Initial feedback for std.experimental.image

[via Digitalmars-d]

On Thursday, 9 July 2015 at 15:05:12 UTC, Rikki Cattermole wrote:
> On 10/07/2015 2:07 a.m., "Gregor =?UTF-8?B?TcO8Y2tsIg==?= 
> <gregormueckl at gmx.de>" wrote:
>> On Thursday, 9 July 2015 at 04:09:11 UTC, Rikki Cattermole 
>> wrote:
>>> On 9/07/2015 6:07 a.m., "Gregor =?UTF-8?B?TcO8Y2tsIg==?=
>>> <gregormueckl at gmx.de>" wrote:
>>>> On Monday, 6 July 2015 at 13:48:53 UTC, Rikki Cattermole 
>>>> wrote:
>>>>>
>>>>> Please destroy!
>>>>>
>>>>
>>>> You asked for it! :)
>>>>
>>>> As a reference to a library that is used to handle images on 
>>>> a
>>>> professional level (VFX industry), I'd encourage you to look 
>>>> at the
>>>> feature set and interfaces of OpenImageIO. Sure, it's a big 
>>>> library and
>>>> some of it is definitely out of scope for what you try to 
>>>> accomplish
>>>> (image tile caching and texture sampling, obviously).
>>>>
>>>> Yet, there are some features I specifically want to mention 
>>>> here to
>>>> challenge the scope of your design:
>>>>
>>>> - arbitrary channel layouts in images: this is a big one. 
>>>> You mention 3D
>>>> engines as a targeted use case in the specification. 3D 
>>>> rendering is one
>>>> of the worst offenders when it comes to crazy channel 
>>>> layouts in
>>>> textures (which are obviously stored as image files). If you 
>>>> have a data
>>>> texture that requires 2 channels (e.g. uv offsets for 
>>>> texture lookups in
>>>> shaders or some crazy data tables), its memory layout should 
>>>> also only
>>>> ever have two channels. Don't expand it to RGB transparently 
>>>> or anything
>>>> else braindead. Don't change the data type of the pixel 
>>>> values wildly
>>>> without being asked to do so. The developer most likely has 
>>>> chosen a 16
>>>> bit signed integer per channel (or whatever else) for a good 
>>>> reason.
>>>> Some high end file formats like OpenEXR even allow users to 
>>>> store
>>>> completely arbitrary channels as well, often with a different
>>>> per-channel data format (leading to layouts like RGBAZ with 
>>>> an
>>>> additional mask channel on top). But support for that really 
>>>> bloats
>>>> image library interfaces. I'd stick with a sane variant of 
>>>> the
>>>> uncompressed texture formats that the OpenGL specification 
>>>> lists as the
>>>> target set of supported in-memory image formats. That mostly 
>>>> matches
>>>> current GPU hardware support and probably will for some time 
>>>> to come.
>>>
>>> As long as the color implementation matches isColor from
>>> std.experimental.color. Then it's a color. I do not handle 
>>> that :)
>>> The rest of how it maps in memory is defined by the image 
>>> storage
>>> types. Any image loader/exporter can use any as long as you 
>>> specify it
>>> via a template argument *currently*.
>>>
>>
>> Hm... in that case you introduce transparent mappings between
>> user-facing types and the internal mapping which may be lossy 
>> in various
>> ways. This works, but the internal type should be discoverable 
>> somehow.
>> This leads down a similar road to OpenGL texture formats: they 
>> have
>> internal storage formats and there's the host formats to/from 
>> which the
>> data is converted when passing back and forth. This adds a lot 
>> of
>> complexity and potential for surprises, unfortunately. I'm not 
>> entirely
>> sure what to think here.
>
> Internal color to an image storage type is well known at 
> compile time.
> Now SwappableImage that wraps another image type. That 
> definitely muddies the water a lot. Since it auto converts from 
> the original format. Which could be, you know messy.
> It's actually the main reason I asked Manu for a gain/loss 
> precision functions. For detecting if precision is being 
> changed. Mostly for logging purposes.
>

I haven't encountered SwappableImage anywhere. Did I miss 
anything?

