CGImageCreate Gives Completly black or no Image or crashes

Question

I have been for some time now attempting to do the following,

• player taps screen
• get the values of the pixel the player has tapped
• Loop through image to find out which pixels are the same Make all
• pixels with the same values have an alpha of 0 create a new image and
• set the UIImage as the newly created one
• ...
• Profit

But it either seems to do as the title says, Completely black screen, or no image whatsoever...Possibly maybe setting all the alpha to 0, or it crashes something about bad access on the stack near this function CGSConvertBGRX8888toRGBA8888

Here is the function

+ (UIImage*)getRGBAsFromImage:(UIImage*)image atX:(int)xx andY:(int)yy

{

// First get the image into your data buffer

CGImageRef imageRef = [image CGImage];

NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
NSUInteger bitsPerComponent = CGImageGetBitsPerComponent(imageRef);
NSUInteger bitsPerPixel = CGImageGetBitsPerPixel(imageRef);
NSUInteger bytesPerRow = CGImageGetBytesPerRow(imageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();


unsigned char *rawData = (unsigned char*) malloc(height * width * 4);


CGContextRef context = CGBitmapContextCreate(rawData, 
                                             width, 
                                             height,
                                             bitsPerComponent, 
                                             bytesPerRow, 
                                             colorSpace,
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);


CGContextClearRect(context, CGRectMake(0, 0, width, height));
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);

//convert phone screen coords to texture coordinates.
xx *= width/320;
yy *= height/480;

int counter = 0;


// Now your rawData contains the image data in the RGBA8888 pixel format.
// Get the byteIndex of pixel tapped.

int byteIndex = (bytesPerRow * yy) + xx * bitsPerPixel;

CGFloat red   = (rawData[byteIndex]     * 1.0) / 255.0;
CGFloat green = (rawData[byteIndex + 1] * 1.0) / 255.0;
CGFloat blue  = (rawData[byteIndex + 2] * 1.0) / 255.0;
CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;
byteIndex += 4;

for(int x = 0; x < width; x++) 
{
    for(int y = 0; y < height; y++) 
    {
        byteIndex = ( width * y ) + x  * bitsPerPixel;

        CGFloat redVal   = ( rawData[byteIndex]     * 1.0) / 255.0;
        CGFloat greenVal = ( rawData[byteIndex + 1] * 1.0) / 255.0;
        CGFloat blueVal  = ( rawData[byteIndex + 2] * 1.0) / 255.0;
        CGFloat alphaVal = ( rawData[byteIndex + 3] * 1.0) / 255.0;
        byteIndex += 4;

        if( alphaVal != 0 )
        {
            if( redVal == red && greenVal == green && blueVal == blue )
            {
                rawData[byteIndex + 3] = 0; 
                counter ++;
            }
        }
    }
}

NSLog(@"Pixels amount: %i", counter);

CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, 
                                                          rawData, 
                                                          width*height*4, 
                                                          NULL);

CGImageRef newCGimage = CGImageCreate( width,  
                                       height, 
                                       bitsPerComponent, 
                                       bitsPerPixel,
                                       bytesPerRow,
                                       colorSpace, 
                                       kCGBitmapByteOrderDefault, 
                                       provider,
                                       NULL, NO, kCGRenderingIntentDefault );

UIImage *newImage = [UIImage imageWithCGImage:newCGimage];

CGDataProviderRelease(provider);
CGImageRelease(newCGimage);
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
free(rawData);

return newImage;

}

I am fairly certain i am obtaining the correct pixels through some tests and outputting the colour selected to a view that part seems to work fine, it seems to be when I'm creating the image after editing the raw data, at least that is where i think the problem lies.

Cheers for any help in advance.

EDIT:

I have now stripped the code so essentially all it does is copy the png to raw data creates a UIImage from it and sets it, essentially i should get the exact same image back and nothing should change. But this time it just dissapears, either from 0 values for everything or due to alpha being 0 I am not sure edited code is as follows

// First get the image into your data buffer
CGImageRef imageRef = [image CGImage];

NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
NSUInteger bitsPerComponent = CGImageGetBitsPerComponent(imageRef);
NSUInteger bitsPerPixel = CGImageGetBitsPerPixel(imageRef);
NSUInteger bytesPerRow = CGImageGetBytesPerRow(imageRef);
CGColorSpaceRef colorSpace = CGImageGetColorSpace(imageRef);
CGBitmapInfo imageInfo = CGImageGetBitmapInfo(imageRef);


unsigned char *rawData = (unsigned char*) malloc(height * width * 4);


