NG_DEBUG ("### pointer before scale[canvas] %lf,%lf", goo_pointer_before.x, goo_pointer_before.y);
////////////////////////////////////////////////////////////////////////////////////////////
goo_canvas_set_scale (GOO_CANVAS (sheet), f * goo_canvas_get_scale (GOO_CANVAS (sheet)));
////////////////////////////////////////////////////////////////////////////////////////////
goo_pointer_after = pixel_pointer;
goo_canvas_convert_from_pixels (GOO_CANVAS (sheet), &goo_pointer_after.x, &goo_pointer_after.y);
NG_DEBUG ("### pointer after scale[canvas] %lf,%lf", goo_pointer_after.x, goo_pointer_after.y);

// this needs to be redone, as scaling might have changed the bar positions
if (sheet_get_adjustments (sheet, &hadj, &vadj)) {
    a.x = gtk_adjustment_get_value (hadj);
    a.y = gtk_adjustment_get_value (vadj);
} else {
    a.x = a.y = 0.;
}
Coords goo_tl2 = {a.x, a.y}; //top left, after scroll (in pixel coords)
goo_canvas_convert_from_pixels (GOO_CANVAS (sheet), &goo_tl2.x, &goo_tl2.y); // to goo canvas coords
NG_DEBUG ("### top left after zoom[canvas] %lf,%lf", goo_tl2.x, goo_tl2.y);

// the delta in goo canvas coords
const Coords goo_delta = {goo_pointer_before.x - goo_pointer_after.x, goo_pointer_before.y - goo_pointer_after.y};
NG_DEBUG ("### delta[canvas] %lf,%lf", goo_delta.x, goo_delta.y)

//apply the offset to the _current_ top left corner
goo_tl2.x -= goo_delta.x;
goo_tl2.y -= goo_delta.y;
goo_canvas_scroll_to (GOO_CANVAS (sheet), goo_tl2.x, goo_tl2.y);
NG_DEBUG ("### top left after scroll[canvas] %lf,%lf", goo_tl2.x, goo_tl2.y);

NG_DEBUG ("\n\n\n");
return;
}

问题是，虽然显示的所有内容都没问题，但是goo_delta某些东西是错误的，指针没有固定在goo坐标上。所以我的猜测是，我需要一个二次校正因子或者现有代码中存在一个bug。

例如 (f=1.1，光标靠近画布左上角)

 sheet_change_zoom +++ ### top left [pixel]  0.000000,0.000000
 sheet_change_zoom +++ ### pointer before scale[canvas] 7.000000,12.000000
 sheet_change_zoom +++ ### pointer after scale[canvas] 6.363636,10.909091
 sheet_change_zoom +++ ### top left after zoom[canvas] 42.045455,30.318182
 sheet_change_zoom +++ ### delta[canvas] 0.636364,1.090909
 sheet_change_zoom +++ ### top left after scroll[canvas] 41.409091,29.227273
 sheet_change_zoom +++ 



 sheet_change_zoom +++ ### top left [pixel]  50.105000,35.365000
 sheet_change_zoom +++ ### pointer before scale[canvas] 47.772727,40.136364
 sheet_change_zoom +++ ### pointer after scale[canvas] 43.429752,36.487603
 sheet_change_zoom +++ ### top left after zoom[canvas] 79.632231,56.789256
 sheet_change_zoom +++ ### delta[canvas] 4.342975,3.648760
 sheet_change_zoom +++ ### top left after scroll[canvas] 75.289256,53.140496
 sheet_change_zoom +++ 



 sheet_change_zoom +++ ### top left [pixel]  110.231000,77.803000
 sheet_change_zoom +++ ### pointer before scale[canvas] 81.074380,63.057851
 sheet_change_zoom +++ ### pointer after scale[canvas] 73.703982,57.325319
 sheet_change_zoom +++ ### top left after zoom[canvas] 110.037566,78.196844
 sheet_change_zoom +++ ### delta[canvas] 7.370398,5.732532
 sheet_change_zoom +++ ### top left after scroll[canvas] 102.667168,72.464313
 sheet_change_zoom +++ 

任何关于解决此特定问题的goocanvas示例的提示和指针。

- drahnr

Answer 1

我最终通过查看goocanvas源代码并了解他们如何实现居中滚动来解决了这个问题。根据那个，我将画布指针位置移动到中心，缩放并将其移回。这里重要的是，移回只是像素位置（这里：仅是指针和中心之间的像素差异！）除以累积缩放因子！（使用goo_canvas_get_scale获取它）

注意：上述描述不包含基于夹紧的问题的更正。

伪代码：

delta = pointer_pos_in_pixel - center_pos_in_pixel
top_left_in_pixel read
top_left_in_pixel -= delta
convert_to_goo_coords (top_left_in_pixel → top_left_in_goo)
scroll_to (top_left_in_goo)

scale (f)

delta /= get_absolute_scale()
top_left_in_pixel read
top_left_in_pixel -= delta
convert_to_goo_coords (top_left_in_pixel → top_left_in_goo)
scroll_to (top_left_in_goo) 

注意：为了以像素为单位计算小部件的中心，请使用gtk_adjustment_get_page_size而不是gtk_widget_get_allocation。

---

补充：

我已经为oregano实现了它，所以对于那些喜欢真正的源代码（GPLv2.0），请参见https://github.com/drahnr/oregano/blob/master/src/sheet/sheet.c中的sheet_change_zoom函数。

- drahnr