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Allow arbitrary rotations by being able to specify an anchor point [qtblend] #1110

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Allow arbitrary rotations by being able to specify an anchor point [qtblend] #1110

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c7d3ccc
Add rotation anchor property to qtblend filter to allow arbitrary rot…
balooii Aug 6, 2025
9b11645
Document new rotate_anchor property
balooii Aug 6, 2025
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18 changes: 16 additions & 2 deletions src/modules/qt/filter_qtblend.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -166,12 +166,22 @@ static int filter_get_image(mlt_frame frame,
if (mlt_properties_get(properties, "rotation")) {
double angle = mlt_properties_anim_get_double(properties, "rotation", position, length);
if (angle != 0.0) {
if (mlt_properties_get_int(properties, "rotate_center")) {
if (mlt_properties_get(properties, "rotate_anchor")) {
mlt_rect anchor = mlt_properties_anim_get_rect(properties, "rotate_anchor", position, length);
// Use custom anchor point (x,y are normalized 0-1 coordinates) where 0, 0 is top left and 1, 1 is bottom right
// negative values are allowed so its possible to rotate around a point outside the rectangle
double anchor_x = anchor.x * rect.w;
double anchor_y = anchor.y * rect.h;
transform.translate(anchor_x, anchor_y);
transform.rotate(angle);
transform.translate(-anchor_x, -anchor_y);
} else if (mlt_properties_get_int(properties, "rotate_center")) {
// old style rotation (from center) to keep compatibility, equivalent to rotate_anchor = 0.5, 0.5
transform.translate(rect.w / 2.0, rect.h / 2.0);
transform.rotate(angle);
transform.translate(-rect.w / 2.0, -rect.h / 2.0);
} else {
// old style rotation (from top left corner) to keep compatibility
// old style rotation (from top left corner) to keep compatibility, equivalent to rotate_anchor = 0, 0
transform.rotate(angle);
}
hasAlpha = true;
Expand Down Expand Up @@ -268,6 +278,10 @@ mlt_filter filter_qtblend_init(mlt_profile profile, mlt_service_type type, const
filter->process = filter_process;
mlt_properties properties = MLT_FILTER_PROPERTIES(filter);
mlt_properties_set_int(properties, "rotate_center", 0);

// Initialize rotate_anchor property with center position (0.5, 0.5)
mlt_rect default_anchor = {0.5, 0.5, 0.0, 0.0, 0.0};
mlt_properties_set_rect(properties, "rotate_anchor", default_anchor);
} else {
mlt_log_error(MLT_FILTER_SERVICE(filter), "Filter qtblend failed\n");

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11 changes: 11 additions & 0 deletions src/modules/qt/filter_qtblend.yml
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Expand Up @@ -56,6 +56,17 @@ parameters:
mutable: yes
widget: checkbox

- identifier: rotate_anchor
title: Rotation anchor point
type: rect
description: >
Keyframable anchor point for rotation. X and Y coordinates define the rotation center.
Values are normalized (0-1 for inside rectangle, negative or >1 for outside).
Default is center (0.5, 0.5). Takes precedence over rotate_center when set.
mutable: yes
animation: yes
default: "0.5 0.5 0 0 0"

- identifier: background_color
title: Background color
type: integer
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