CTB file format: header and tables

CBD-Tech does not publish the format; its files carry a notice forbidding reverse engineering. What is known comes from independent study of the files by the open source community, now gathered in UVtools, whose reader and writer (ChituboxFile.cs) this page follows. Field names are UVtools’; fields whose purpose nobody has worked out are marked unknown. All numbers are little-endian; floats are IEEE 754 single precision.

Magic numbers and versions

The same layout serves the older .cbddlp (and .photon) files and .ctb. The first four bytes tell them apart, and the version at offset 4 says which optional blocks are present.

Magic	Bytes	Meaning
0x12FD0019	`19 00 FD 12`	CBDDLP (and Photon): antialiasing by repeated 1-bit layers, no encryption
0x12FD0086	`86 00 FD 12`	CTB versions 2 and 3
0x12FD0106	`06 01 FD 12`	CTB versions 4 and 5
0xFF220810	`10 08 22 FF`	CTB for UniFormation GKtwo printers (version 4 layout, longer notice)
0x12FD0107	`07 01 FD 12`	Encrypted CTB, a different layout; see Encrypted CTB

Version	Adds
1	Header, previews, layer table, images
2	Print parameters block; in CTB, slicer info and machine name
3	An 84-byte extended layer record before each layer image (per-layer lift and wait settings)
4	Copyright notice and version 4 parameters
5	Resin parameters

Order of blocks

Header (112 bytes)
Large preview: record and image data
Small preview: record and image data
Print parameters (version 2 and later)
Slicer info, then the machine name (CTB version 2 and later)
Copyright notice, then version 4 parameters (version 4 and later)
Resin parameters (version 5)
Layer table
Layer images, each preceded in version 3 and later by its extended layer record

Every block is found through offsets in the header or the slicer info, so readers should follow the offsets rather than assume this order.

Header

Offset	Size	Type	Name	Meaning
0	4	uint32	Magic	See above
4	4	uint32	Version	1 to 5
8	4	float	BedSizeX	Build area width, mm
12	4	float	BedSizeY	Build area depth, mm
16	4	float	BedSizeZ	Build height, mm
20	8	uint32 × 2	Unknown	Not understood
28	4	float	TotalHeight	Height of the model, mm
32	4	float	LayerHeight	Layer height used when slicing, mm (the layer table has the real positions)
36	4	float	ExposureTime	Normal layer exposure, s
40	4	float	BottomExposureTime	Bottom layer exposure, s
44	4	float	LightOffDelay	Light-off time between normal layers, s
48	4	uint32	BottomLayerCount	Number of bottom layers
52	4	uint32	ResolutionX	Screen width, pixels
56	4	uint32	ResolutionY	Screen height, pixels
60	4	uint32	LargePreviewOffset	Offset of the large preview record
64	4	uint32	LayerTableOffset	Offset of the layer table
68	4	uint32	LayerCount	Number of layers
72	4	uint32	SmallPreviewOffset	Offset of the small preview record
76	4	uint32	PrintTime	Estimated print time, s
80	4	uint32	ProjectorType	0 normal, 1 mirrored projection (LCD printers count as mirrored)
84	4	uint32	PrintParametersOffset	Offset of the print parameters (version 2 and later)
88	4	uint32	PrintParametersSize	Their size, 60
92	4	uint32	AntiAliasLevel	Number of image planes per layer: 1 for CTB; 1, 2, 4 or 8 for CBDDLP
96	2	uint16	LightPWM	UV light power on normal layers, 0 to 255
98	2	uint16	BottomLightPWM	UV light power on bottom layers, 0 to 255
100	4	uint32	EncryptionKey	Seed for the layer image XOR; 0 means not encrypted (always 0 in CBDDLP)
104	4	uint32	SlicerInfoOffset	Offset of the slicer info (CTB), 0 if none
108	4	uint32	SlicerInfoSize	Size of the slicer info without the machine name, 76

Preview records

Offset	Size	Type	Name	Meaning
0	4	uint32	ResolutionX	Width, pixels
4	4	uint32	ResolutionY	Height, pixels
8	4	uint32	ImageOffset	Offset of the image data, normally right after this record
12	4	uint32	ImageLength	Its length in bytes
16	16	uint32 × 4	Unknown	Not understood; zero

The image data is 16-bit run-length colour; see Image encoding. The printer shows the previews when the user chooses a file, so it need not render the model itself.

Print parameters (60 bytes)

Offset	Size	Type	Name	Meaning
0	4	float	BottomLiftHeight	Platform lift after bottom layers, mm
4	4	float	BottomLiftSpeed	Its speed, mm/min
8	4	float	LiftHeight	Platform lift after normal layers, mm
12	4	float	LiftSpeed	Its speed, mm/min
16	4	float	RetractSpeed	Speed back down to the vat, mm/min
20	4	float	VolumeMl	Resin needed, ml
24	4	float	WeightG	Its weight, g
28	4	float	Cost	Its cost, in the user’s currency
32	4	float	BottomLightOffDelay	Light-off time after bottom layers, s
36	4	float	LightOffDelay	Light-off time after normal layers, s
40	4	uint32	BottomLayerCount	Number of bottom layers (again)
44	16	uint32 × 4	Padding	Zero

Slicer info (76 bytes plus the machine name)

