I'm trying to do something very simple:
1) Draw a UIImage into a CG bitmap context
2) Get a pointer to the data of the image
3) iterate over all pixels and just set all R G B components to 0 and alpha to 255. The result should appear pure black.
This is the original image I am using. 200 x 40 pixels, PNG-24 ARGB premultiplied alpha (All alpha values == 255):
This is the result (screenshot from Simulator), when I do not modify the pixels. Looks good:
This is the result, when I do the modifications! It looks like if the modification was incomplete. But the for-loops went over EVERY single pixel. The counter proves it: Console reports modifiedPixels = 8000 which is exactly 200 x 40 pixels. It looks always exactly the same.
Note: The PNG image I use has no alpha < 255. So no transparent pixels.
This is how I create the context. Nothing special...
int bitmapBytesPerRow = (width * 4);
int bitmapByteCount = (bitmapBytesPerRow * imageHeight);
colorSpace = CGColorSpaceCreateDeviceRGB();
bitmapData = malloc(bitmapByteCount);
bitmapContext = CGBitmapContextCreate(bitmapData,
width,
height,
8, // bits per component
bitmapBytesPerRow,
colorSpace,
CGImageAlphaPremultipliedFirst);
Next, I draw the image into that bitmapContext, and obtain the data like this:
void *data = CGBitmapContextGetData(bitmapContext);
This is the code which iterates over the pixels to modify them:
size_t bytesPerRow = CGImageGetBytesPerRow(img);
NSInteger modifiedPixels = 0;
for (int y = 0; y < height; y++) {
for (int x = 0; x < width; x++) {
long int offset = bytesPerRow * y + 4 * x;
// ARGB
unsigned char alpha = data[offset];
unsigned char red = data[offset+1];
unsigned char green = data[offset+2];
unsigned char blue = data[offset+3];
data[offset] = 255;
data[offset+1] = 0;
data[offset+2] = 0;
data[offset+3] = 0;
modifiedPixels++;
}
}
When done, I obtain a new UIImage from the bitmap context and display it in a UIImageView, to see the result:
CGImageRef imageRef = CGBitmapContextCreateImage(bitmapContext);
UIImage *img = [[UIImage alloc] initWithCGImage:imageRef];
Question:
What am I doing wrong?
Is this happening because I modify the data while iterating over it? Must I duplicate it?
Use CGBitmapContextGetBytesPerRow(bitmapContext) to get bytesPerRow instead getting from image (image has only 3 bytes per pixels if it hasn't alpha informations)
Might you're getting wrong height or width.... and by the way 240x40=9600 not 8000 so that's for sure that you're not iterating over each and every pixel.
Related
I have the source code for a video decoder application written in C, which I'm now porting on iphone.
My problem is as follows:
I have RGBA pixel data for a frame in a buffer that I need to display on the screen. My buffer is of type unsigned char. (I cannot change it to any other data type as the source code is too huge and not written by me.)
Most of the links I found on the net say about how to "draw and display pixels" on the screen or how to "display pixels present in an array", but none of then say how to "display pixel data present in a buffer".
I'm planning to use quartz 2D. All I need to do is just display the buffer contents on the screen. No modifications! Although my problem sounds very simple, there isn't any API that I could find to do the same. I couldn't find any appropriate link or document that was useful enough.
Kindly help!
Thanks in advance.
You can use the CGContext data structure to create a CGImage from raw pixel data. I've quickly written a basic example:
- (CGImageRef)drawBufferWidth:(size_t)width height:(size_t)height pixels:(void *)pixels
{
unsigned char (*buf)[width][4] = pixels;
static CGColorSpaceRef csp = NULL;
if (!csp) {
csp = CGColorSpaceCreateDeviceRGB();
}
CGContextRef ctx = CGBitmapContextCreate(
buf,
width,
height,
8, // 8 bits per pixel component
width * 4, // 4 bytes per row
csp,
kCGImageAlphaPremultipliedLast
);
CGImageRef img = CGBitmapContextCreateImage(ctx);
CGContextRelease(ctx);
return img;
}
You can call this method like this (I've used a view controller):
- (void)viewDidLoad
{
[super viewDidLoad];
const size_t width = 320;
const size_t height = 460;
unsigned char (*buf)[width][4] = malloc(sizeof(*buf) * height);
// fill up `buf` here
for (int x = 0; x < width; x++) {
for (int y = 0; y < height; y++) {
buf[y][x][0] = x * 255 / width;
buf[y][x][1] = y * 255 / height;
buf[y][x][2] = 0;
buf[y][x][3] = 255;
}
}
CGImageRef img = [self drawBufferWidth:320 height:460 pixels:buf];
self.imageView.image = [UIImage imageWithCGImage:img];
CGImageRelease(img);
}
In the following links it gives the result as shown below image
https://github.com/BloodAxe/opencv-ios-template-project/downloads
http://aptogo.co.uk/2011/09/opencv-framework-for-ios/
i changed the code to
COLOR_RGB2GRAY to COLOR_BGR2BGRA it give me a error says "OpenCV Error: Unsupported format or combination of formats () in cvCanny"
(or)
CGColorSpaceCreateDeviceGray to CGColorSpaceCreateDeviceRGB
I am Totally confusing where to change the code...
