How to draw many gradient lines quickly - ios

Question

I'm developing an app that has to draw 320 vertical gradient lines on a portrait iPhone screen where each gradient line is either 1px or 2px wide (non-retina vs retina). Each gradient line has 1000 positions, with each position able to have a unique color. These 1000 colors (floats) sit in a C-style 2D array (an array of arrays, 320 arrays of 1000 colors)
Currently, the gradient lines are drawn in a For Loop inside the drawRect method of a custom UIView. The problem I'm having is that it takes longer than ONE second to cycle through the For Loop and draw all 320 lines. Within that ONE second, I have another thread that's updating the color arrays and but since it takes longer than ONE second to draw, I don't see every update. I see every second or third update.
I'm using the exact same procedure in my Android code, which has no problems drawing 640 gradient lines (double the amount) multiple times in a second using a SurfaceView. My Android app never misses an update.
If you look at the Android code, it actually draws gradient lines to TWO separate canvases. The array size is dynamic and can be up to half the landscape resolution width of an Android phone (ex 1280 width = 1280/2 = 640 lines). Since the Android app is fast enough, I allow landscape mode. Even with the double the data as an iPhone and drawing to two separate canvases, the Android code runs multiple times a second. The iPhone code with half the number of lines and only drawing to a single context can not draw in under a second.
Is there a faster way to draw 320 vertical gradient lines (each with 1000 positions) on an iPhone?
Is there a hardware accelerated SurfaceView equivalent for iOS that can draw many gradients really fast?
//IPHONE - drawRect method
int totalNumberOfColors = 1000;
int i;
CGFloat *locations = malloc(totalNumberOfColors * sizeof locations[0]);
for (i = 0; i < totalNumberOfColors; i++) {
float division = (float)1 / (float)(totalNumberOfColors - 1);
locations[i] = i * division;
}
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
for (int k = 0; k < 320; k++) {
CGFloat * colorComponents = arrayOfFloatArrays[k];
CGGradientRef gradient = CGGradientCreateWithColorComponents(
colorSpace,
colorComponents,
locations,
(size_t)(totalNumberOfColors));
CGRect newRect;
if (currentPositionOffset >=320) {
newRect = CGRectMake(0, 0, 1, CGRectGetMaxY(rect));
} else {
newRect = CGRectMake(319 - (k * 1), 0, 1, CGRectGetMaxY(rect));
}
CGContextSaveGState(ctx);
//NO CLIPPING STATE
CGContextAddRect(ctx, newRect);
CGContextClip(ctx);
//CLIPPING STATE
CGContextDrawLinearGradient(
ctx,
gradient,
CGPointMake(0, 0),
CGPointMake(0, CGRectGetMaxY(rect)),
(CGGradientDrawingOptions)NULL);
CGContextRestoreGState(ctx);
//RESTORE TO NO CLIPPING STATE
CGGradientRelease(gradient);
}
//ANDROID - public void run() method on SurfaceView
for (i = 0; i < sonarData.arrayOfColorIntColumns.size() - currentPositionOffset; i++) {
Paint paint = new Paint();
int[] currentColors = sonarData.arrayOfColorIntColumns.get(currentPositionOffset + i);
//Log.d("currentColors.toString()",currentColors.toString());
LinearGradient linearGradient;
if (currentScaleFactor > 1.0) {
int numberOfColorsToUse = (int)(1000.0/currentScaleFactor);
int tmpTopOffset = currentTopOffset;
if (currentTopOffset + numberOfColorsToUse > 1000) {
//shift tmpTopOffset
tmpTopOffset = 1000 - numberOfColorsToUse - 1;
}
int[] subsetOfCurrentColors = new int[numberOfColorsToUse];
System.arraycopy(currentColors, tmpTopOffset, subsetOfCurrentColors, 0, numberOfColorsToUse);
linearGradient = new LinearGradient(0, tmpTopOffset, 0, getHeight(), subsetOfCurrentColors, null, Shader.TileMode.MIRROR);
//Log.d("getHeight()","" + getHeight());
//Log.d("subsetOfCurrentColors.length","" + subsetOfCurrentColors.length);
} else {
//use all colors
linearGradient = new LinearGradient(0, 0, 0, getHeight(), currentColors, null, Shader.TileMode.MIRROR);
//Log.d("getHeight()","" + getHeight());
//Log.d("currentColors.length","" + currentColors.length);
}
paint.setShader(linearGradient);
sonarData.checkAndAddPaint(paint);
numberOfColumnsToDraw = i + 1;
}
//Log.d(TAG,"numberOfColumnsToDraw " + numberOfColumnsToDraw);
currentPositionOffset = currentPositionOffset + i;
if (currentPositionOffset >= sonarData.getMaxNumberOfColumns()) {
currentPositionOffset = sonarData.getMaxNumberOfColumns() - 1;
}
if (numberOfColumnsToDraw > 0) {
Canvas canvas = surfaceHolder.lockCanvas();
if (AppInstanceData.sonarBackgroundImage != null && canvas != null) {
canvas.drawBitmap(AppInstanceData.sonarBackgroundImage, 0, getHeight()- AppInstanceData.sonarBackgroundImage.getHeight(), null);
if (cacheCanvas != null) {
cacheCanvas.drawBitmap(AppInstanceData.sonarBackgroundImage, 0, getHeight()- AppInstanceData.sonarBackgroundImage.getHeight(), null);
}
}
for (i = drawOffset; i < sizeToDraw + drawOffset; i++) {
Paint p = sonarData.paintArray.get(i - dataStartOffset);
p.setStrokeWidth(2);
//Log.d("drawGradientLines", "canvas.getHeight() " + canvas.getHeight());
canvas.drawLine(getWidth() - (i - drawOffset) * 2, 0, getWidth() - (i - drawOffset) * 2, canvas.getHeight(), p);
if (cacheCanvas != null) {
cacheCanvas.drawLine(getWidth() - (i - drawOffset) * 2, 0, getWidth() - (i - drawOffset) * 2, canvas.getHeight(), p);
}
}
surfaceHolder.unlockCanvasAndPost(canvas);
}

Answer 1

No comment on the CG code — it's been a while since I've drawn any gradients — but a couple of notes:
You shouldn't be doing that in drawRect because it's called a lot. Draw into an image and display it.
There's no matching free for the malloc, so you're leaking memory like crazy.

Answer 2

It'll have a learning curve, but implement this using OpenGL ES 2.0. I previously took something that was drawing a large number of gradients as well, and reimplemented it using OpenGL ES 2.0 and custom vertex and fragment shaders. It is way faster than the equivalent drawing done using Core Graphics, so you will probably see a big speed boost as well.
If you don't know any OpenGL yet, I would suggest finding some tutorials for working with OpenGL ES 2.0 (has to be 2.0 because that's what offers the ability to write custom shaders) on iOS, to learn the basics. Once you do that, you should be able to significantly increase the performance of your drawing, way above that of the Android version, and maybe would be incentive to make the Android version use OpenGL as well.