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Graphics essentials in Smart Mobile Studio 3

JavaScript and the DOM has a few quirks that can be a bit tricky for Delphi developers to instinctively understand. And while our RTL covers more or less everything, I would be an idiot if I said we havent missed a spot here and there. A codebase as large as Smart is like a living canvas; And with each revision we cover more and more of our blind-spots.

Where did TW3Image.SaveToStream vanish?

We used to have a SaveToStream method in TW3Image that took the raw DIB data (raw RGBA pixel data) and emitted that to a stream. That method was never really meant to save a picture in a compliant format, but to make it easy for game developers to cache images in a buffer and quickly draw the pixel-data to a canvas (or push it to localstorage, good if you are making a paint program). This should have been made more clear in the RTL unit, but sadly it escaped me. I apologize for that.

But in this blog-post we are going to make a proper Save() function, one that saves to a proper format like PNG or JPG. It should be an interesting read for everyone.

Resources are global in scope

Before we dig in, a few words about how the browser treats resources. This is essential because the browser is a resource oriented system. Just think about it: HTML loads everything it needs separately, things like pictures, sounds, music, css styles — all these resources are loaded as the browser finds them in the code – and each have a distinct URI (uniform resource identifier) to represent them.

So no matter where in your code you are (even a different form), if you have the URI for a resource – it can be accessed. It’s important to not mix terminology here because URI is not the same as a URL. URI is a unique identifier, an URL (uniform resource location) defines “where” the browser can find something (it can also contain the actual data).

If you look at the C/C++ specs, the URL class inherits from URI. Which makes sense.

Once a resource is loaded and is assigned an URI, it can be accessed from anywhere in your code. It is global in scope and things like forms or parent controls in the RTL means nothing to the underlying DOM.

Making new resources

When you are creating new resources, like generating a picture via the canvas, that resource doesn’t have an URI. Thankfully, generating and assigning an URI so it can be accessed is very simple — and once we have that URI the user can download it via normal mechanisms.

But the really cool part is that this system isn’t just for images. It’s also for raw data! You can actually assign a URI to a buffer and make that available for download. The browsers wont care about the content.

If you open the RTL unit SmartCL.System.pas and scroll down to line 107 (or there about), you will find the following classes defined:

  (* Helper class for streams, adds data encapsulation *)
  TAllocationHelper = class helper for TAllocation
    function  GetObjectURL: string;
    procedure RevokeObjectURL(const ObjectUrl: string);

  TW3URLObject = static class
    class function  GetObjectURL(const Text, Encoding, ContentType, Charset: string): string; overload;
    class function  GetObjectURL(const Text: string): string; overload;
    class function  GetObjectURL(const Stream: TStream): string; overload;
    class function  GetObjectURL(const Data: TAllocation): string; overload;
    class procedure RevokeObjectURL(const ObjectUrl: string);

    // This cause a download in the browser of an object-url
    class procedure Download(const ObjectURL: string; Filename: string); overload;
    class procedure Download(const ObjectURL: string; Filename: string;
          const OnStarted: TProcedureRefS); overload;

The first class, TAllocationHelper, is just a helper for a class called TAllocation. TAllocation is the base-class for objects that allocate raw memory, and can be found in the unit System.Memory.Allocation.pas.
TAllocation is really central and more familiar classes like TMemoryStream expose this as a property. The idea here being that if you have a memory stream with something, making the data downloadable is a snap.

Hopefully you have gotten to know the central buffer class, TBinaryData, which is defined in System.Memory.Buffer. This is just as important as TMemoryStream and will make your life a lot easier when talking to JS libraries that expects an untyped buffer handle (for example) or a blob (more on that later).

The next class, TW3URLObject, is the one that is of most interest here. You have probably guessed that TAllocationHelper makes it a snap to generate URI’s for any class that inherits from or expose a TAllocation instance (read: really handy for TMemoryStream). But TW3URLObject is the class you want.

