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eia608: several fixes to interrupt-drive Line 21 drawing
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@rasky
rasky committed Dec 9, 2024
1 parent c8d82cc commit e954ef1
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Showing 2 changed files with 81 additions and 28 deletions.
13 changes: 8 additions & 5 deletions include/eia608.h
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Original file line number Diff line number Diff line change
Expand Up @@ -21,17 +21,20 @@
* *any* change to VI configuration, make sure to call #eia608_stop first,
* and then call #eia608_start again.
*
* To generate captions, you can use #eia608_caption to display a UTF-8
* string on the screen. The function will take care of encoding the string
* into EIA-608 format, doing some basic formatting (word-wrap, and centering),
* and hiding it after the specified number of seconds.
* To generate captions, you can use #eia608_caption_prepare and #eia608_caption_show
* to display a UTF-8 string on the screen. These functions will take care of
* encoding the string into EIA-608 format, doing some basic formatting (word-wrap,
* and centering), and hiding it after the specified number of seconds.
*
* This module also provides a way to display raw EIA-608 data, via
* #eia608_write_raw. One possible use case for this function is to
* display captions saved into the Scenarist SCC format, which is a very
* simple text format that encodes captions as sequence of raw 16-bit
* EIA-608 words.
*
* @note The runtime performance impact of this module is around ~100 µs per
* VI frame (so 60 Hz), irrespective of the actual game speed.
*
* @note EIA-608 is a standard for NTSC video signals. It is not compatible
* with PAL or MPAL TVs.
*
Expand Down Expand Up @@ -233,7 +236,7 @@ void eia608_write_ctrl_raw(eia608_ctrl_t ctrl);
void eia608_caption_prepare(eia608_channel_t cc, const char *utf8_str, eia608_captionparms_t *parms);

/**
* @brief Show a caption that was previously prepared with #eia608_caption
* @brief Show a caption that was previously prepared with #eia608_caption_prepare
*
* This shows a caption that was prepared by #eia608_caption_prepare. The caption
* will be displayed on the screen for the specified duration, and then hidden.
Expand Down
96 changes: 73 additions & 23 deletions src/eia608.c
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,22 @@
#include <assert.h>
#include <string.h>

/**
* @brief Activate debugging mode for EIA608 signal
*
* This macro will do a few things:
* - Allow PAL
* - Stick the EIA608 line to the top of the output window
* - Make the EAI608 line 8 lines big so that it's more visible
*
* This would allow to visually debug the implementation by configuring large
* borders, so that EIA608 becomes visible. Obviously it will likely break
* *actual* captions.
*/
#define DEBUG_EIA608 0

#define EIA608_IRQ_SAFEMARGIN 4 ///< How many halflines to trigger the IRQ before the actual line

#define SIG_ON 0x8421 ///< IRE 50 (50% intensity RGBA16)
#define SIG_OFF 0x0000 ///< IRE 0

Expand All @@ -36,12 +52,13 @@ static int force_clear_timer;
static int irq_errors;

static struct {
bool interlaced; ///< Whether the parameters refer to interlaced mode or not
uint32_t reg_ctrl; ///< Value to set for VI_CTRL
uint32_t reg_h_video; ///< Value to set for VI_H_VIDEO
uint32_t reg_xscale; ///< Value to set for VI_X_SCALE
int v_line_start; ///< Lines to draw
int v_line_end; ///< Lines to draw
bool interlaced; ///< Whether the parameters refer to interlaced mode or not
uint32_t reg_ctrl; ///< Value to set for VI_CTRL
uint32_t reg_h_video; ///< Value to set for VI_H_VIDEO
uint32_t reg_xscale; ///< Value to set for VI_X_SCALE
int v_line_start; ///< Lines to draw
int v_line_end; ///< Lines to draw
int out_x0, out_y0, out_x1, out_y1; ///< Previous output window
} sigparms;

static uint16_t* linebuffer_write_bits(uint16_t* buffer, bool on, int nbits)
Expand Down Expand Up @@ -110,6 +127,7 @@ static void recalc_parms(void)
ctrl |= !sys_bbplayer() ? VI_PIXEL_ADVANCE_DEFAULT : VI_PIXEL_ADVANCE_BBPLAYER;
ctrl |= VI_CTRL_TYPE_16_BPP; // 16-bit color
ctrl |= VI_AA_MODE_RESAMPLE; // resample AA mode
ctrl |= vi_read(VI_CTRL) & VI_CTRL_SERRATE; // keep interlace mode if active

bool interlaced = ctrl & VI_CTRL_SERRATE;

Expand Down Expand Up @@ -143,7 +161,17 @@ static void recalc_parms(void)
sigparms.reg_h_video = (h_start << 16) | h_end;
sigparms.reg_xscale = (xoffset_fx << 16) | xscale_fx;
sigparms.v_line_start = v_halfline;
sigparms.v_line_end = v_halfline + interlaced ? 2 : 1;
sigparms.v_line_end = v_halfline + (interlaced ? 2 : 1);

