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[GTK] Fix the image size cache never hitting at zoom != 100 - #3512

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[GTK] Fix the image size cache never hitting at zoom != 100#3512
vogella wants to merge 2 commits into
eclipse-platform:masterfrom
vogella:perf/imageatsize-units

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@vogella vogella commented Aug 13, 2026

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Stacked on #3510. This branch contains that commit as well, so review only the second one, [GTK] Fix the image size cache never hitting at zoom != 100. Draft until #3510 merges, then I will rebase and mark it ready.

CachedImageAtSize.refresh converted the requested draw size to pixels with DPIUtil.pointToPixel and handed that to isReusable, which compared it against the cached image's width and height.
Those are points: the cached image is built with new Image(device, imageData, getDeviceZoom()) and Image.init(ImageData, zoom) divides by the scale factor.
At zoom 200 the comparison is scaledWidth/2 == scaledWidth, never true, so the single-entry size cache never hit and every scaled draw reloaded the image.
This remembers the requested size in pixels alongside the cached image and compares against that, as the win32 HandleAtSize already does.

Opens per draw over 100 draws of one Image at a stable draw size through the 5 argument overload, SVG, strace -f -e trace=openat on Linux/GTK:

Device zoom Before After
100 0.02 0.02
200 2.00 0.02

Zoom 100 is unchanged and zoom 200 drops to the same one-miss-then-hits behaviour.
Rendering is identical, verified as matching SHA-256 of the drawn ImageData across both overloads, three files and zoom 100, 150 and 200.

Cocoa has a textually identical isReusable but is not affected: its cached image is built with new Image(device, imageData), which passes zoom 100 to init and so keeps the value in pixels.

The new test needs #3510, since without it the getImageData decode re-reads the deleted file at zoom 200 and the test would fail for an unrelated reason.

Fixes #3511

The internal drawImage that every public overload funnels into read the
source dimensions from srcImage.getImageData(), which is getImageData(100).
At currentDeviceZoom != 100 that misses the zoom == currentDeviceZoom fast
path and loads new ImageData(fileName): a full open and full decode, for
SVG a full re-parse and re-rasterize, to obtain two integers the Image
already knows. The result was used for nothing else, the drawing itself
goes through srcImage.surface. An ImageGcDrawer image paid the same way,
by running the drawer callback once per draw.

Take the dimensions from the Image, falling back to ImageData for images
wrapped around a native handle by Image.gtk_new, which carry none. Those
have no provider, so the fallback reads the cairo surface and never a
file. Cocoa already takes its dimensions from NSImage.size() and win32
from getBounds(), so this brings GTK in line with both.

Opens per draw over 100 draws of one Image at zoom 200, strace on
Linux/GTK:

  9 arg overload, PNG    1.00 -> 0.00
  5 arg overload, PNG    2.00 -> 1.00

The open left on the 5 arg overload is the CachedImageAtSize path of
issue 3505.

Rendering is unchanged wherever the Image dimensions agree with the
decoded ones, verified as identical SHA-256 of the drawn ImageData over
both overloads, three files and zoom 100, 150 and 200, and again for an
ImageFileNameProvider returning one path at every zoom.

Two cases change, both where getBounds() and getImageData() already
disagreed. For an asset set that is not exactly proportional, 16 pixels
at 100% and 33 at 200%, the width field is round(33/2) = 17 while
getImageData(100) gives 16, so at zoom 200 the unscaled draw now paints
the last point row and column that were previously clipped. For a
provider handing out the same file at every zoom, a 16 pixel file at zoom
200 gives bounds 8 and getImageData 16, so passing getImageData()
dimensions as the source rectangle now raises ERROR_INVALID_ARGUMENT
where it previously drew. In both cases the new value is the one that
agrees with getBounds().

Fixes eclipse-platform#3507
CachedImageAtSize.refresh converted the requested draw size to pixels with
DPIUtil.pointToPixel and handed that to isReusable, which compared it
against the cached image's width and height. Those are points: the cached
image is built with new Image(device, imageData, getDeviceZoom()) and
Image.init(ImageData, zoom) divides by the scale factor. At zoom 200 the
comparison is scaledWidth/2 == scaledWidth, never true, so the cache never
hit and every scaled draw reloaded the image.

Remember the requested size in pixels alongside the cached image and
compare against that, as the win32 HandleAtSize already does.

Opens per draw over 100 draws of one Image at a stable draw size through
the 5 argument overload, SVG, strace on Linux/GTK:

  zoom 100    0.02 -> 0.02
  zoom 200    2.00 -> 0.02

Rendering is unchanged, verified as identical SHA-256 of the drawn
ImageData over both overloads, three files and zoom 100, 150 and 200.

Cocoa has the same comparison but is not affected: its cached image is
built with new Image(device, imageData), which passes zoom 100 to init and
so keeps the value in pixels.

Fixes eclipse-platform#3511
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Test Results

  211 files   -  1    211 suites   - 1   27m 15s ⏱️ - 1m 12s
4 839 tests  - 55  4 817 ✅  - 53   21 💤  - 3  0 ❌ ±0  1 🔥 +1 
7 020 runs   - 45  6 854 ✅  - 43  165 💤  - 3  0 ❌ ±0  1 🔥 +1 

For more details on these errors, see this check.

Results for commit ee4ddac. ± Comparison against base commit 9ca368d.

This pull request removes 57 and adds 2 tests. Note that renamed tests count towards both.
org.eclipse.swt.graphics.ImageWin32Tests ‑ testDisposeDrawnImageBeforeRequestingTargetForOtherZoom
org.eclipse.swt.graphics.ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[1] true
org.eclipse.swt.graphics.ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[2] false
org.eclipse.swt.graphics.ImageWin32Tests ‑ testImageDataForDifferentFractionalZoomsShouldBeDifferent
org.eclipse.swt.graphics.ImageWin32Tests ‑ testImageShouldHaveDimesionAsPerZoomLevel
org.eclipse.swt.graphics.ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[1] true
org.eclipse.swt.graphics.ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[2] false
org.eclipse.swt.graphics.ImageWin32Tests ‑ test_getImageData_fromCopiedImage
org.eclipse.swt.graphics.ImageWin32Tests ‑ test_getImageData_fromImageForImageDataFromImage
org.eclipse.swt.tests.win32.Test_org_eclipse_swt_dnd_DND ‑ testByteArrayTransfer
…
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_graphics_GC ‑ test_drawImage_doesNotReReadImageFileAtNonDefaultZoom
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_graphics_Image ‑ test_drawImageAtSize_cacheIsReusedAtNonDefaultZoom

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CachedImageAtSize never hits at device zoom != 100 (points compared against pixels)

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