From Newsgroup: comp.lang.javascript

Common code for applying a transition effect where the image being
transitioned to has some matrix transform applied:

function def_transform_effect_common(transform, swap_parms)
/* function factory for transform-related effect functions. */
{
return function(ctx, parms, t)
{
ctx.save()
ctx.reset()
ctx.resetTransform()
ctx.drawImage(swap_parms ? parms.image_bitmap_2 : parms.image_bitmap_1, 0, 0)
ctx.setTransform(transform(t))
ctx.drawImage(swap_parms ? parms.image_bitmap_1 : parms.image_bitmap_2, 0, 0)
ctx.restore()
}
} /*def_transform_effect_common*/

A specific matrix transform: stretch/shrink the image vertically,
from/to a specified y-coordinate:

function def_transform_stretch_shrink(stretch, y0)
{
return function(t)
{
const mtx =
{
"a" : 1,
"c" : 0,
"e" : 0,
"b" : 0,
"d" : (stretch ? t : 1 - t),
"f" : %(height)d * y0 * (stretch ? 1 - t : t),
}
return mtx
}
} /*def_transform_stretch_shrink*/

Put them together, to define a rCLshrink to bottomrCY effect:

window.effects = /* exported effects functions */
{
...
"shrink_to_bottom" :
def_transform_effect_common(def_transform_stretch_shrink(false, 1), true),
...
}
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From Newsgroup: comp.lang.javascript

On Fri, 24 Jul 2026 06:43:10 -0000 (UTC), I wrote:

Put them together, to define a rCLshrink to bottomrCY effect:

window.effects = /* exported effects functions */
{
...
"shrink_to_bottom" :
def_transform_effect_common(def_transform_stretch_shrink(false, 1), true),
...
}

Just to make the code-reuse factor clear, here are 6 different visual
effects, each defined with just one line of code:

window.effects = /* exported effects functions */
{
...
"stretch_from_bottom" :
def_transform_effect_common(def_transform_stretch_shrink(true, 1), false),
"stretch_from_middle" :
def_transform_effect_common(def_transform_stretch_shrink(true, 0.5), false),
"stretch_from_top" :
def_transform_effect_common(def_transform_stretch_shrink(true, 0), false),
"shrink_to_bottom" :
def_transform_effect_common(def_transform_stretch_shrink(false, 1), true),
"shrink_to_middle" :
def_transform_effect_common(def_transform_stretch_shrink(false, 0.5), true),
"shrink_to_top" :
def_transform_effect_common(def_transform_stretch_shrink(false, 0), true),
...
}
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