| name | iot-color-byte-order-diagnosis |
| description | Diagnose ARGB vs ABGR byte-order mismatches when IoT device colors render wrong after an RGB setColor call. |
| domain | iot-drivers |
| confidence | high |
| source | earned |
Context
This skill applies when an IoT device driver packs a color as a 32-bit integer and sends it to a cloud API, but the rendered color on the physical device differs from what was requested.
Diagnostic Pattern: Red→Blue, Green→Green, Blue→Red
When you observe:
- Red renders as blue (or a bluish tint)
- Green renders as green (correct)
- Blue renders as red
…this is the exact empirical signature of an ARGB vs ABGR byte-order mismatch.
Why green is invariant
In a 32-bit color word 0xAA_XX_GG_YY, green always occupies the middle byte (bits 15–8). Swapping the R and B byte positions does not move green — so green-channel colors appear correct while red and blue are swapped.
Root cause
- Driver packs:
0xAARRGGBB (ARGB)
- API wire format:
0xAABBGGRR (ABGR)
- Result: R and B channels are swapped on the device
Fix
Swap the R and B byte positions in every color-packing function:
// WRONG (ARGB): R at bits 23-16, B at bits 7-0
return ((0xFFL << 24) | ((r & 0xFFL) << 16) | ((g & 0xFFL) << 8) | (b & 0xFFL)) as Long
// CORRECT (ABGR): B at bits 23-16, R at bits 7-0
return ((0xFFL << 24) | ((b & 0xFFL) << 16) | ((g & 0xFFL) << 8) | (r & 0xFFL)) as Long
Also reverse the assignment in any color-unpacking function (reading ABGR back into r/g/b variables):
// ABGR unpack: high data byte is B, low data byte is R
float b = ((argbLong >> 16) & 0xFFL) / 255.0f
float g = ((argbLong >> 8) & 0xFFL) / 255.0f
float r = (argbLong & 0xFFL) / 255.0f
Confirmed by Real Device
Gemstone Lights cloud API (AWS API Gateway, mytpybpq12.execute-api.us-west-2.amazonaws.com) confirmed ABGR wire format in v0.4.5 empirical testing (2026-05-16). Pure red 0xFFFF0000 packed as ARGB → device displayed blue. Fix: swap to ABGR encoding.
Quick Reference
| Sent (ARGB packed) | Device interprets as (ABGR) | Visible color |
|---|
0xFFFF0000 (red) | R=0, G=0, B=255 | Blue |
0xFF00FF00 (green) | R=0, G=255, B=0 | Green ✓ |
0xFF0000FF (blue) | R=255, G=0, B=0 | Red |
