How do I Render Individual Pixels in Flutter?

Question

Setup

I am using a custom RenderBox to draw.
The canvas object in the code below comes from the PaintingContext in the paint method.

Drawing

I am trying to render pixels individually by using Canvas.drawRect.
I should point out that these are sometimes larger and sometimes smaller than the pixels on screen they actually occupy.

for (int i = 0; i < width * height; i++) {
  // in this case the rect size is 1
  canvas.drawRect(
      Rect.fromLTWH(index % (width * height), 
          (index / (width * height)).floor(), 1, 1), Paint()..color = colors[i]);
}

Storage

I am storing the pixels as a List<List<Color>> (colors in the code above). I tried differently nested lists previously, but they did not cause any noticable discrepancies in terms of performance. The memory on my Android Emulator test device increases by 282.7MB when populating the list with a 999x999 image. Note that it only temporarily increases by 282.7MB. After about half a minute, the increase drops to 153.6MB and stays there (without any user interaction).

Rendering

With a resolution of 999x999, the code above causes a GPU max of 250.1 ms/frame and a UI max of 1835.9 ms/frame, which is obviously unacceptable. The UI freezes for two seconds when trying to draw a 999x999 image, which should be a piece of cake (I would guess) considering that 4k video runs smoothly on the same device.

CPU

I am not exactly sure how to track this properly using the Android profiler, but while populating or changing the list, i.e. drawing the pixels (which is the case for the above metrics as well), CPU usage goes from 0% to up to 60%. Here are the AVD performance settings:

Cause

I have no idea where to start since I am not even sure what part of my code causes the freezing. Is it the memory usage? Or the drawing itself?
How would I go about this in general? What am I doing wrong? How should I store these pixels instead.

Efforts

I have tried so much that did not help at all that I will try to only point out the most notable ones:

• I tried converting the List<List<Color>> to an Image from the dart:ui library hoping to use Canvas.drawImage. In order to do that, I tried encoding my own PNG, but I have not been able to render more than a single row. However, it did not look like that would boost performance. When trying to convert a 9999x9999 image, I ran into an out of memory exception. Now, I am wondering how video is rendered as all as any 4k video will easily take up more memory than a 9999x9999 image if a few seconds of it are in memory.

• I tried implementing the image package. However, I stopped before completing it as I noticed that it is not meant to be used in Flutter but rather in HTML. I would not have gained anything using that.

• This one is pretty important for the following conclusion I will draw: I tried to just draw without storing the pixels, i.e. is using Random.nextInt to generate random colors. When trying to randomly generate a 999x999 image, this resulted in a GPU max of 1824.7 ms/frames and a UI max of 2362.7 ms/frame, which is even worse, especially in the GPU department.

Conclusion

This is the conclusion I reached before trying my failed attempt at rendering using Canvas.drawImage: Canvas.drawRect is not made for this task as it cannot even draw simple images.

How do you do this in Flutter?

Notes

• This is basically what I tried to ask over two months ago (yes, I have been trying to resolve this issue for that long), but I think that I did not express myself properly back then and that I knew even less what the actual problem was.

• The highest resolution I can properly render is around 10k pixels. I need at least 1m.

• I am thinking that abandoning Flutter and going for native might be my only option. However, I would like to believe that I am just approaching this problem completely wrong. I have spent about three months trying to figure this out and I did not find anything that lead me anywhere.

Answer 1

Solution

dart:ui has a function that converts pixels to an Image easily: decodeImageFromPixels

• Example implementation

• Issue on performance

• Does not work in the current master channel

I was simply not aware of this back when I created this answer, which is why I wrote the "Alternative" section.

Alternative

Thanks to @pslink for reminding me of BMP after I wrote that I had failed to encode my own PNG.
I had looked into it previously, but I thought that it looked to complicated without sufficient documentation. Now, I found this nice article explaining the necessary BMP headers and implemented 32-bit BGRA (ARGB but BGRA is the order of the default mask) by copying Example 2 from the "BMP file format" Wikipedia article. I went through all sources but could not find an original source for this example. Maybe the authors of the Wikipedia article wrote it themselves.

Results

Using Canvas.drawImage and my 999x999 pixels converted to an image from a BMP byte list, I get a GPU max of 9.9 ms/frame and a UI max of 7.1 ms/frame, which is awesome!

|  ms/frame |  Before (Canvas.drawRect) | After (Canvas.drawImage) |
|-----------|---------------------------|--------------------------|
|  GPU max  | 1824.7                    | 9.9                      |
|  UI max   | 2362.7                    | 7.1                      |

Conclusion

Canvas operations like Canvas.drawRect are not meant to be used like that.

Instructions

First of, this is quite straight-forward, however, you need to correctly populate the byte list, otherwise, you are going to get an error that your data is not correctly formatted and see no results, which can be quite frustrating.

You will need to prepare your image before drawing as you cannot use async operations in the paint call.

In code, you need to use a Codec to transform your list of bytes into an image.

final list = [
            0x42, 0x4d, // 'B', 'M'
            ...];
// make sure that you either know the file size, data size and data offset beforehand
//        or that you edit these bytes afterwards

final Uint8List bytes = Uint8List.fromList(list);

final Codec codec = await instantiateImageCodec(bytes));

final Image image = (await codec.getNextFrame()).image;

You need to pass this image to your drawing widget, e.g. using a FutureBuilder.
Now, you can just use Canvas.drawImage in your draw call.