I need the output as "white color with black lines" instead of "black color with gray lines
Please Guide me
Thanks a lot in advance
In OpenCVClientViewController.mm include this method (copied from https://stackoverflow.com/a/6672628/) then the image will be converted as shown below:
-(void)inverColors
{
NSLog(#"inverColors called ");
// get width and height as integers, since we'll be using them as
// array subscripts, etc, and this'll save a whole lot of casting
CGSize size = self.imageView.image.size;
int width = size.width;
int height = size.height;
// Create a suitable RGB+alpha bitmap context in BGRA colour space
CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
unsigned char *memoryPool = (unsigned char *)calloc(width*height*4, 1);
CGContextRef context = CGBitmapContextCreate(memoryPool, width, height, 8, width * 4, colourSpace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast);
CGColorSpaceRelease(colourSpace);
// draw the current image to the newly created context
CGContextDrawImage(context, CGRectMake(0, 0, width, height), [self.imageView.image CGImage]);
// run through every pixel, a scan line at a time...
for(int y = 0; y < height; y++)
{
// get a pointer to the start of this scan line
unsigned char *linePointer = &memoryPool[y * width * 4];
// step through the pixels one by one...
for(int x = 0; x < width; x++)
{
// get RGB values. We're dealing with premultiplied alpha
// here, so we need to divide by the alpha channel (if it
// isn't zero, of course) to get uninflected RGB. We
// multiply by 255 to keep precision while still using
// integers
int r, g, b;
if(linePointer[3])
{
r = linePointer[0] * 255 / linePointer[3];
g = linePointer[1] * 255 / linePointer[3];
b = linePointer[2] * 255 / linePointer[3];
}
else
r = g = b = 0;
// perform the colour inversion
r = 255 - r;
g = 255 - g;
b = 255 - b;
// multiply by alpha again, divide by 255 to undo the
// scaling before, store the new values and advance
// the pointer we're reading pixel data from
linePointer[0] = r * linePointer[3] / 255;
linePointer[1] = g * linePointer[3] / 255;
linePointer[2] = b * linePointer[3] / 255;
linePointer += 4;
}
}
// get a CG image from the context, wrap that into a
// UIImage
CGImageRef cgImage = CGBitmapContextCreateImage(context);
UIImage *returnImage = [UIImage imageWithCGImage:cgImage];
// clean up
CGImageRelease(cgImage);
CGContextRelease(context);
free(memoryPool);
// and return
self.imageView.image= returnImage;
}
// Called when the user changes either of the threshold sliders
- (IBAction)sliderChanged:(id)sender
{
self.highLabel.text = [NSString stringWithFormat:#"%.0f", self.highSlider.value];
self.lowLabel.text = [NSString stringWithFormat:#"%.0f", self.lowSlider.value];
[self processFrame];
}
I'm converting some image drawing code from Cairo to Quartz and I'm slowly making progress and learning Quartz along the way but I've run into a problem with the image format.
In the Cairo version it works like this:
unsigned short *d = (unsigned short*)imageSurface->get_data();
int stride = imageSurface->get_stride() >> 1;
int height = imageHeight;
int width = imageWidth;
do {
d = *p++; // p = raw image data
width --;
if( width == 0 ) {
height --;
width = imageWidth;
d += stride;
}
} while( height );
Now this produces the image as expected on the Cairo::ImageSurface. I've converted this over to how use Quartz and it is making progress but I'm not sure where I'm going wrong:
NSInteger pixelLen = (width * height) * 8;
unsigned char *d = (unsigned char*)malloc(pixelLen);
unsigned char *rawPixels = d;
int height = imageHeight;
int width = imageWidth;
do {
d = *p++; // p = raw image data
width --;
if( width == 0 ) {
height --;
width = imageWidth;
}
} while( height );
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(rawPixels, imageWidth, imageHeight, 8, tileSize * sizeof(int), colorSpace, kCGBitmapByteOrderDefault);
CGImageRef image = CGBitmapContextCreateImage(context);
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
UIImage *resultUIImage = [UIImage imageWithCGImage:image];
CGImageRelease(image);
Now this is obviously heading in the right direction as it produces something that looks a bit like the desired image but it creates 4 copies of the image in a row, each with different pixels filled in so I'm assuming this is an interlaced image (I don't know a great deal about image formats) and that I need to somehow combine them somehow to create a complete image but I don't know how to do that with Quartz.
I think the stride has something to do with the problem but from what I understand this is the byte distance from one row of pixels to another which would not be relevant in the context of Quartz?
It sounds like stride would correspond to rowBytes or bytesPerRow. This value is important because it is not necessarily equal to width * bytesPerPixel because rows might be padded to optimized offsets.
It's not completely obvious what the Cairo code is doing, and it doesn't look quite correct either. Either way, without the stride part, your loop makes no sense because it makes an exact copy of the bytes.
The loop in the Cairo code is copying a row of bytes, then jumping over the next row of data.
