C中的双三次插值

Question

我正在尝试处理 c 中的 双三次图像插值。因此我构建了这个小脚本。

1. "resize_image"-函数：

void resize_image(PPMImage *source_image, PPMImage *destination_image, float scale) { uint8_t sample[3]; int y, x; destination_image->x = (long)((float)(source_image->x)*scale); destination_image->y = (long)((float)(source_image->y)*scale); for (y = 0; y < destination_image->y; y++) { float v = (float)y / (float)(destination_image->y - 1); for (x = 0; x < destination_image->x; ++x) { float u = (float)x / (float)(destination_image->x - 1); sample_bicubic(source_image, u, v, sample); destination_image->data[x+((destination_image->y)*y)].red = sample[0]; destination_image->data[x+((destination_image->y)*y)].green = sample[1]; destination_image->data[x+((destination_image->y)*y)].blue = sample[2]; } } }

2。 "sample_bicubic"-函数

void sample_bicubic(PPMImage *source_image, float u, float v, uint8_t sample[]) { float x = (u * source_image->x)-0.5; int xint = (int)x; float xfract = x-floor(x); float y = (v * source_image->y) - 0.5; int yint = (int)y; float yfract = y - floor(y); int i; uint8_t p00[3]; uint8_t p10[3]; uint8_t p20[3]; uint8_t p30[3]; uint8_t p01[3]; uint8_t p11[3]; uint8_t p21[3]; uint8_t p31[3]; uint8_t p02[3]; uint8_t p12[3]; uint8_t p22[3]; uint8_t p32[3]; uint8_t p03[3]; uint8_t p13[3]; uint8_t p23[3]; uint8_t p33[3]; // 1st row get_pixel_clamped(source_image, xint - 1, yint - 1, p00); get_pixel_clamped(source_image, xint + 0, yint - 1, p10); get_pixel_clamped(source_image, xint + 1, yint - 1, p20); get_pixel_clamped(source_image, xint + 2, yint - 1, p30); // 2nd row get_pixel_clamped(source_image, xint - 1, yint + 0, p01); get_pixel_clamped(source_image, xint + 0, yint + 0, p11); get_pixel_clamped(source_image, xint + 1, yint + 0, p21); get_pixel_clamped(source_image, xint + 2, yint + 0, p31); // 3rd row get_pixel_clamped(source_image, xint - 1, yint + 1, p02); get_pixel_clamped(source_image, xint + 0, yint + 1, p12); get_pixel_clamped(source_image, xint + 1, yint + 1, p22); get_pixel_clamped(source_image, xint + 2, yint + 1, p32); // 4th row get_pixel_clamped(source_image, xint - 1, yint + 2, p03); get_pixel_clamped(source_image, xint + 0, yint + 2, p13); get_pixel_clamped(source_image, xint + 1, yint + 2, p23); get_pixel_clamped(source_image, xint + 2, yint + 2, p33); // interpolate bi-cubically! for (i = 0; i < 3; i++) { float col0 = cubic_hermite(p00[i], p10[i], p20[i], p30[i], xfract); float col1 = cubic_hermite(p01[i], p11[i], p21[i], p31[i], xfract); float col2 = cubic_hermite(p02[i], p12[i], p22[i], p32[i], xfract); float col3 = cubic_hermite(p03[i], p13[i], p23[i], p33[i], xfract); float value = cubic_hermite(col0, col1, col2, col3, yfract); CLAMP(value, 0.0f, 255.0f); sample[i] = (uint8_t)value; printf("sample[%d]=%d\n",i,sample[i]); } }

3。 "interpolation helpers"

float cubic_hermite(float A, float B, float C, float D, float t) { float a = -A / 2.0f + (3.0f*B) / 2.0f - (3.0f*C) / 2.0f + D / 2.0f; float b = A - (5.0f*B) / 2.0f + 2.0f*C - D / 2.0f; float c = -A / 2.0f + C / 2.0f; float d = B; return a*t*t*t + b*t*t + c*t + d; } void get_pixel_clamped(PPMImage *source_image, int x, int y, uint8_t temp[]) { CLAMP(x, 0, source_image->x - 1); CLAMP(y, 0, source_image->y - 1); temp[0] = source_image->data[x+(W*y)].red; temp[1] = source_image->data[x+(W*y)].green; temp[2] = source_image->data[x+(W*y)].blue; }

我已经上传了包含所有内容的完整代码 here。

执行此代码没有语法错误。

但是输出图像让我很困惑。

输入图像（21x20像素）：

此输入图像按比例放大 2（42x40 像素）：

插值在某些点上似乎工作正常，但图像看起来像素发生了偏移。

有人可以告诉我我做错了什么吗？这个脚本是在以下帮助下制作的： http://blog.demofox.org/2015/08/15/resizing-images-with-bicubic-interpolation/

谢谢大家！

（请不要考虑这段代码的效率......我知道它很棒）

Answer 1

来自您的 resize_image() 函数：

destination_image->data[x+((destination_image->y)*y)].red   = sample[0];

应该是

destination_image->data[x+((destination_image->x)*y)].red   = sample[0];

有助于调试此类情况的方法是使用一些 "magic color" 实际数据中不存在的目标图像（例如一些糟糕的粉红色:-)）。然后您可能会注意到在 resize_image() 调用之后某些目标像素仍然具有该颜色。这提示了问题。

Answer 2

我测试了你的代码，发现还有一个问题。

来自您的 init_destination_image() 函数：

img->data = (PPMPixel*)malloc(W * H * (int)scale * sizeof(PPMPixel));

应该是：

img->data = (PPMPixel*)malloc(W * H * (int)scale * (int)scale * sizeof(PPMPixel));

例如，W 和 H 正在缩放。

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