text: allow combining transforms with different anchors

CHANGELOG.md

@@ -35,6 +35,8 @@ Versioning](https://semver.org/spec/v2.0.0.html) for `libnopegl`.
 - `Render*` nodes are renamed to `Draw*`
 - `TextEffect.transform` now default to an anchor in the center of the
   characters instead of a distant bottom-left position
+- `TextEffect.transform` are now combined on overlapping text effects instead of
+  replacing the previous one
 
 ### Removed
 - `Text.aspect_ratio`, it now matches the viewport aspect ratio

libnopegl/src/text.c

@@ -349,11 +349,13 @@ static int set_transform(float *dst, struct texteffect_opts *effect_opts,
     ngli_mat4_translate(tmreloc0,  anchor_x,  anchor_y, 0.f);
     ngli_mat4_translate(tmreloc1, -anchor_x, -anchor_y, 0.f);
 
-    float *tmp = tmreloc0;             // go to anchor
+    NGLI_ALIGNED_MAT(tmp);
+    memcpy(tmp, dst, sizeof(tm));      // start from the existing position
+    ngli_mat4_mul(tmp, tmp, tmreloc0); // go to anchor
     ngli_mat4_mul(tmp, tmp, tm);       // apply user transform matrix
     ngli_mat4_mul(tmp, tmp, tmreloc1); // go back from anchor
 
-    memcpy(dst, tmp, 4 * 4 * sizeof(*dst));
+    memcpy(dst, tmp, sizeof(tmp));
 
     return 0;
 }

tests/texteffect.py

@@ -205,6 +205,45 @@ def texteffect_vp_anchor(cfg: ngl.SceneCfg):
     return ngl.Text("ABC", bg_color=(1, 0, 0), box=(-1, 0, 2, 1), effects=effects)
 
 
+@test_fingerprint(width=640, height=360, keyframes=10, tolerance=1)
+@ngl.scene()
+def texteffect_combined_diff_anchors(cfg: ngl.SceneCfg):
+    cfg.duration = 3
+
+    animkf_rotate = [
+        ngl.AnimKeyFrameFloat(0, -180),
+        ngl.AnimKeyFrameFloat(0.5, 0, "linear"),
+    ]
+    animkf_scale = [
+        ngl.AnimKeyFrameVec3(0, (0.0, 0.0, 1.0)),
+        ngl.AnimKeyFrameVec3(1, (1.0, 1.0, 1.0), "linear"),
+    ]
+    rotate = ngl.Rotate(ngl.Identity(), angle=ngl.AnimatedFloat(animkf_rotate))
+    scale = ngl.Scale(ngl.Identity(), factors=ngl.AnimatedVec3(animkf_scale))
+    effects = [
+        ngl.TextEffect(
+            start=0,
+            end=cfg.duration,
+            target="char",
+            overlap=0.5,
+            transform=scale,
+            anchor=(0, -1),
+            anchor_ref="viewport",
+        ),
+        ngl.TextEffect(
+            start=0,
+            end=cfg.duration,
+            target="char",
+            overlap=0.0,
+            transform=rotate,
+            anchor=(0, 1),
+            anchor_ref="char",
+        ),
+    ]
+
+    return ngl.Text("ABC", bg_color=(1, 0, 0), box=(-1, 0, 2, 1), effects=effects)
+
+
 @test_fingerprint(width=640, height=360, keyframes=10, tolerance=1)
 @ngl.scene()
 def texteffect_chars_space_nospace(cfg: ngl.SceneCfg):