>>>> - window regions: now this not quite your average image 
>>>> format feature,
>>>> but relevant for some use cases. The gist of it is that the 
>>>> image file
>>>> may define a coordinate system for a whole image frame but 
>>>> only contain
>>>> actual data within certain regions that do not cover the 
>>>> whole frame.
>>>> These regions may even extend beyond the defined image frame 
>>>> (used e.g.
>>>> for VFX image postprocessing to have properly defined pixel 
>>>> values to
>>>> filter into the visible part of the final frame). Again, the 
>>>> OpenEXR
>>>> documentation explains this feature nicely. Again, I think 
>>>> this likely
>>>> is out of scope for this library.
>>>
>>> Ugh based upon what you said, that is out of scope of the 
>>> image
>>> loader/exporters that I'm writing. Also right now it is only 
>>> using
>>> unsigned integers for coordinates. I'm guessing if it is 
>>> outside the
>>> bounds it can go negative then.
>>> Slightly too specialized for what we need in the general case.
>>>
>>
>> Yes, this is a slightly special use case. I can think of quite 
>> a lot of
>> cases where you would want border regions of some kind for 
>> what you are
>> doing, but they are all related to rendering and image 
>> processing.
>
> You have convinced me that I need to add a subimage struct 
> which is basically SwappableImage. Just with offset/size 
> different to original.
>

Again, I unfortunately fail to follow. Sorry.

I wanted to argue that window regions are out of scope, unlike 
subimages (which actually are completely separate images within 
the same file - they do not share a single coordinate system).

>>>> My first point also leads me to this criticism:
>>>>
>>>> - I do not see a way to discover the actual data format of a 
>>>> PNG file
>>>> through your loader. Is it 8 bit palette-based, 8 bit per 
>>>> pixel or 16
>>>> bits per pixel? Especially the latter should not be 
>>>> transparently
>>>> converted to 8 bits per pixel if encountered because it is a 
>>>> lossy
>>>> transformation. As I see it right now you have to know the 
>>>> pixel format
>>>> up front to instantiate the loader. I consider that bad 
>>>> design. You can
>>>> only have true knowledge of the file contents after the 
>>>> image header
>>>> were parsed. The same is generally true of most actually 
>>>> useful image
>>>> formats out there.
>>>
>>> The reasoning is because this is what I know I can work with. 
>>> You
>>> specify what you want to use, it'll auto convert after that. 
>>> It makes
>>> user code a bit simpler.
>>>
>>
>> I can understand your reasoning and this is why libraries like 
>> FreeImage
>> make it very simple to get the image data converted to the 
>> format you
>> want from an arbitrary input. What I'd like to see is more of 
>> an
>> extension of the current mechanism: make it possible to query 
>> the data
>> format of the image file. That way, the application can make a 
>> wiser
>> decision on the format in which it wants to receive the data, 
>> but it
>> always is able to get the data in a format it understands. The 
>> format
>> description for the file format would have to be quite complex 
>> to cover
>> all possibilities, though. The best that I can come up with is 
>> a list of
>> tuples of channel names (as strings) and data type (as enums).
>> Processing those isn't fun, though.
>
> The problem here is simple. You must know what color type you 
> are going to be working with. There is no guessing. If you want 
> to change to match the file loader better, you'll have to load 
> it twice and then you have to understand the file format 
> internals a bit more.
> This is kinda where it gets messy.
>

Messy in what way? My conceptual view of the user code is that it 
could do either of two things:

1. ask for a image in a format the user specifies - the loader 
must meet that request exactly and do all required conversions
2. ask the loader for the image data format contained in the 
file, choose a representation that best matches it in the user's 
view and only then ask the loader as in 1.

Note that the user gets exactly what he asked for in each case, 
but makes an informed decision in 2. only because he chose to.

> But, would it be better if you could just parse the headers? So 
> it doesn't initialize the image data. I doubt it would be all 
> that hard. It's just disabling a series of features.
>

That's what this would boil down to, I believe: a separate entry 
point for the loader that just parses the headers and returns 
file format information to the user, but not the image data 
itself. The user-facing interface would just be an extra 
interface method. Internally, it would share most of the code 
with the actual image loading code path.

