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bitter_laser
Visitor II
June 15, 2026
Question

rawPartialBitmap draws Y-overhang rows at dirty-rect top instead of screenY (ABGR2222 external bitmap, TouchGFX 4.26.1)

  • June 15, 2026
  • 0 replies
  • 21 views

Environment

  • TouchGFX 4.26.1 (Engine + prebuilt library)
  • LCD config: 8bpp, `LCD8bpp_ABGR2222`, 240x240
  • Reproduced on Windows simulator (MSVC), Linux simulator, and STM32U595 hardware. Same TouchGFX source across all three, so this is not platform-specific.

Summary

When an ABGR2222 dynamic external bitmap is drawn with `LCD::drawPartialBitmap` and the source rect extends past the dirty (clip) rect on the Y axis, the clipping path writes the overhanging rows starting at the dirty rect's top (`dirtyRect.y`) instead of at the requested `screenY`. The result is a duplicated copy of those rows at the wrong vertical position.

X-axis clipping is correct: `screenX` can be negative or off-screen with no issue. The defect is specific to vertical overhang.

 

Trigger conditions (all must hold)

  • Bitmap format `Bitmap::ABGR2222` (the other three 8bpp 2222 formats compile to identical code, so they are likely affected too)
  • Bitmap registered with `Bitmap::dynamicBitmapCreateExternal`
  • Drawn via `LCD::drawPartialBitmap`, or `LCD::blitCopy` with a full-bitmap blitRect
  • Source rect extends past the dirty rect on Y: `screenY + height > dirtyRect.y + dirtyRect.height`, or `screenY < dirtyRect.y`
  • `screenY != dirtyRect.y` (typically `screenY != 0`)

Non-trigger: a non-zero `screenY` on its own is fine when the whole bitmap fits inside the dirty rect (verified at `screenY = 1`).

 

Minimal reproduction

A 50x50 ABGR2222 external bitmap drawn at (60, 220) on a 240x240 screen, so it overhangs the bottom edge by 30 px.

// ABGR2222: 1 byte/pixel, bits MSB->LSB are AABBGGRR.
// Opaque red = alpha 3, blue 0, green 0, red 3 = 0b11000011 = 0xC3.
static uint8_t  pattern[50 * 50];
static uint16_t pool[2048];

for (size_t i = 0; i < sizeof(pattern); ++i)
    pattern[i] = 0xC3;

Bitmap::setCache(pool, sizeof(pool), 1);
BitmapId id = Bitmap::dynamicBitmapCreateExternal(
    50, 50, pattern, Bitmap::ABGR2222);

// Inside a Widget::draw():
Bitmap bmp(id);
Rect src(0, 0, bmp.getWidth(), bmp.getHeight());
HAL::lcd().drawPartialBitmap(bmp, 60, 220, src, 255, true);

 

Expected: 

a single red strip about 20 rows tall near the bottom of the screen.

Observed

that strip, plus a spurious second red strip near the top. The bottom-overhang rows are written at `y = 0` (`dirtyRect.y`) instead of `y = 220` (`screenY`).

Impact

Any widget that scrolls a bitmap into or out of view, anchors one near a screen edge, or otherwise clips it vertically will show this artifact.

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