Android ImageView and Drawable with SVG Support

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Introduction

This article shows how to use SVG images in Android. The goal is to use SVG images just like any other images, using Drawable and ImageView classes. New classes are introduced for this purpose: SvgDrawable and SvgImageView.

These classes support all Android scale types for images: fitXY, fitStart, fitEnd, centerCrop and so on. What is important, the image is stretched in vector format, before rasterization. I believe this improves overall performance.

Background

You are encouraged to read the article by Igor Kushnarev beforehand. Carl D. Worth, Anton Persson and Igor Kushnarev have done a great job. This article is a continuation of their work.

I needed to change their implementation because of the following reasons:

• Actually a lot (if not all) SVG images have the desired width and height. They are written in the file. And I want to use them too. The original implementation did not care about SVG width and height.
• I need a Drawable class to work with image, not just an ImageView. Because Drawable can be used in many other Android classes.
• I want to use advantage of scale types for images: fitXY, fitStart, fitEnd, centerCrop and so on.
• FitXY scale type requires non-uniform scaling of image. This is required but was not supported by the original native code.
• I do not want standard ImageView class to scale rasterized bitmap in case of SVG Drawable. I want that vector image would be scaled according to scale type before rasterization. This will improve performance and quality of image.

Prerequisites

Well, if you have read the article by Igor Kushnarev, you should already have this:

• Eclipse with Java, Android and CDT support
• Android SDK
• Android NDK
• Sequoyah Android Native Support in Eclipse

Points of Interest

Width and height of SVG image

Please look at /jni/libsvg-android/svg_android.c.

I've added two functions that return width and height of SVG image to Java class SvgRaster:

• Java_com_libsvg_SvgRaster_svgAndroidGetWidth(),
• Java_com_libsvg_SvgRaster_svgAndroidGetHeight().

They use values from DOM model, which were set on parsing the XML structure.

Scaling Support

I've been selecting from two basic approaches to implement Android scale types support:

1. Rewrite the whole ImageView class. Because it was written specially for raster bitmaps, it takes the whole work to make matrix transformation, not letting the Drawable itself to take care about scaling.
2. Cheat on ImageView class. Make it believe that Drawable has a good size and should not be scaled using Matrix transform. Let the SVG image make scaling in vector, before rasterization.

I've chosen the second approach because it is easier.

Before ImageView class prepares matrix transform, we let SvgDrawable to adjust its width and height to match those from ImageView with respect to scaleType.

For this purpose, we rewrite setScaleType(), setImageDrawable() and onSizeChanged().

@Override
public void setScaleType (ImageView.ScaleType scaleType) {
    super.setScaleType(scaleType);
    if (! mIsSvg) {
        return;
    } 

    SvgDrawable svg = (SvgDrawable) getDrawable(); 
    // Let SVG scale itself!
    super.setImageDrawable(null); 
    svg.setScaleType(scaleType);
    int vWidth = getWidth() - getPaddingLeft() - getPaddingRight();
    int vHeight = getHeight() - getPaddingTop() - getPaddingBottom();
    svg.adjustToParentSize(vWidth, vHeight);
    super.setImageDrawable(svg);
} 

Notice that we first call super.setImageDrawable(null). Then we change SVG image scaling. And then return it with super.setImageDrawable(svg). This trick works!

SvgDrawable Class

This is a rather simple implementation based on ideas of Igor Kushnarev.

Scaling support required new function:

public void adjustToParentSize(int vWidth, int vHeight) 

It is called from SvgImageView when onSizeChanged(), setScaleType() and on load.

Please also notice that we choose how to render svg image in onBoundsChange():

• using non-uniform scaling - SvgRaster.svgAndroidRenderToArea()
• using uniform scaling - SvgRaster.svgAndroidRenderToAreaUniform

Non-uniform Scaling Support in libsvgandroid

Please look at /jni/libsvg-android/svg_android_render.c.

I've changed function:

svg_status_t svgAndroidRenderToArea(JNIEnv *env, 
svg_android_t *svg_android, jobject android_canvas, int x, int y, int w, int h) { 
    ... 
    svg_android->fit_to_area = -1; 
    svg_android->fit_uniform = 0; 
    ... 
    return svg_render (svg_android->svg, &SVG_ANDROID_RENDER_ENGINE, svg_android);
}

svg_android->fit_uniform is a newly added member, which is used in: _svg_android_set_viewport_dimension().

In svgAndroidRenderToArea(), it is set to zero (false).

In svgAndroidRenderToAreaUniform(), it is set to -1 (true).

Using SvgImageView

Look at /res/layout/main2.xml.

Part of this file:

<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/TableView" 
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent"
    android:stretchColumns="1">
    <TableRow
        android:layout_weight="1.0"
        android:layout_height="fill_parent">
        <com.libsvg.SvgImageView
            android:src="@drawable/smile01" 
            android:background="#FF806440"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent" 
            android:layout_gravity="center"
            android:scaleType="fitXY" 
            />
        <com.libsvg.SvgImageView
            android:src="@drawable/smile02" 
            android:background="#FF758040"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent" 
            android:layout_gravity="center" 
            android:scaleType="fitStart" 
            />

Notice the tags <com.libsvg.SvgImageView>. They have all the attributes of standard ImageView.

Result

Upon successful compile and run, you should see the following picture of smiles in a table.
Table has two rows with 3 images on each row.

It demonstrates different scale types for SVG images.

Currently, from left to right, from top to bottom:
fitXY, fitStart, fitEnd,
centerCrop, center, centerInside

Conclusion

Hope this article will help to improve Android development. If you have something to add - please write here and share your code!

--
Sincerely yours,
Pavel B. Chernov

History

• 14^th December, 2010: Initial post