pixel_mode.c

Understanding pixel formats and RGB packing.

Understanding pixel formats and RGB packing.Pixel formats define how RGB colour values are packed into integers. Different formats trade off memory usage vs colour precision. This example demonstrates how the same RGB values produce different packed integers depending on the pixel format.

 
#include <banjo/log.h>
#include <banjo/main.h>
#include <banjo/pixel.h>
 
void display_value(bj_pixel_mode mode, uint8_t red, uint8_t green, uint8_t blue) {
    // bj_get_pixel_value() converts 8-bit RGB components (0-255) into a packed
    // integer value according to the specified pixel format. The packing order
    // and bit depth vary by format.
    const uint32_t val = bj_get_pixel_value(mode, red, green, blue);
 
    // Convert the packed value to binary string for visualization of bit layout.
    char binarystr[33] = {0};
    for(size_t b = 0 ; b < 32 ; ++b) {
        binarystr[31-b] = '0' + ((val >> b) & 0x01);
    }
 
    bj_info("R:%d, G:%d, B:%d -[0x%08x]--> %ld\t0x%08x\t0b%s",
        red, green, blue, mode, 0, val, binarystr
    );
}
 
int main(int argc, char* argv[]) {
    (void)argc;
    (void)argv;
 
    // RGB565: 16-bit format with 5 bits red, 6 bits green, 5 bits blue.
    // Uses less memory (2 bytes per pixel) but lower colour precision.
    // Green gets 6 bits because human eyes are most sensitive to green.
    display_value(BJ_PIXEL_MODE_RGB565, 255, 0, 0);
    display_value(BJ_PIXEL_MODE_RGB565, 0, 255, 0);
    display_value(BJ_PIXEL_MODE_RGB565, 0, 0, 255);
 
    // XRGB1555: 16-bit format with 1 unused bit, then 5 bits each for RGB.
    // Also 2 bytes per pixel but with equal precision across all channels.
    display_value(BJ_PIXEL_MODE_XRGB1555, 255, 0, 0);
    display_value(BJ_PIXEL_MODE_XRGB1555, 0, 255, 0);
    display_value(BJ_PIXEL_MODE_XRGB1555, 0, 0, 255);
 
    // XRGB8888: 32-bit format with 8 bits per channel (full precision).
    // Uses 4 bytes per pixel but retains all 8 bits of input colour data.
    // The 'X' byte is unused padding for alignment.
    display_value(BJ_PIXEL_MODE_XRGB8888, 255, 0, 0);
    display_value(BJ_PIXEL_MODE_XRGB8888, 0, 255, 0);
    display_value(BJ_PIXEL_MODE_XRGB8888, 0, 0, 255);
 
    return 0;
}