ImageBitmap Options

This page is a Work in Progress

Problem Statement Loading and preparing image resources for rendering to 2D canvas contexts and WebGL contexts is often inefficient using existing web APIs. Many resource preparation steps are implementable in JavaScript, but could be performed much faster and more conveniently with proper API support and native implementations provided by the browser. Examples of such tasks: color correction, image flipping, alpha premultiplication.

Use Case Description

• A web application needs to load an image and may or may not need to flip the image vertically, depending on whether client-side code has selected to draw using a 2D canvas context or a WebGL context.
• WebGL applications often use image media for transferring non-image data (illumination maps, vector maps, etc.) For such applications the image data must usually be decoded without colorspace conversions and without alpha premultiplications.

Current Usage and Workarounds

Currently, there are several ways to apply the transformations on the client side, all of which involve intermediate copies of the image data. Some solutions involve DataURLs, temporary canvas, or direct manipulation of image data arrays, all of which are inefficient. Preparing the resources asynchronously also presents an additional challenge to the web developer.

WebGL applications can request that colorspace conversions be skipped at texture upload time by specifying pixel storage parameters, but this currently causes a synchronous image decode, because it is not known in advance that the image needs to be decoded in a special way. This undue latency can cause animation to be janky.

Benefits

With the right API support, these image preparation steps could be performed by native code, and in many case they can be integrated directly into the browser's image decoding process without any additional memory consumption and little additional computational burden.

Requests for this Feature

• Source

  I would like this feature ...

Proposed Solutions

ImageBitmap Construction Options

Add an optional options argument to createImageBitmap to specify image treatments to be applied when creating the ImageBitmap object.

Strongly desired options

• boolean flipY, default = false

  If set to true, the image is flipped vertically

• boolean colorspaceConversion, default = true

  If set to false, the ImageBitmap must be created by decoding the source image media without performing any colorspace conversions. This means that the image decoding algorithm must ignore color profile meta-data embedded in the image source as well as the display device color profile. colorspaceConversion only applies when creating ImageBitmaps from bitmap media data (i.e. Blobs and Image elements)

• boolean premultiplyAlpha, default = true

  If set to false, the image is decoded without multiplying color components by the alpha component. Only applies when creating ImageBitmaps from bitmap media data. Alpha multiplication is never performed type of image sources

Other potentially useful options

• frameIndex: for creating an image bitmap from multi-frame image media
• flipX: like flipY but horizontal
• colorspace: to decode images to a colorspace other than the device colorspace (sRGB, Adobe RGB, YUV, YCbCr, etc.)
• custom color tranforms: color matrix, 3D Look up tables, per component look up tables, bias, gain, ...

Processing Model

TBD Explanation of the changes introduced by this solution. It explains how the document is processed, and how errors are handled. This should be very clear, including things such as event timing if the solution involves events, how to create graphs representing the data in the case of semantic proposals, etc.

Limitations

TBD Cases not covered by this solution in relation to the problem description; other problems with this solution, if any.

Implementation

TBD Description of how and why browser vendors would take advantage of this feature.

Adoption

• Easier to develop asychronous image resource loading and preparation in JavaScript
• Better performance that what can be implemented using current APIs