问题
My application needs to capture some pictures of a given size (lets say WxH) in a portrait orientation mode.
In a general case the size WxH I want is not supported by the camera therefore I will need to crop the captured picture in order to match my specifications.
This apparently simple program is driving me crazy for the problem of "good" corrispondence among preview and picture sizes and format. Let me explain:
I need a format (let's give some numbers: 800x600) for the output image, and I have to take pictures in a portrait screen orientation. My camera by default is in Landscape mode therefore it takes pictures with a Width much larger than the height. But since I want a portrait preview I need to rotate the image and as a consequence I get images with an height much larger than the width (the transpose of the original image I guess).
In this scenario I need to cut a horizontally extended rectangle from a bigger vertically extended rectangle and I would like to do that by having an accettable large preview.
the problem of cropping the out image from the picture does not scare me (for the moment), the mean problem is the matching among what the user sees into the preview and what the camera actually captures.
for each possible phone I need to:
- chose a suitable camera picture size with respect the desired image format
- chose a suitable camera preview size with respect to the picture size and format.
- hide the preview parts that will be cropped.
And with the constraints of no distortion and large preview.
How to do it in general?
What I thought and tried:
the main algorithm steps are:
- get the optimal picture size once known the desired format
- get the optimal preview size once known the picture size
- hide the parts not capturable of the preview
- crop the image
tried method 1)
A) I get the optimal picture size by minimizing the area difference (I could also check the aspect ratio affinity is not very important). (Size is a custom type different from Camera.Size)
public Size getOptimalPictureSize(List<Camera.Size> sizes) {
Size opt = new Size();
float objf = Float.MAX_VALUE;
float v;
for(Camera.Size s : sizes){
if(s.height<target_size.width || s.width<target_size.height)
continue;
v = (s.height-target_size.width)*s.width + (s.width-target_size.height)*target_size.width;
if(v<objf){
opt.width=s.width;
opt.height=s.height;
objf=v;
}
}
return opt;
}
B) I get the optimal preview size by finding the best compromise among different aspect ratio (with respect to the picture size) :
@Override
public Size getOptimalPreviewSize(Size picSize,List<android.hardware.Camera.Size> sizes) {
Size opt = new Size();
double objf = Double.MAX_VALUE;
double aspratio = picSize.getAspectRatio();
double v;
for(Camera.Size s : sizes){
v = Math.abs( ((double)s.width)/((double)s.height) - aspratio )/(Math.max(((double)s.width)/((double)s.height), aspratio));
if(v<objf){
objf=v;
opt.width=s.width;
opt.height=s.height;
}
}
return opt;
}
C) hiding methods for displaying only capturable parts....(discussed later)
** Trial 2) **
A) I get the picture and preview sizes by minimizing an optimality functions that weights at the same time the misfit among camera image aspect ratio and desired one and the misfit among the preview and picture aspect ratio.
public void setOptimalCameraSizes(List<Camera.Size> preview_sizes,List<Camera.Size> picture_sizes,Size preview_out, Size picture_out) {
double objf=Double.MAX_VALUE;
double tmp;
for(Camera.Size pts : picture_sizes){
for(Camera.Size pws : preview_sizes){
tmp = percv(((double)pws.height)/((double)pws.width),target_size.getAspectRatio())
+ percv(((double)pws.width)/((double)pws.height),((double)pts.width)/((double)pts.height));
if(tmp<objf){
preview_out.set(pws.width, pws.height);
picture_out.set(pts.width, pts.height);
objf=tmp;
}
}
}
}
where
percv(a,b) = |a-b|/max(|a|,|b|) measures the relative deviation (and thus is dimensionless).
C) some hiding methods...
Ok this two sizes selection methods are the best I found and chose good sizes, but they have a physiological problem that comes from the camera landscape orientation... they can only produce vertical rectangular images and this implies that when I draw the preview I can get two cases:
1. I set the surface dimensions so as not distort the preview image -> due to the huge height this reflects in a very small valid area in which the image is visible (so the user experience is hurted)
2. I set the maximum possible width -> I can obtain (it depends on the preview aspect ratio) distorted previews but much bigger than in case 1.
How to avoid these problems??
What I though is work on phase C) of the algorithm (hiding phase) and I tried to:
trial 1: make the camera preview go beyond the screen sizes. This will allow me to make an arbitrary zoom in the area of interest and make the screen crop the preview. I tried using a scrollview but it didn't work and I don't know why. The topology was simple a root scrollview and inside a FrameLayout with the attached surfaceview but the surface always filled the screen leading to horrible distortions.
trial 2: capture the camera frame and manipulate them directly by overriding the onPreviewFrame(.) method: I got a misteryous error in locking the canvas (IllegalArgumentException)
How can I solve this?
来源：https://stackoverflow.com/questions/24491431/android-camera-preview-size-picture-size-cropping-and-distortions