Loading images as contiguous memory

[ThomasMiz]

I've been using ImageSharp for quite some time to load images in a graphics library I've been developing.

ImageSharp has been a great cross-platform solution for decoding images in different formats and getting a contiguous span of data I can pass to OpenGL and load into a texture.

However, in a recent update, the image.GetPixelSpan() function was removed.

First, I tried using image.GetPixelRowSpan(0) and that seemed to work, and looking at this commit from ImageSharp.Textures this seemed to be the way to go.

But then I saw the image.TryGetSinglePixelSpan() function and sure enough, in my test project that loads a single texture I can get a Span with the entire image as desired. Unfortunately for me, the syntax of this function clearly implies that the memory of images can no longer be guaranteed to be contiguous, which is something I need.

Is there a good solution to this? It would sure be nice to have a way to ensure an Image's memory is contiguous and I imagine I'm not the only one having this issue.

[antonfirsov]

Discontiguous buffers have been introduced in #1109 to enable handling very large images. The size of a single contiguous buffer is limited by ArrayPoolMemoryAllocator.BufferCapacityInBytes. It's current (arbitrary) default value is int.MaxValue/4 which means that an Image<Rgba32> below the size of ~134 megapixels (~ 12k x 11k pixels) will always fit into a single buffer with the default setup. However, we may decide to lower this value, so if you want to make sure your images fit into a single buffer, I'd suggest to create a custom ArrayPoolMemoryAllocator instance, and assign it to Configuration.Default.MemoryAllocator (or to the same property on your custom Configuration instance).

Unfortunately, at the moment there's no other way than to use the full constructor of ArrayPoolMemoryAllocator, feeding the defaults to preceding parameters:

ImageSharp/src/ImageSharp/Memory/Allocators/ArrayPoolMemoryAllocator.cs

Lines 85 to 90 in ab8ab39

	public ArrayPoolMemoryAllocator(
	int maxPoolSizeInBytes,
	int poolSelectorThresholdInBytes,
	int maxArraysPerBucketLargePool,
	int maxArraysPerBucketNormalPool,
	int bufferCapacityInBytes)

[ThomasMiz]

Does this mean, assuming ArrayPoolMemoryAllocator.BufferCapacityInBytes is int.MaxValue/4, that I can guarantee that any Image<Rgba32> below ~134 megapixels will be contiguous in memory?

There's no such thing as a non-contiguous 1024x1024 Image<Rgba32>?

[antonfirsov]

Exactly.

We may lower this threshold if we find out that more intense usage of noncontigous blocks improves throughput for the most common resize scenarios. But I think we'll never go below 1MP.

[ThomasMiz]

Alright, thanks!

By the way, the SixLabors set of libraries are incredible, so special thanks to everyone who worked on these for their amazing work! Open source projects like these hold our personal projects together!