// For now, we only support default NTSC output where the real output
// begins on line 23, just one line after the EIA-608 signal.
vi_get_output(&sigparms.out_x0, &sigparms.out_y0, &sigparms.out_x1, &sigparms.out_y1);
#if DEBUG_EIA608
sigparms.v_line_start = sigparms.out_y0 - 16;
sigparms.v_line_end = sigparms.out_y0;
#endif
assertf(sigparms.v_line_end == sigparms.out_y0,
"EIA-608: unimplemented support for borders");
}

static void __eia608_interrupt(void)
Expand All @@ -170,53 +198,63 @@ static void __eia608_interrupt(void)
// Draw the data into the linebuffer
linebuffer_write(&linebuffer, data);
}

// The VI should still be drawing the line *before* the one that we want
// to draw on. If we're not there, it means that the interrupt was late,
// so we'll just skip this frame.
if ((*VI_V_CURRENT|1) != sigparms.v_line_start-2) {
if ((*VI_V_CURRENT|1) >= (sigparms.v_line_start|1)-2) {
#if DEBUG_EIA608
debugf("EIA-608: IRQ error %ld %d\n", *VI_V_CURRENT, sigparms.v_line_start);
#endif
++irq_errors;
return;
}

// Fetch the current VI configuration
uint32_t old_origin = *VI_ORIGIN;
uint32_t old_width = *VI_WIDTH;
uint32_t old_ctrl = *VI_CTRL;
uint32_t old_h_video = *VI_H_VIDEO;
uint32_t old_x_scale = *VI_X_SCALE;
uint32_t old_y_scale = *VI_Y_SCALE;

// Wait for the line to match the one we're expecting
while ((*VI_V_CURRENT|1) != sigparms.v_line_start) {}
uint32_t new_origin = (uint32_t)(linebuffer.buffer);
uint32_t new_ctrl = sigparms.reg_ctrl;
uint32_t new_h_video = sigparms.reg_h_video;
uint32_t new_x_scale = sigparms.reg_xscale;
uint32_t new_y_scale = 0; // FIXME: this seems to revert the interlacing fix?

// Wait for the line to match the one we're expecting
while ((*VI_V_CURRENT|1) < (sigparms.v_line_start|1)-2) {}

// We are now in hblank. Quickly switch VI configuration to draw the
// linebuffer.
*VI_ORIGIN = PhysicalAddr(linebuffer.buffer);
*VI_WIDTH = linebuffer.width;
*VI_CTRL = sigparms.reg_ctrl;
*VI_H_VIDEO = sigparms.reg_h_video;
*VI_X_SCALE = sigparms.reg_xscale;
*VI_Y_SCALE = 0;
// linebuffer. Notice that we don't need to set VI_WIDTH because Y scale is 0 anyway.
*VI_ORIGIN = new_origin;
*VI_CTRL = new_ctrl;
*VI_H_VIDEO = new_h_video;
*VI_X_SCALE = new_x_scale;
*VI_Y_SCALE = new_y_scale;
MEMORY_BARRIER();

// Wait for the EIA-608 signal to be displayed
// Displaying one line takes less than 60us, so we realistically don't
// have time to do much else, and it would be a waste to retrigger another
// interrupt.
while ((*VI_V_CURRENT|1) < sigparms.v_line_end) {}
while ((*VI_V_CURRENT|1) < (sigparms.v_line_end|1)) {}

// Restore the original VI configuration
*VI_ORIGIN = old_origin;
*VI_WIDTH = old_width;
*VI_CTRL = old_ctrl;
*VI_H_VIDEO = old_h_video;
*VI_X_SCALE = old_x_scale;
*VI_Y_SCALE = old_y_scale;
MEMORY_BARRIER();
}

void eia608_init(void)
{
#if !DEBUG_EIA608
assertf(get_tv_type() == TV_NTSC, "EIA-608 is only supported on NTSC");
#endif
assert(ring_buffer == NULL);
ring_buffer = malloc(RING_BUFFER_SIZE * 2);
rb_rpos = rb_wpos = 0;
Expand All @@ -243,8 +281,20 @@ void eia608_start(void)
for (int i=0; i<30; i++)
ring_buffer[rb_wpos++] = EIA608_NOP;

// Enable the line interrupts
vi_set_line_interrupt(sigparms.v_line_start-2, __eia608_interrupt);
vi_write_begin();
// Enable the line interrupt
vi_set_line_interrupt(sigparms.v_line_start-EIA608_IRQ_SAFEMARGIN, __eia608_interrupt);
// Increase the output window to include the EIA-608 signal
vi_set_output(sigparms.out_x0, sigparms.v_line_start, sigparms.out_x1, sigparms.out_y1);
vi_write_end();
}

void eia608_stop(void)
{
vi_write_begin();
vi_set_output(sigparms.out_x0, sigparms.out_y0, sigparms.out_x1, sigparms.out_y1);
vi_set_line_interrupt(sigparms.v_line_start-EIA608_IRQ_SAFEMARGIN, NULL);
vi_write_end();
}

static uint8_t odd_parity(uint8_t value)
Expand Down

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