Errors and Limits

The condition

(define-condition slug-error (error) ...)
(slug-error-message condition) => string

Every error cl-slug signals for bad data or a full store is a slug-error. Its message says what went wrong and what to do. An argument outside the type its function declares, such as a store that is not a store, signals a type-error instead, which SBCL signals on entry and words itself; where the argument is a constant, SBCL warns when the call is compiled. That includes a fill rule given to store-add that is not :nonzero or :even-odd, a real given to half-float-bits that rounds to no finite half, the bits of an infinity or a NaN given to half-float-value, and a row past 65535 or a count past 65536 given to copy-curve-rows or copy-band-rows.

Errors adding a shape

Message	Cause	What to do
A shape's curves are six numbers each, X0 Y0 CX CY X1 Y1, but n numbers were given.	The vector's length is not a multiple of 6.	Check how the curves were packed.
A shape's curves are numbers, not x.	An element is not a real.
x is beyond the largest half-precision float, 65504, so it cannot be stored in the curve texture. Give curves in ems, as for a font of size 1.	A coordinate is too large for the curve texture.	Scale the shape to ems.
x is not a finite number, which a curve's coordinates must be.	An infinity.
A fill rule is :NONZERO or :EVEN-ODD, not x.	The fill rule store-find-or-add's function returned.
A band of n curves does not fit in a row of the band texture, of 4096 texels. Split the shape into smaller shapes.	More than 4096 curves cross one band.	Split the shape; draw the parts together.
A shape's bands need more than 65535 texels of the band texture. Split it into smaller shapes.	The shape's band lists reach too far from its headers.	Split the shape.
store is full: its textures may have n rows. Make the store with a larger :MAX-ROWS, up to what the device's textures take, or put fewer shapes in it.	Either texture would need more than max-rows rows.	A larger :max-rows, if the GPU takes taller textures; or a second store.

The first five are found before the store's lock is taken, and leave the store as it was. The last three are found while placing the shape: what was placed of it stays in the store, unused, and the store still works for shapes that fit.

Other errors

Function	Message	Cause
write-shape-data	Writing a shape's 8 floats from start overruns a vector of n.	Fewer than eight floats left in the vector after start.

Limits

Limit	Value	Why
Texture width	4096 texels	The reference shaders' layout.
Rows per texture	max-rows, 4096 by default, at most 65536	16-bit places in the band texture.
Bands per shape, each way	32	Enough for the largest glyphs; the band count is 8 bits in the reference format.
Curves in one band	4096	A list is read along one row.
A shape's band data	65536 texels from its place	16-bit offsets.
Curve coordinates	±65504	Half floats.
Precision	about 1/2048 em near 1 em	Half floats' 11 bits.

Real fonts come nowhere near the band limits: the largest CJK glyphs have a few hundred curves, spread over many bands.

What is not an error

• An empty shape, such as a space: shape-empty-p is true, and it takes no texels.
• Curves of no length: left out.
• Open contours: not checked. The fill is wrong where a contour is open, as in any renderer.
• Self-intersecting or overlapping contours: filled by the fill rule, as fonts expect.
• The same curves added twice with store-add: added twice. Use store-find-or-add to share.