ForeignPngFilter

[randyridge]

add ForeignPngFilter

[kleisauke]

Thanks! I've added a few inline suggestions, but otherwise this looks good to me! The reason that this "enum" was not semi-generated is because VipsForeignPngFilter is not an enum, but a bit field.

@jcupitt Do you think the other bindings can also benefit from this? I had a similar issue with wasm-vips, see for example:
https://github.com/kleisauke/pyvips/tree/flags-helper
https://github.com/kleisauke/wasm-vips/blob/master/build/gen_type_declarations.py#L228-L230
https://github.com/kleisauke/wasm-vips/blob/master/build/gen_type_declarations.py#L251-L253
https://github.com/kleisauke/wasm-vips/blob/master/build/gen_type_declarations.py#L283

[jcupitt]

Yes, the flags/enum thing is used in some places, but not worked through all the bindings. It might be useful to fix this.

[kleisauke]

Thank you Randy, this will be in v1.2.5.

@jcupitt As far as I know VipsForeignPngFilter is the only flag/bitfield used in libvips. I noticed that it is also missing in pyvips.

[jcupitt]

There are a few unimportant ones, eg.:

john@yingna ~/GIT/libvips/libvips/include/vips (master) $ grep "/*< flag" *.h
foreign.h:typedef enum /*< flags >*/ {
foreign.h:typedef enum /*< flags >*/ {
object.h:typedef enum /*< flags >*/ {
operation.h:typedef enum /*< flags >*/ {

gtk-doc uses the magic /*< .. >*/ comment to spot them, and you can find them by introspection.

I wonder if VipsForeignPngFilter is a true bitfield. Does libpng really support avg | paeth? I suspect it behaves more like a simple enum, though I'm too lazy to check.

[kleisauke]

I'll also add the corresponding Enums.ForeignPngFilter? filter = null overload for Pngsave{,Buffer,Target,Stream} allowing you to do this (for example):

var im = Image.NewFromFile("lichtenstein.jpg", access: Enums.Access.Sequential);
im.Pngsave("lichtenstein.png", filter: /*(int)*/Enums.ForeignPngFilter.All);

(it currently requires casting an enum to int, which is a bit clumsy)

[kleisauke]

@jcupitt I'm not not entirely sure if you could do avg | paeth, but none | sub | up | avg | paeth is possible (which is just a synonym for all). Anyways, it's defined here:
https://github.com/glennrp/libpng/blob/dbe3e0c43e549a1602286144d94b0666549b18e6/png.h#L1459-L1471

[kleisauke]

I can confirm that avg | paeth also works. It generates a PNG file with exclusively avg or paeth as row filter, see for example:

$ pngcheck -vv lichtenstein.png | grep filters -A 1 | tail -5
    row filters (0 none, 1 sub, 2 up, 3 avg, 4 paeth):
      3 4 4 4 4 4 4 (2415 out of 2421)
--
    row filters (0 none, 1 sub, 2 up, 3 avg, 4 paeth):
      4 3 4 4 4 4 (2421 out of 2421)

[jcupitt]

Huh how curious. Thanks for checking.

[kleisauke]

NetVips v2.0.0 is now available with this improvement included.