CGContextRef context = CGBitmapContextCreate(rawData, 
                                             width, 
                                             height,
                                             bitsPerComponent, 
                                             bytesPerRow, 
                                             colorSpace,
                                             imageInfo);

//convert phone screen coords to texture coordinates.
xx *= (width/[[UIScreen mainScreen] bounds].size.width);
yy *= (height/[[UIScreen mainScreen] bounds].size.height);

NSLog(@"converted X pos: %i",xx);
NSLog(@"converted Y pos: %i",yy);


int counter = 0;


CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, 
                                                          rawData, 
                                                          width*height*4, 
                                                          NULL);
CGImageRef newCGimage = CGImageCreate( width,  
                                       height, 
                                       bitsPerComponent, 
                                       bitsPerPixel,
                                       bytesPerRow,
                                       colorSpace, 
                                       imageInfo, 
                                       provider,
                                       NULL, NO, kCGRenderingIntentDefault );

UIImage *newImage = [[UIImage alloc]initWithCGImage:newCGimage];

CGDataProviderRelease(provider);
CGImageRelease(newCGimage);
CGContextRelease(context);
free(rawData);

return newImage;

Answer 1

Well I have fixed it, Following others examples sometimes just end up you spending a whole day trying to figure out something you would rather not...Either way the code above is fine...with the exception of

for(int y = 0; y < height; y++) 
{
    byteIndex = ( width * y ) + x  * bitsPerPixel;

    CGFloat redVal   = ( rawData[byteIndex]     * 1.0) / 255.0;
    CGFloat greenVal = ( rawData[byteIndex + 1] * 1.0) / 255.0;
    CGFloat blueVal  = ( rawData[byteIndex + 2] * 1.0) / 255.0;
    CGFloat alphaVal = ( rawData[byteIndex + 3] * 1.0) / 255.0;
    byteIndex += 4;

    if( alphaVal != 0 )
    {
        if( redVal == red && greenVal == green && blueVal == blue )
        {
            rawData[byteIndex + 3] = 0; 
            counter ++;
        }
    }
}

I am incrementing the byte index before using it again in my pixel check, so move that after the test and that is all good.

As for the picture display issue, remove "free(rawData);" apparently the memory dislikes being freed.

Answer 2

In your EDIT post, the image is all black because you are not doing anything to the context. You just create a blank context and turn it into an image.

Also, I made the main part of your algorithm a little more efficient. When iterating over an array, you always want to put your for loops in row-major order (in this case, outer loop for height, then inner loop for width) so that index calculations are much easier.

int byteIndex = (bytesPerRow * yy) + (xx * (bitsPerPixel / 8));  //you want bytes, not bits, so divide the bitsPerPixel by 8

//you don't need to do the "* 1.0 / 255.0" part if you are just
//going to compare the values with other values in the same 0-255 range.
//Only do it if something requires the values to be from 0-1 or if that
//is how you want it to print.
CGFloat red   = (rawData[byteIndex++];
CGFloat green = (rawData[byteIndex++];
CGFloat blue  = (rawData[byteIndex++];
CGFloat alpha = (rawData[byteIndex++];

//because the 2-dimensional data is in a 1-dimensional array,
//the lines wrap as they go down, so the data reads like a book
//with height = 0 at the top and width = 0 on the left.
//This for loop configuration allows for easy iteration by just
//incrementing the index while iterating.

byteIndex = 0;

for (int y = 0; y < height; y++)
{
    for(int x = 0; x < width; x++) 
    {
        //Incrementing byteIndex like this makes it go to the correct
        //position for the next loop cycle.

        //Do not multiply by one unless you want maximum readability.
        //If you were trying to get out of doing integer division,
        //multiplying by one isn't necessary because the 255.0 not an integer.
        //Because multiplying by one is executed at run-time, it hogs
        //the CPU for a little bit. To overcome this, cast if you really
        //have to, because casting is a compile time operation,
        //not a run-time one, giving more performance.

        CGFloat redVal   = rawData[byteIndex++];
        CGFloat greenVal = rawData[byteIndex++];
        CGFloat blueVal  = rawData[byteIndex++];
        CGFloat alphaVal = rawData[byteIndex++];

        //you can combine the if statements into one because of short-circuiting

        if( alphaVal != 0 && redVal == red && greenVal == green && blueVal == blue )
        {
            rawData[byteIndex-1] = 0; 
            counter ++;
        }
    }
}

Hope it helps!