There is currently an ongoing discussion with the Khronos Web3D group to
develop a proposal that solves these problems in canvas, over the past few
weeks we have converged on a solution that I think is pretty solid. I am in
the process of writing-up the HTML (non-WebGL) part of the proposal and I
intend to post it to the WICG shortly so that we can incubate it further,
with a broader audience. When that happens, I will update this thread.
On Sat, Apr 30, 2016 at 2:07 PM, Rik Cabanier <caban...@gmail.com> wrote:
> [Sorry to revive this old thread]
> with the advent of DCI-P3 compliant monitors and Apple's Safari doing
> color managing to the device, we're seeing some issues in this area.
> - Currently, WebKit sets the profile of the canvas backing store to sRGB
> regardless of the output device. Because of this, high gamut images are
> always clipped to sRGB. 
> It would be ideal if we can specify that the canvas backing store is in
> the device profile.
> Alternatively, we could add an API to attach a color profile to the canvas.
> - The spec currently states that toDataURL should not include a profile.
> However, if the backing store is in device pixels, the generated image
> should include the correct profile. Otherwise if you draw the bitmap in a
> compliant browser (ie Safari), the colors will look too saturated.
> If we agree that canvas is in the device space, I'd like to see the spec
>  clarified to state that compositing on the canvas should match
> compositing on the HTML surface.
> The canvas
> must perform colour correction at only two points: when rendering images
> with their own gamma correction and colour space information onto a bitmap,
> to convert the image to the colour space used by the bitmaps (e.g. using
> the 2D Context's drawImage()
> with an HTMLOrSVGImageElement
> and when rendering the actual canvas bitmap to the output device.
> The canvas
> must perform colour correction at only one point: when rendering content
> with its own gamma correction and colour space information onto a bitmap to
> the colour space used by the bitmaps (e.g. using the 2D Context's
> with an HTMLOrSVGImageElement
> ToDataURL and ToBlob  should also be enhanced so they include the
> device profile if it is different from sRGB.
> It would also be great if the browser could let us know what profile (if
> any) it was using.
> On Thu, May 22, 2014 at 12:21 PM, Justin Novosad <ju...@google.com> wrote:
>> tl;dr: The color space of canvas backing stores is undefined, which
>> causes problems for many web devs, but also has non-negligible advantages.
>> So be careful what you wish for.
>> I saw some confusion and questions needing answers in the "WebGL and
>> ImageBitmaps" thread regarding color management. I will attempt to clarify
>> to the best of my abilities. Though I am knowledgeable on the subject, I am
>> not an absolute authority, so others are welcome to correct me if I am
>> wrong about anything.
>> Color management... To make a long story short, there are two types of
>> color profiles : input profiles and output profiles for characterizing
>> input devices (cameras, scanners) and output devices (displays, printers)
>> Image files will usually encode their color information in a standard
>> color space or in a an input device dependent space. If colors are encoded
>> in a color space that is different from the format's default, then a color
>> profile or a color space identifier must be encoded into the image
>> resource's metadata.
>> To present color-managed image content on screen, the image needs to be
>> converted from whatever color space the image was encoded into into a
>> standard "connection space" using the color profile or color space metadata
>> from the image resource. Then the colors need to be converted from the
>> profile connection space to the output space, which is provided by the
>> OS/display driver. Depending on the OS and hardware configuration, the
>> output space may be a standard color space (like sRGB), or a
>> device-specific color profile.
>> Currently, many color-managed software applications rely on the codec to
>> take care of the entire color-management process for image and video
>> content, meaning that the decoded image data is in output-referred color
>> space (i.e. the display's profile was applied). There are practical
>> reasons for this, the most important ones being color fidelity and memory
>> consumption. Let me explain. The profile connection space is typically CIE
>> XYZ or CIE L*a*b. I wont get into the technical details of how these work
>> except to say that they are device independent and allow for an accurate
>> representation of the whole spectrum of human-visible colors. This makes it
>> possible to map colors from a wide gamut camera to a wide gamut display
>> with high color fidelity for all the colors that are located in the
>> intersection of the color gamuts of both the input and output devices. If
>> we were forced to convert the image to an intermediate sRGB representation,
>> the colors in the image would be clamped to the sRGB gamut (which is
>> narrower than the gamuts of many devices). Currently, most browsers avoid
>> doing that for <img>, and therefore provide (more or less) optimal image
>> and video color fidelity for users of wide gamut devices. Also, an
>> intermediate representation in 8-bit sRGB means loss of precision due to
>> rounding errors, as opposed to the profile connection space which uses
>> higher precision registers for intermediate color values to avoid precision
>> issues caused by rounding. To avoid perceptible precision issues in an
>> intermediate sRGB representation, we'd have to increase the bit depth and
>> therefore use more RAM for storing decoded image data.
>> All of this is to say that there are good reasons for the current
>> situation where we deal with decoded images that have the output device's
>> color profile pre-applied: color fidelity and memory consumption.
>> In the case of 2D canvas, the color space for the backing store is
>> unspecified, and many implementations have chosen to use the output
>> device's color space, which has many advantages:
>> * images and videos are already decoded directly into that space
>> * no color conversion is necessary when presenting the canvas on screen
>> (good for performance)
>> * there is no loss of precision due the use of a limited-precision
>> intermediate color space.
>> * the color gamut is not constrained by an intermediate color space (like
>> And disadvantages:
>> * Compositing operations produce incorrect results because most of them
>> (including source-over) are affected by the color space.
>> * direct pixel manipulation using put/getImageData exposes data in a
>> color space that is undefined, making it extremely challenging to perform
>> many types of image processing and image generation tasks in a
>> device-independent way.
>> * The device-dependent behavior of a drawImage/getImageData round trip is
>> a known fingerprinting vector.
>> Right now, I am hearing a lot of complaints regarding the lack of a
>> standardized color space for canvases, and in particular the impact this
>> has on applications that try to do cool things with put/getImageData, or
>> generate images procedurally. I want to make sure everyone understands
>> there is a trade-off to fixing this, so be careful what you wish for.
>> I am especially concerned about the issue of color gamut clamping, which
>> will increase in relevance as wide gamut devices become more widespread.
>> If we were to decide that canvas backing stores had to be in sRGB, that
>> would mean that a wide gamut image viewed on a wide gamut display would
>> look best when displayed in an <img> and would be duller when drawn through
>> a <canvas>, whether it be 2D or WebGL. Is that something we are willing to
>> live with in the name of standardizing the color space of ImageData? I
>> must admit I was in favor of moving canvases to sRGB until I reviewed some
>> of Noel Gordon's recent work which brought the gamut and precision issues
>> to my attention.
>> Rik: to answer your question about your experiment: there is no issue
>> with a put/getImageData round trip. You will get back the same color you
>> put in (at least for opaque colors, but that is another story). The issue
>> is with a drawImage/getImageData round trip. For the same source image,
>> getImageData will return different values depending on the display profile
>> of the system you are running on.
>> Ken: You mentioned to me off thread that built-in support for sRGB render
>> buffers in OpenGL ES 3 may make it easier to move rendering to sRGB on next
>> gen devices. I tend to agree, and I think it will also mitigate the loss of
>> precision issue, but it still implies clamping wide gammut media to the
>> sRGB range.