>>>> - Could support for image data alignment be added by 
>>>> defining a new
>>>> ImageStorage subclass? The actual in-memory data is not 
>>>> exposed to
>>>> direct access, is it? Access to the raw image data would be 
>>>> preferable
>>>> for those cases where you know exactly what you are doing. 
>>>> Going through
>>>> per-pixel access functions for large image regions is going 
>>>> to be
>>>> dreadfully slow in comparison to what can be achieved with 
>>>> proper
>>>> processing/filtering code.
>>>
>>> I ugh... had this feature once. I removed it as if you 
>>> already know
>>> the implementation why not just directly access it?
>>> But, if there is genuine need to get access to it as e.g. 
>>> void* then I
>>> can do it again.
>>>
>>>> - Also, uploading textures to the GPU requires passing raw 
>>>> memory blocks
>>>> and a format description of sorts to the 3D API. Being 
>>>> required to
>>>> slowly copy the image data in question into a temporary 
>>>> buffer for this
>>>> process is not an adequate solution.
>>>
>>> Again for previous answer, was possible. No matter what the 
>>> image
>>> storage type was. But it was hairy and could and would cause 
>>> bugs in
>>> the future. Your probably better off knowing the type and 
>>> getting
>>> access directly to it that way.
>>>
>>
>> This is where the abstraction of ImageStorage with several 
>> possible
>> implementations becomes iffy. The user is at the loader's 
>> mercy to
>> hopefully hand over the right implementation type. I'm not 
>> sure I like
>> that idea. This seems inconsistent with making the pixel 
>> format the
>> user's choice.  Why should the user have choice over one thing 
>> and not
>> the other?
>
> If the image loader uses another image storage type then it is 
> miss behaving. There is no excuse for it.
>

Am I looking at an old version? The PNG loader I see documented 
returns a specific ImageStorage implementation.

> Anyway the main thing about this to understand is that if the 
> image loader does not initialize, then it would have to resize 
> and since not all image storage types have to support 
> resizing...
>

OK, why do you abstract image storage at all? What's the 
rationale there? Storage formats other than the trivial 
row-by-row storage are only used in very few cases in my 
experience, mostly when it involves compression (compressed 
textures for games - effectively read only after compression) or 
certain kinds of out-of-core image processing. This seems to be 
mostly out of scope this library IMO.

>>>
>>> Some very good points that I believe definitely needs to be 
>>> touched
>>> upon where had.
>>>
>>> I've had a read of OpenImageIO documentation and all I can 
>>> say is irkkk.
>>> Most of what is in there with e.g. tiles and reflection 
>>> styles methods
>>> are out of scope out right as they are a bit specialized for 
>>> my
>>> liking. If somebody wants to add it, take a look at the offset
>>> support. It was written as an 'extension' like ForwardRange 
>>> is for
>>> ranges.
>>>
>>
>> I mentioned OpenImageIO as this library is full-featured and 
>> very
>> complete in a lot of areas. It shows what it takes to be as 
>> flexible as
>> possible regarding the image data that is processed. Take it 
>> as a
>> catalog of things to consider, but not as template.
>>
>>> The purpose of this library is to work more for GUI's and 
>>> games then
>>> anything else. It is meant more for the average programmer 
>>> then
>>> specialized in imagery ones. It's kinda why I wrote the 
>>> specification
>>> document. Just so in the future if somebody comes in saying 
>>> its awful
>>> who does know and use these kinds of libraries. Will have to
>>> understand that it was out of scope and was done on purpose.
>>
>> Having a specification is a good thing and this is why I 
>> entered the
>> discussion. Although your specification is still a bit vague 
>> in my
>> opinion, the general direction is good. The limitation of the 
>> scope
>> looks fine to me and I'm not arguing against that. My point is 
>> rather
>> that your design can still be improved to match that scope 
>> better.
>
> Yeah indeed. Any tips for specification document improvement?
> I would love to make it standard for Phobos additions like this.

This is a really big can of worms, especially if you want to set 
standards for the future. You can do anything from informal and 
lax to extreme level of detail.

For example, we require students to write specifications that 
include numbered lists of *all* functional and non-functional 
requirements for their (rather small) projects, which fill 
endless pages in the end. Additionally they have to describe the 
development tools and environment, the target audience, manual 
and automatic tests, and so on. It's the classic waterfall model 
with all the problems it has (don't ask - decision from the 
top...).

In your case I'd start by defining a target audience and 
describing target use cases in a high-level manner (e.g. 
"identify the format and required loader for an image file", 
"load the image so that the user sees a representation which he 
understands", ...). Stating non-goals or non-features is a good 
thing to limit scope (your spec already mentions some).

The list of use cases can be checked against relevance for the 
target audience. Also, the use cases can be translated into test 
cases of various kinds:

- example snippets demonstrating ease of use of the design (or 
lack thereof)
- automatic tests (e.g. unit tests)
- manual tests of complex scenarios requiring user interaction 
(not relevant in this case, but e.g. for UI libraries)

A very formal approach could number the use cases and tests and 
keep cross-references between them to make it easier to check for 
completeness. But for the most part, this would be tedious and 
boring for little gain.