The class contains 3 methods with various overloading:

  • GetObjectURL
  • RevokeObjectURL
  • Download

I think these are self explanatory, but in short they deliver the following:

  • GetObjectURL creates an URI for a resource
  • RevokeObjectURL removes a previously made URI from a resource
  • Download triggers the “SaveAs” dialog so users can, well, save the data to their local disk

The good news for graphics is that the canvas object contains a neat method that does this automatically, namely the ToDataUrl() function, which is a wrapper for the raw JS canvas method with the same name. Not only will it encode your picture in a normal picture format (defaults to png but supports all known web formats), it will also return the entire image as a URI encoded string.

This saves us the work of having to manually call GetObjectURL() and then invoke the save dialog.

Making some offscreen graphics

TW3Image is not meant for drawing, it’s like Delphi’s TImage and is a graphics container. So before we put a TW3Image on our form we are going to create the actual graphics to display. And we do this by creating an off-screen graphics context, assign a canvas to it, draw the graphics, and then encode the data via ToDataUrl().

To make things easier, lets use the Delphi compatible TBitmap and TCanvas classes. These can be found in SmartCL.Legacy. They are as compatible as I could make them.

  • Browsers only support 32 bit graphics, so only pf32bit is allowed
  • I havent implemented checkered, diagonal or other patterns – so bsSolid and bsClear are the only brush modes for canvas (and pen style as well).
  • Brush doesn’t have a picture property (yet), but this will be added later at some point. I have to replace the built-in linedraw() method with the Bresham algorithm for that to happen (and all the other primitives).
  • When drawing lines you have to call Stroke() to render. The canvas buffers up all the drawing operations and removes overlapping pixels to speed up the final drawing process — this is demanded by the browser sadly.

Right, with that behind us, lets create an off-screen bitmap, fill the background red and assign it to a TW3Image control.

To replicate this example please use the following recipy:

  1. Start a new “visual components project”
  2. Add the following units to the uses clause:
    1. System.Colors
    2. System.Types.Graphics
    3. SmartCL.Legacy
  3. Add a TW3Button to the form
  4. add a TW3Image to the form
  5. Save your project
  6. Double-Click on the button. This creates a code entry point for the default event, which for a button is OnClick.

Let’s populate the entry point with the following:

procedure TForm1.W3Button1Click(Sender: TObject);
  LBitmap:  TBitmap;
  LRect:    TRect;
  LBitmap := TBitmap.Create;
    LBitmap.Allocate(640, 480);
    LRect := TRect.Create(0, 0, LBitmap.width-1, LBitmap.Height-1);
    LBitmap.Canvas.Brush.Color := clRed;

    w3image1.LoadFromUrl( LBitmap.Canvas.ToDataURL('image/png') );


The code above creates a bitmap, which is an off-screen (not visible) graphics context. We then set a background color to use (red) and fill the bitmap with that color. When this is done we load the picture-data directly into our TW3Image control so we can see it.

Triggering a download

With the code for creating graphics done, we now move on to the save mechanism. We want to download the picture when the user clicks the button.


Offscreen graphics is quite fun once you know how it works

Since the image already have an URI, which it get’s when you call the ToDataURL() method, we don’t need to mess around with blob buffers and generating the URI manually. So forcing a download could not be simpler:

procedure TForm1.W3Button1Click(Sender: TObject);
  LBitmap:  TBitmap;
  LRect:    TRect;
  LBitmap := TBitmap.Create;
    LBitmap.Allocate(640, 480);
    LRect := TRect.Create(0, 0, LBitmap.width-1, LBitmap.Height-1);
    LBitmap.Canvas.Brush.Color := clRed;

    var LEncodedData:= LBitmap.Canvas.ToDataURL('image/png');

    TW3URLObject.Download( LEncodedData, 'picture.png');


Note: The built-in browser in Smart doesn’t allow save dialogs, so when you run this example remember to click the “open in browser” button on the execute window. Then click the button and voila — the image is downloaded directly.

Well, I hope this has helped! I will do a couple of more posts on graphics shortly because there really is a ton of cool features here. We picked heavily from various libraries when we implemented TW3Canvas and TCanvas, so if you like making games or display data – then you are in for a treat!

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