Offset	Size	Type	Name	Meaning
0	4	float	BottomLiftHeight2	Second, slower stage of the bottom lift, mm (two-stage motion)
4	4	float	BottomLiftSpeed2	Its speed, mm/min
8	4	float	LiftHeight2	Second stage of the normal lift, mm
12	4	float	LiftSpeed2	Its speed, mm/min
16	4	float	RetractHeight2	Second stage of the retract, mm
20	4	float	RetractSpeed2	Its speed, mm/min
24	4	float	RestTimeAfterLift	Wait after lifting, s
28	4	uint32	MachineNameOffset	Offset of the machine name
32	4	uint32	MachineNameSize	Its length in bytes
36	1	uint8	AntiAliasFlag	CTB: 7 without, 15 (0x0F) with antialiasing; CBDDLP: 0 or 8
37	2	uint16	Padding	Zero
39	1	uint8	PerLayerSettings	0 per-layer settings not allowed; 0x10 CBDDLP; 0x20 CTB v2; 0x30 v3; 0x40 v4; 0x50 v5 (only partly understood)
40	4	uint32	ModifiedMinutes	Time of slicing, in minutes since 1 January 1970 UTC
44	4	uint32	AntiAliasLevel	Antialiasing level chosen by the user (for CTB, informational)
48	4	uint32	SoftwareVersion	Chitubox version, one byte each for major, minor and patch from the top: 0x01060300 is 1.6.3, 0x01090000 1.9.0, 0x02000000 2.0.0
52	4	float	RestTimeAfterRetract	Wait after retracting, s
56	4	float	RestTimeAfterLift2	Wait after the second lift stage, s
60	4	uint32	TransitionLayerCount	Layers over which exposure steps from bottom to normal (not all printers)
64	4	uint32	PrintParametersV4Offset	Offset of the version 4 parameters (version 4 and later)
68	8	uint32 × 2	Padding	Zero
76	varies	chars	MachineName	Printer model from the slicer profile, ASCII in all known files; not null-terminated, except in version 5

Version 4 parameters

After the machine name comes the copyright notice, 320 bytes of text (323 bytes for GKtwo files), then this block at PrintParametersV4Offset:

Offset	Size	Type	Name	Meaning
0	4	float	BottomRetractSpeed	Retract speed after bottom layers, mm/min
4	4	float	BottomRetractSpeed2	Second stage, mm/min
8	4	uint32	Padding	Zero
12	4	float	Unknown	Always 4.0 in known files
16	4	uint32	Padding	Zero
20	4	float	Unknown	Always 4.0 in known files
24	4	float	RestTimeAfterRetract	Wait after retracting, s
28	4	float	RestTimeAfterLift	Wait after lifting, s
32	4	float	RestTimeBeforeLift	Wait before lifting, s
36	4	float	BottomRetractHeight2	Second stage of the bottom retract, mm
40	8	uint32 × 2	Unknown	Vary between files; not understood
48	4	uint32	Unknown	Usually 5
52	4	uint32	LastLayerIndex	LayerCount − 1
56	16	uint32 × 4	Padding	Zero
72	4	uint32	DisclaimerOffset	Offset of the notice
76	4	uint32	DisclaimerLength	Its length, 320 or 323
80	4	uint32	ResinParametersOffset	Offset of the resin parameters (version 5), else 0
84	380	bytes	Reserved	Zero

Resin parameters (version 5)

Offset	Size	Type	Name	Meaning
0	4	uint32	Padding	Zero
4	4	uint8 × 4	ResinColour	Blue, green, red, alpha
8	4	uint32	MachineNameOffset	Offset of the machine name below
12	4	uint32	ResinTypeLength	Length of the resin type
16	4	uint32	ResinTypeOffset	Its offset
20	4	uint32	ResinNameLength	Length of the resin name
24	4	uint32	ResinNameOffset	Its offset
28	4	uint32	MachineNameLength	Length of the machine name
32	4	float	ResinDensity	g/ml
36	4	uint32	Padding	Zero
40	varies	chars	ResinType, ResinName, MachineName	The three strings, in that order

Layer table

LayerCount × AntiAliasLevel records of 36 bytes. In CBDDLP files with antialiasing the table holds one full set of layers for each image plane, plane 0 first; CTB always has one set.

Offset	Size	Type	Name	Meaning
0	4	float	PositionZ	Platform height for this layer, mm
4	4	float	ExposureTime	Exposure of this layer, s
8	4	float	LightOffDelay	Light off after this layer, s
12	4	uint32	DataOffset	Offset of the image data within its 4 GiB page
16	4	uint32	DataSize	Length of the image data
20	4	uint32	PageNumber	Which 4 GiB page: the real offset is PageNumber × 2³² + DataOffset
24	4	uint32	TableSize	36, or 84 in version 3 and later (this record plus the extension)
28	8	uint32 × 2	Unknown	Zero

Extended layer record (version 3 and later)

Directly before each layer’s image data: a copy of its 36-byte table record, then 48 more bytes.

Offset	Size	Type	Name	Meaning
0	36	record	LayerDef	Copy of the layer table record
36	4	uint32	TotalSize	Size of this record plus the image data
40	4	float	LiftHeight	Lift after this layer, mm
44	4	float	LiftSpeed	mm/min
48	4	float	LiftHeight2	Second lift stage, mm
52	4	float	LiftSpeed2	mm/min
56	4	float	RetractSpeed	mm/min
60	4	float	RetractHeight2	Second retract stage, mm
64	4	float	RetractSpeed2	mm/min
68	4	float	RestTimeBeforeLift	s
72	4	float	RestTimeAfterLift	s
76	4	float	RestTimeAfterRetract	s
80	4	float	LightPWM	UV power for this layer, 0 to 255, stored as a float

Identical CBDDLP layer images (version 2 and earlier) may share one copy of their data: several table records point at the same offset.

.CTB File Format – Header and tables