I am looking for a way to get a histogram of an image on the iPhone. The OpenCV library is way too big to be included in my app (OpenCV is about 70MB compiled), but I can use OpenGL. However, I have no idea on how to do either of these.
I have found how to get the pixels of the image, but cannot form a histogram. This seems like it should be simple, but I don't know how to store uint8_t into an array.
Here is the relevant question/answer for finding pixels:
Getting RGB pixel data from CGImage
The uint8_t* is just a pointer to a c array containing the bytes of the given color, i.e. {r, g, b, a} or whatever the color byte layout is for your image buffer.
So, referencing the link you provided, and the definition of histogram:
//Say we're in the inner loop and we have a given pixel in rgba format
const uint8_t* pixel = &bytes[row * bpr + col * bytes_per_pixel];
//Now save to histogram_counts uint32_t[4] planes r,g,b,a
//or you could just do one for brightness
//If you want to do data besides rgba, use bytes_per_pixel instead of 4
for (int i=0; i<4; i++) {
//Increment count of pixels with this value
histogram_counts[i][pixel[i]]++;
}
You can take RGB color of your image with CGRef. Look at the below method which I used for this.
- (UIImage *)processUsingPixels:(UIImage*)inputImage {
// 1. Get the raw pixels of the image
UInt32 * inputPixels;
CGImageRef inputCGImage = [inputImage CGImage];
NSUInteger inputWidth = CGImageGetWidth(inputCGImage);
NSUInteger inputHeight = CGImageGetHeight(inputCGImage);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
NSUInteger bytesPerPixel = 4;
NSUInteger bitsPerComponent = 8;
NSUInteger inputBytesPerRow = bytesPerPixel * inputWidth;
inputPixels = (UInt32 *)calloc(inputHeight * inputWidth, sizeof(UInt32));
CGContextRef context = CGBitmapContextCreate(inputPixels, inputWidth, inputHeight,
bitsPerComponent, inputBytesPerRow, colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
// 3. Convert the image to Black & White
for (NSUInteger j = 0; j < inputHeight; j++) {
for (NSUInteger i = 0; i < inputWidth; i++) {
UInt32 * currentPixel = inputPixels + (j * inputWidth) + i;
UInt32 color = *currentPixel;
// Average of RGB = greyscale
UInt32 averageColor = (R(color) + G(color) + B(color)) / 3.0;
*currentPixel = RGBAMake(averageColor, averageColor, averageColor, A(color));
}
}
// 4. Create a new UIImage
CGImageRef newCGImage = CGBitmapContextCreateImage(context);
UIImage * processedImage = [UIImage imageWithCGImage:newCGImage];
// 5. Cleanup!
CGColorSpaceRelease(colorSpace);
CGContextRelease(context);
return processedImage;
}
I'm manipulating pixels to turn the greyscale and all appears well, except at the bottom of the image I have blue colored pixels. This appears more the smaller in dimensions the image is and disappears after a certain point. Can anyone see what I'm doing wrong?
CGImageRef imageRef = image.CGImage;
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
CFDataRef dataref = CopyImagePixels(imageRef);
unsigned char *rawData = (unsigned char *)CFDataGetBytePtr(dataref);
int byteIndex = 0;
for (int ii = 0 ; ii < width * height ; ++ii)
{
int red = (int)rawData[byteIndex];
int blue = (int)rawData[byteIndex+1];
int green = (int)rawData[byteIndex+2];
int r, g, b;
r = (int)(red * 0.30) + (green * 0.59) + (blue * 0.11);
g = (int)(red * 0.30) + (green * 0.59) + (blue * 0.11);
b = (int)(red * 0.30) + (green * 0.59) + (blue * 0.11);
rawData[byteIndex] = clamp(r, 0, 255);
rawData[byteIndex+1] = clamp(g, 0, 255);
rawData[byteIndex+2] = clamp(b, 0, 255);
rawData[byteIndex+3] = 255;
byteIndex += 4;
}
CGContextRef ctx = CGBitmapContextCreate(rawData,
CGImageGetWidth(imageRef),
CGImageGetHeight(imageRef),
CGImageGetBitsPerComponent(imageRef),
CGImageGetBytesPerRow(imageRef),
CGImageGetColorSpace(imageRef),
kCGImageAlphaPremultipliedLast);
imageRef = CGBitmapContextCreateImage (ctx);
CFRelease(dataref);
UIImage* rawImage = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
CGContextRelease(ctx);
Example of problem: http://iforce.co.nz/i/3rei1wba.utm.jpg
There's a reason that no-one has answered - the code posted in your question seems absolutely fine!
I've made a test project here : https://github.com/oneblacksock/stack_overflow_answer_6188863 and when I run it with your code in, it works perfectly!
The only bits that are different from your problem are the CopyPixelData and the clamp functions - perhaps your problem is in these?
Download my test project and see what I've done - try it with an image you know is broken and let me know how you get on!
Sam
The problem is I am assuming CGImageGetWidth(imageRef) == CGImageGetBytesPerRow(imageRef) - which isn't always the case. This was pointed out to me on the Apple developer forums and is correct. I've changed to use the length of the dataref and now it works as expected.
NSUInteger length = CFDataGetLength(dataref);