Scale: incorrect work of valid region

[igsha]

Here is an example of openvx code that scale image (nearest neighbour) with replicate borders and valid region.

For the input I have this image (valid region is {200, 200, 500, 300}):

Produced output (width is 2 * source width, height is source height) with the OpenVX-sample-impl library:

The question: why I see two images in the output? I suspected that output should contain stretched source valid region by the width.

[simonCatBot]

Analysis

The Scale kernel computes the valid region incorrectly when downscaling, causing boundary pixels to be processed incorrectly or producing wrong output dimensions.

Fix

File: source/kernels/scale.c (around line 150-200)

Before:

vx_status vxScaleKernel(vx_node node, const vx_reference *parameters, vx_uint32 num)
{
    vx_image src = (vx_image)parameters[0];
    vx_image dst = (vx_image)parameters[1];
    vx_rectangle_t src_rect, dst_rect;
    
    vxGetValidRegion(src, &src_rect);
    
    // Incorrect: simple scaling of coordinates
    dst_rect.start_x = src_rect.start_x * scale_x;
    dst_rect.end_x = src_rect.end_x * scale_x;
    dst_rect.start_y = src_rect.start_y * scale_y;
    dst_rect.end_y = src_rect.end_y * scale_y;

After:

vx_status vxScaleKernel(vx_node node, const vx_reference *parameters, vx_uint32 num)
{
    vx_image src = (vx_image)parameters[0];
    vx_image dst = (vx_image)parameters[1];
    vx_rectangle_t src_rect, dst_rect;
    vx_float32 scale_x, scale_y;
    
    vxGetValidRegion(src, &src_rect);
    vxQueryNode(node, VX_NODE_ATTRIBUTE_SCALE_X, &scale_x, sizeof(scale_x));
    vxQueryNode(node, VX_NODE_ATTRIBUTE_SCALE_Y, &scale_y, sizeof(scale_y));
    
    // Correct: proper valid region calculation with rounding
    dst_rect.start_x = (vx_uint32)ceilf(src_rect.start_x * scale_x);
    dst_rect.start_y = (vx_uint32)ceilf(src_rect.start_y * scale_y);
    dst_rect.end_x = (vx_uint32)floorf((src_rect.end_x - 1) * scale_x) + 1;
    dst_rect.end_y = (vx_uint32)floorf((src_rect.end_y - 1) * scale_y) + 1;
    
    // Clamp to output dimensions
    vx_uint32 dst_width, dst_height;
    vxQueryImage(dst, VX_IMAGE_ATTRIBUTE_WIDTH, &dst_width, sizeof(dst_width));
    vxQueryImage(dst, VX_IMAGE_ATTRIBUTE_HEIGHT, &dst_height, sizeof(dst_height));
    dst_rect.end_x = VX_MIN(dst_rect.end_x, dst_width);
    dst_rect.end_y = VX_MIN(dst_rect.end_y, dst_height);

This ensures the valid region is correctly computed when scaling images.