Aspect ratio of an image resized with Opencv?

Question

I have a project where I need to identify objects in an image, currently using the Opencv library in C++. I used the function warpPerspective() Opencv on a 605x141 size image to resize your perspective to a 674x35 size image. In doing so the object of the image was distorted as in the illustration of Example in the image below:

Let’s say the width of the object in the first image is 100, for example. How can I identify the new width of the object in the new distorted image? It seems obvious but I tried to use the basic rule of three, using the area and aspect ratio of the two images, but the result is not clicking when I manually compare the width in an image editor.

Note: If this question is not compatible with any forum policy, please let me know so I can try to rewrite it.

EDIT:

I do not know exactly if it will help to solve the problem, but here is the code where I perform the perspective transformation:

void MyClass::TransformPerspec(){

Mat mat = cvCreateMat(3, 3, CV_8UC1);
this->fileira = cvCreateMat(35, 674, CV_8UC1); // matriz de saida da fileira será de tamanho 674x35

// Input Quadilateral or Image plane coordinates
Point2f inputQ[4];
// Output Quadilateral or World plane coordinates
Point2f outputQ[4];

inputQ[0].x = this->X1; inputQ[0].y = this->Y1;
inputQ[1].x = this->X2; inputQ[1].y = this->Y2;
inputQ[2].x = this->X3; inputQ[2].y = this->Y3;
inputQ[3].x = this->X4; inputQ[3].y = this->Y4;

outputQ[0].x = 0 ; outputQ[0].y = 0;
outputQ[1].x = 674;outputQ[1].y = 0;
outputQ[2].x = 0;  outputQ[2].y = 35;
outputQ[3].x = 674;outputQ[3].y = 35;

mat = getPerspectiveTransform(inputQ, outputQ);
warpPerspective(this->imagem_proc, this->fileira, mat, this->fileira.size());

imwrite("Teste.bmp", this->fileira);

}

After perspective transformation I just call this method:

reconheceObject(int largura);

where I use the object width of the first image for recognition, but since the object is distorted after the perspective transform, the 'width' value needs to be recalculated based on the new image. So I initially thought I could use a rule of three to find the new width. But now I know that a possible solution is not so simple.

Answer 1

Hey, I think you need the job resize(src,dst,size), where src is your original image and dst is a new image variable that you will create and size is the final image size that you can declare as such: Size(100,100).

Answer 2

To maintain the proportions of the original image without any distortions you can only choose arbitrarily the size of one of the new dimensions, the other dimension will increase or decrease according to the change in the chosen dimension, So if your image has 100 height and you want to increase it to 140 (40% growth), your new width should be 40% larger. This is valid for the choice of both dimensions. The following code snippet gives an idea of how to solve.

    double novaLargura = antigaLargura * novaAltura / antigaAltura;
    resize(img, img, Size(novaLargura, novaAltura));